<![CDATA[JayWink Solutions - Blog]]>Wed, 12 Jun 2024 06:18:41 -0400Weebly<![CDATA[Occupational Soundscapes – Part 17:  Total Soundscape Management]]>Wed, 12 Jun 2024 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-17-total-soundscape-management     Total Soundscape Management (TSM) is a comprehensive program of occupational, recreational, and community noise control, hearing conservation and health monitoring, communication, regulatory compliance, and an overarching awareness and education campaign.  That is, TSM integrates the activities, responsibilities, and all subject matter presented throughout the “Occupational Soundscapes” series into a holistic multidisciplinary program administered by organizational (i.e. employer) leaders.
     To realize its full potential, a TSM initiative requires the engagement of all members of an organization, irrespective of their enrollment status in a hearing loss prevention program (HLPP), and the surrounding community.  The tenets of TSM must be lived; that is, adherence to its principles pursued in all aspects of life – at work, at play, and at rest.
An Integrated Approach
     Total Soundscape Management (TSM) resembles other “total” management programs, such as Total Quality Management (TQM) or Total Productive Maintenance (TPM), in that a multidisciplinary approach is adopted.  This type of program takes a broader view of responsibilities, influences, and outcomes, recognizing that performance improvement on any metric, or the attainment of objectives, can rarely be attributed to a single entity, be it a person, discipline, or department.  TSM differs from these programs in that it extends beyond professional responsibilities, encompassing all activities in which one might engage.
     This presentation of TSM is the result of evolutionary forces that have acted on the “Occupational Soundscapes” series throughout its development.  At the outset, noise-induced hearing loss (NIHL) and its prevention in workplaces was the declared focus.  However, as the series progressed, topics on the periphery were found to be too intricately linked, and too important, to exclude.  Perhaps a new title would be appropriate for a revised edition of the series.  For now, the title reflects the centrality of occupational noise to any discussion of hearing health, as it poses the greatest risk to hearing health for a large portion of the population.
     A Total Soundscape Management program integrates the following components:
  • Regulatory Compliance
  • Hearing Loss Prevention Program (HLPP)
  • Ototoxicity and Health Interactions
  • Sound as an Ingredient of Wellness
  • Noise Control
  • Quiet Product Development and Equipment Procurement
  • Communication System Design
  • Community Noise Management
  • Personal Soundscape Management
  • Awareness and Education
  • Continuous Improvement
Each component of TSM is discussed below, with references given to previous installments of this series where relevant topics are presented in greater detail.  Additional information is provided to reinforce the connections between subjects as interdependent components of a holistic approach to the management of sound and health.

Regulatory Compliance
     Compliance with all regulatory and statutory requirements of the various agencies and jurisdictions to which one might be subject is only the first milestone of a successful TSM program.  Detrimental effects of noise can be experienced despite full compliance; legal liability for those detriments, be they individual or community, remains with the generator (i.e. source) of noise and comes at potentially great expense.
     In the USA, noise-relevant regulations include:
  • OSHA defines a 90-dB criterion level and a 5-dB exchange rate; NIOSH recommends an 85-dB criterion level and a 3-dB ER (see Part 6).
  • OSHA requires a baseline audiogram be acquired within six months of an employee’s first exposure and subsequent audiometric testing on an annual basis thereafter.  However, NIHL can occur on a much shorter timeline (see Part 5).
  • OSHA permits “age-corrected” determinations of threshold shifts; NIOSH no longer recommends this practice for fear that NIHL will be underreported and left untreated.
  • EPA requires hearing protection devices (HPDs) to be clearly labeled with a Noise Reduction Rating (NRR), though it cannot be assured that users will attain the cited attenuation (see Part 14).
  • EPA requires products to be labeled with a Noise Rating (NR) and an approximate noise range of comparable products.
  • Community noise ordinances are established by municipalities or other local jurisdictional authority and can vary widely (see Part 15).  Similar regulatory schemes are in place in various countries around the world.  For any site location, local, regional, national, and international standards, regulations, ordinances, and statutes must be consulted to ensure compliance.
  • Industry-specific requirements are established by Department of Labor (OSHA, MSHA), Department of Transportation (FRA, FMCSA), Department of Defense (collectively and each military branch individually), Department of Energy, NASA, and so on.  Classification of industries and activities defines the portion(s) of the web of regulations that are relevant.
  • The Americans with Disabilities Act (ADA) requires “reasonable accommodations” be made to allow otherwise-capable individuals to perform their duties.  However, functional hearing requirements are acceptable in some situations (see Part 9).

Hearing Loss Prevention Program (HLPP)
     An effective hearing loss prevention program (HLPP) or hearing conservation program (HCP) involves a variety of disciplines, requiring medical expertise, engineering, and various supervisory and administrative skills.  It consists of several essential components, including:
  • sound level measurement
  • noise control
  • hearing protection
  • auditory health monitoring
  • motivation and education
  • recordkeeping
  • program review and modification
     Sound level measurement is required to determine the types, sources, and levels of sound exposures.  The measurements are used to identify at-risk employees that must be enrolled in an HLPP.  Compiling this information on a hazard map or sound survey map facilitates selection and prioritization of noise control projects.
     Though listed as a single component of an HLPP, noise control consists of a wide variety of activities, including both physical and behavioral changes (i.e. engineering and administrative controls; see Parts 11, 12, 13, 14).  The sound survey map is a critical input to the source-path-receiver (SPR) model (see Part 11).  Once a source is identified, research begins on potential sound level reductions that can be achieved by each component of the model.
     Hearing protection comprises a substantial portion of receiver noise control efforts (see Part 14).  Sourcing, evaluating, and ensuring proper use of hearing protection devices (HPDs) are critical components of program success and, thus, warrant significant attention.  Encouraging use of HPDs is advisable at the first suspicion that a noise-exposure issue may exist.  Ideally, some form of HPD will always be available to protect personnel from occasional exposures due to nonroutine activities and to accommodate those with greater sensitivity.  Previous use or familiarity with HPDs may also facilitate a transition to compulsory use when criterion-level exposures have been confirmed (see Part 6).
     Audiometric testing is often the most conspicuous element of auditory health monitoring, but it is not the entirety.  Infections and injuries resulting from HPD use, contamination, etc. must also be treated appropriately when they occur.
     Motivation and education exhibit a type of reciprocity; each fuels the other.  Motivated individuals are more receptive to information and suggestions for improvement.  Educated individuals tend to be more enthusiastic about protecting auditory health and improving conditions.  Education is typically associated with the “how” of hearing conservation, while motivation is usually derived from the “why.”
     Recordkeeping is important to both internal participants of a program and to external parties, such as regulatory agencies.  Program records are used for compliance, research, and assessments of performance.
     Internally, documentation of sound survey data, designs of noise control mechanisms and results obtained, equipment calibration history, and similar information relevant to hearing-conservation efforts add value to the program.  Audiometric test results must be made available to employees’ general practitioner for use in health assessments; this is usually handled between medical professionals to ensure privacy protection.  External reporting may include confirmed occurrences of noise-induced hearing loss (NIHL), verification of compliance with regulations, and similar information.
     Establishing an HLPP is only the beginning.  Program review and modification ensures continued regulatory compliance and program efficacy.  All aspects of a program should be evaluated periodically, including frequency of audiometric testing, effectiveness of controls and HPDs, clarity and accessibility of records, content and delivery of educational material, and financial impacts of the program. 

Ototoxicity and Health Interactions
     The effects of chemical exposure (i.e. ototoxicity, neurotoxicity) require additional attention when experienced in conjunction with high-intensity noise.  Likewise, health conditions, such as cardiovascular disease or diabetes, may be exacerbated by or increase sensitivity to noise.  Additional medical expertise is required to evaluate and treat synergistic conditions (see Part 8).

Sound as an Ingredient of Wellness
     Wellness encompasses a number of interrelated and seemingly unrelated influences on one’s life.  Extra-auditory effects of noise (see Part 8) can impact one’s physical and mental health in profound ways.  This is true during exposure (e.g. annoyance, communication difficulty, stress, sleep disturbance) and after (e.g. hearing loss, tinnitus, depression, cognitive decline).  Wellness programs often emphasize diet and exercise, regular checkups, and mindfulness.  The short- and long-term effects of noise exposure should be given similar attention, as they are all important, related topics.
     Exposure to infrasound or ultrasound can cause a stress response; because these frequencies are inaudible, the cause of discomfort may be difficult to diagnose or the source difficult to identify.  Fortunately, exposures of sufficient intensity to cause health concerns (infrasound > 120 dB SPL; ultrasound > 105 dB SPL) are rare.  Despite their rarity, maintaining awareness of possible infrasound and ultrasound exposures and potential effects ensures a thorough wellness program.  After all, these are merely extensions of the audible soundscape.

Noise Control
     Though it has been presented as one of the essential components of an HLPP, noise control deserves a place at the top level of TSM.  It is a critical component of soundscape management in any context that can be named, including occupational, community, or recreational noise.  Noise control is an umbrella term that encompasses a vast array of techniques, requiring four installments of this series (Parts 11, 12, 13, and 14) to provide an adequate introduction to the subject; the thread continues into a fifth (Part 15).  This fact, alone, renders further defense of this position moot.

Quiet Product Development and Equipment Procurement
     One lever of control in an occupational setting is the specification of equipment requirements.  Setting limits on noise levels of custom machinery ensures that noise-exposure concerns are addressed in the design phase and verified prior to installation.  Specifying acceptable sound levels of standard equipment directs buyers to establish procurement agreements only with suppliers of compliant products.  While this may limit the number of potential sources, advantageous terms may be obtainable as a result of consolidated purchasing.
     Manufacturers of consumer products may find that lower noise emissions improve subjective assessments and increase sales.  In the long-term, it can bolster a favorable reputation for the product and company, extending the boost in sales.  This places pressure on competitors to reduce sound levels – a scenario in which everyone wins.  If competitive pressure sufficiently reduces sound levels of entire product categories, additional regulation may be unnecessary.  Should regulations be enacted, prior development efforts reduce the burden of achieving compliance (see Part 12).

Communication System Design
     Communication system design includes the selection of verbal and nonverbal auditory signals and any equipment needed to transmit or receive the signals.  Nonverbal signals require specification of tone or spectral composition, intensity, duration, and temporal variation.  The significance of each signal must be clear to all personnel to avoid confusion, inappropriate responses, or inaction.
     Specification of verbal signals can be more complicated.  To maximize effectiveness of speech communication in high-intensity noise, a limited vocabulary can be defined.  Doing so prevents misidentification of spoken words (i.e. confusion of similar words).  Use of high-quality electronic devices can prevent the reduced clarity of speech that may occur with increased vocal effort and the discomfort of vocal strain.
     Auditory signals may be supplemented with other forms of communication, such as visual signals.  When this is done, the various signal types must be compatible to ensure that intended messages are clear.  A number of considerations for communication and system design are presented in Part 9 and Part 10 of this series.

Community Noise Management
     It is important to consider community noise (see Part 15) in conjunction with occupational noise.  Both require sound level measurements, noise control mechanisms, and continued monitoring.  The similarity of tasks simplifies addressing both simultaneously; the need to do so arises from the influence of occupational noise, and efforts to manage it, on surrounding communities.  For example, relocating noisy equipment from inside a facility to outside, or to an adjoining structure, may reduce noise exposure for employees while causing community noise to exceed acceptable levels.  The two are intricately linked and should be treated accordingly.
     A summary of effects and corresponding community response when the day-night average sound level (see Part 15) exceeds 55 dBA is provided in Exhibit 1.  The effects of a 55-dBA DNL are described in the table, while the graph depicts the increase in community response that occurs as average sound levels rise above 55 dBA.  The two vertical scales indicate the difference between annoyance and complaints; many that are annoyed by noise do not openly complain, suggesting that noise concerns are greater than the frequency of complaints reveals.
Personal Soundscape Management
     Management of one’s personal soundscape involves taking responsibility for all sound-related risks to hearing and well-being that one can control or influence.  Perhaps the greatest opportunity to reduce long-term hearing health risk is to actively manage recreational noise.  Recreational noise may come from a number of sources, depending on one’s interests and activities, including music, sporting events, gunfire, engines, and various equipment used in pursuit of a hobby.  Engaging in these activities by choice causes many to willfully neglect their hearing health and even to dispute use of the term “noise.”
     Transportation noise poses another potentially significant risk to hearing that, in most cases, can be managed on a personal level without great difficulty.  Whether a traveler or a bystander, use of HPDs may be advisable; when possible, distancing oneself from the noise source is usually a better option.
     Addition of colored noise (see Part 16) to an environment is quintessential personal soundscape management; music may also be used to mask intrusive sounds.  Experimentation may be necessary to generate a custom soundscape that does not create new annoyances or risks to hearing or well-being.

Awareness and Education
     Awareness and education are perpetual pursuits, involving all personnel in an organization.  While production personnel are usually at highest risk for hearing loss in a factory, others also need to be trained to protect their hearing and to support those at higher risk.  Engineers and technicians should be familiar with sound level measurements and noise control techniques.  Buyers must understand noise-related specifications to purchase appropriate equipment.  Other personnel may be exposed to noise only occasionally, but must be trained in proper HPD use to protect themselves during periods of exposure.
     Awareness is important to all members of an organization because exposure is not confined to workplaces, though they are the most intuitive sources.  Everyone should be familiar with both occupational and nonoccupational sound-related risks and responses, including the unmonitored and unregulated nature of many recreational noise sources.
     Formal training is only one component of an education program.  Hearing conservation should be included in team meetings and other safety discussions.  Demonstration of proper HPD use, such as earplug insertion technique, should be part of formal presentations and more-frequent informal refreshers.  Those at highest risk for NIHL should receive training with the greatest depth and frequency to combat complacency.
     The most powerful information is why all of the activities under the TSM umbrella are important.  The most compelling facts are often those pertaining to long-term effects of NIHL and detriments to quality-of-life.  If information presented is personally relatable, it is more likely to be internalized, recalled, and put into action.  However, the obligatory nature of HPD use in some settings must also be made explicit, including the consequences of defying the policy.
     Various formats of presentation should be developed to accommodate differences in learning styles and levels of comfort with technologies used.  The workforce at any site could include members that vary widely in age and background, including native language.  Various print, audio, and video formats should be considered to engage a diverse audience effectively.

Continuous Improvement
     All aspects of an organization’s TSM program should be routinely reviewed for effectiveness and opportunity for improvement.  This is done at the level of specific measures, such as audiometric testing frequency, enclosure attenuation performance (e.g. degradation over time), selection of HPDs, communication signals, and so on.  A thorough review also includes financial analyses, such as comparisons of internal vs. external administration of program elements (e.g. audiometric testing, sound level measurement), program cost projections vs. costs of offsetting noise controls, and so on.
     At a higher level, a maturity assessment scores the TSM program as a whole.  The lowest level of maturity occurs at the initiation of a program, when all elements have not yet been fully implemented; even regulatory compliance may be uncertain.  Program maturity increases as the proportion of employees trained increases, and as the level of training rises.  Training level definitions are flexible, as roles within organizations differ; examples include awareness (akin to a Six Sigma white belt), sound measurement technician, noise control specialist, and TSM administrator.
     Other maturity assessment variables include sound level reductions, occurrences of NIHL and workers’ compensation claims, proportion of equipment meeting “buy quiet” standards, community noise complaints, and standardization across sites, among others.  Dramatic improvement on every metric at every review cannot be expected, nor should any relevant variable be ignored.  Documentation of efforts and results can be used to sustain momentum and support for TSM at all levels of an organization; visibility engenders motivation, while obscurity begets indifference.
     A metareview should also be conducted to determine if the review process effectively identifies improvement opportunities and drives implementation.  Membership and expertise of the review team, meeting frequency, evaluation procedures, etc. should be included in the metareview.

A Socio-Ecological Model
     Sobel and Martin’s Socio-Ecological Model of Hearing Health Promotion (The Noise Manual, 6ed, Chapter 16) consists of several concentric spheres of influence, with the individual at its core.  The spheres of the model, depicted in Exhibit 2, represent the influences on one’s beliefs and behaviors.  Changes in these beliefs and behaviors are more likely to occur when multiple spheres are simultaneously active.
     In the individual sphere, personal experience, knowledge, and attitudes shape our behavior.  Knowing someone that has suffered significant hearing loss is a particularly salient experience; the closer the relationship with that person, the stronger its influence is likely to be.
     Attitudes and behaviors of the people with whom we interact regularly, such as family, friends, and colleagues, influence our own in the interpersonal sphere.  The greater respect, admiration, or deference bestowed upon a person or group, the more influential that person or group is.  This type of influence can be especially powerful, as it is often exerted on a subconscious level; behaviors can be emulated without conscious awareness or recognition.
     A corporate employer is a typical embodiment of the organizational sphere, whether a single-site operation or a conglomerate.  A military unit, educational institution, political party, social club, activist group, or any collective also exerts organizational influence.  An organization’s influence can be derived from formal, binding means, such as bylaws, or by loosely-defined objectives and guiding principles.  An organization’s influence typically becomes salient through the words and deeds of its leaders; if these are inconsistent, the influence on member behavior may not be as intended.
     The community sphere often refers to a group of people in geographic proximity, such as constituents of a single jurisdiction.  However, online communities have become prevalent, effective influencers.  Geographically disparate communities can coalesce around an ethnic or religious background, profession, hobby, societal concern, or any shared interest, personal attribute, or activity.
     The outermost sphere in the model, public policy, is where the legal framework is found.  It is comprised of all statutes, regulations, ordinances, and any other names given to requirements established or enforced by a jurisdictional authority.  This includes the workplace regulations from OSHA, MSHA, etc., EPA’s product-labeling requirements, and local noise ordinances discussed in previous installments of this series.
     It is easy to see how a socio-ecological model can be applied to far more than the promotion of hearing health; similar models are, in fact, used widely to support various political and social agendas.  Many have discovered that messages received from multiple spheres of influence, when in agreement, are much more effective than those received from a single source, even when that message is compelling.  It must also be recognized that influence can guide people to unexpected conclusions when messages, embodied in words or deeds, are inconsistent.
     The Socio-Ecological Model of Hearing Health Promotion and Total Soundscape Management are complementary constructs.  While TSM defines relevant subject areas and related activities, the Socio-Ecological Model provides guidance in the key area of awareness and education, where momentum is built and sustained.  Momentum, in the context of TSM efforts, is defined by support, including funding, participation, and outcomes.
     Stated another way, the Socio-Ecological Model describes how a culture develops; the influence of each sphere can strengthen or weaken a safety culture.  Messages must be carefully crafted; actions must be consistent with other forms of communication.  Ultimately, individuals’ pursuit of hearing conservation at work and outside the workplace is responsible for creating a domino effect that becomes apparent in other spheres and drives TSM.

     The importance of culture to the success of an expansive program, such as TSM, cannot be overstated; it is the key reason for inclusion of the Socio-Ecological Model in this discussion.  A well-developed safety culture supports all aspects of health and wellness; hearing conservation is only one component, albeit a salient one.  An important function of safety culture is to assimilate newcomers rapidly.  Doing so maximizes individual and group safety in both short and long terms.
     For those new to occupational noise, rapid assimilation may be especially important.  Individuals are most susceptible to NIHL during the first 5 – 10 years of exposure; there are two key reasons.  First, a healthy (i.e. undamaged) auditory system “has the most to lose” in the same way a full bucket is more susceptible to spillage than one that is half-full.  Second, a lack of prior experience with noise limits awareness or appreciation of its damaging effects.  Sometimes, it is simply a feeling of invulnerability, such as that inherent in youth, that causes exposed individuals to neglect their hearing health until effects are apparent, at which time prevention is no longer an option.
     Participation of all members of an organization in an HLPP should be encouraged.  Promotion of healthy habits throughout an organization generates a type of network effect that influences behavior in positive ways.  When optional participation is common, it tends to reduce resistance among those whose participation is compulsory.  It also increases the number of information-sharing channels available, strengthening the interpersonal and organizational spheres of influence.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

[Link] “Information On Levels Of Environmental Noise Requisite To Protect Public Health and Welfare With An Adequate Margin Of Safety.”  The U.S. Environmental Protection Agency Office of Noise Abatement and Control; March 1974.
[Link] E-A-R Log Series.  Elliot H. Berger.  Aearo Company; 1980 – 1993.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications; 1996.
[Link] “Preventing Occupational Hearing Loss – A Practical Guide.”  John R. Franks, Mark R. Stephenson, and Carol J. Merry, Eds.  NIOSH Publication 96-110; June 1996.
[Link] “Noise Exposures:  Effects on Hearing and Prevention of Noise Induced Hearing Loss.”  Sally L. Lusk.  American Association of Occupational Health Nurses Journal; August 1997.
[Link] “Criteria for a Recommended Standard - Occupational Noise Exposure, Revised Criteria 1998.”  Publication No. 98-126, NIOSH, June 1998.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] Environmental Noise.  Brüel & Kjær; 2001.
[Link] Engineering Noise Control – Theory and Practice, 4ed.  David A. Bies and Colin H. Hansen.  Taylor & Francis; 2009.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] ”29 CFR 1910.95 - Occupational noise exposure.’  OSHA.
[Link] 40 CFR Part 211 – Product Noise Labeling. EPA.
[Link] US Code Title 42:  The Public Health and Welfare; Chapter 65 – Noise Control.
[Link] US Code Title 42:  The Public Health and Welfare; Chapter 85, Subchapter IV – Noise Pollution

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 16:  A Rainbow of Noise]]>Wed, 29 May 2024 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-16-a-rainbow-of-noise     There are several sounds, with defined spectral compositions, that have been assigned colors for identification.  These colors invoke the rainbow of the visible light spectrum to facilitate comparative references to the spectral nature of various sounds.  The analogy is not completely accurate, however.
     Each named color in the visible light spectrum consists of a single frequency of radiation, the equivalent of a pure tone sound.  A named color of sound, in contrast, refers to a specific combination of frequencies.  It should also be noted that the characteristics and effects associated with certain colors by traditional color theory are not applicable to sound.  There are some interesting parallels, however, between color theory and colored noise.
     This installment differs significantly from others in the “Occupational Soundscapes” series.  The series is loaded with information derived from research and refined by experience.  Recommendations provided were developed with scientific and engineering rigor.  The information related in this installment, in contrast, lacks the empirical data necessary to gain universal acceptance.
     Though research has been conducted on the effects of colored noise exposure, it simply is not thorough enough to claim “clinically proven” benefits.  Anecdotal evidence suggests that some people may benefit from colored noise “therapy;” therefore, this sidebar installment is deemed worthy of inclusion in the series.
     A note on usage of the terms sound and noise in this installment should be made.  Throughout the series, noise has been loosely defined as “unwanted sound.”  The difference between sound and noise remains a matter of perspective when discussing colored noise.  In this context, the term noise is applied to indicate a lack of recognizable, or coherent, sounds, such as speech or music, though it is intentionally added (i.e. “wanted”) to the soundscape.

Hear the Rainbow
     A brief summary of several named sound/noise colors is provided below.  A description of the spectral composition, source examples, and potential benefits of employing each are presented.  Visual representations of some colored noise spectra are shown in Exhibit 1
White Noise
     Readers are likely familiar with the term “white noise,” though it is often misused; many times, any random collection of sound frequencies is called white noise.  Its precise definition is a broadband sound consisting of all frequencies in the audible range at equal intensity.  If this were the only named color of sound, the analogy to light would be accurate.
     White noise sources include fans, vacuum cleaners, and older TVs and radios when tuned to an unused channel.  Claimed benefits include improved sleep and memory, mitigation of tinnitus (see Part 8), and improved cognitive performance in children with attention-deficit hyperactivity disorder (ADHD).  Some relief from tinnitus is attained by way of masking (see Part 9) the internal noise with an external white noise.  Cognitive performance can be improved by adding white noise to increase the S/N (see Part 9) of the desired message; this phenomenon is called stochastic resonance.
Pink Noise
     The intensity of pink noise constituent frequencies decreases at a rate of 3 dB/octave. This bias toward lower frequencies results in a deeper sound than white noise.  Sometimes called “the sound of nature,” pink noise sources include rainfall, rustling leaves, and crashing waves.
     Claimed benefits include increased attention span, improved focus, memory, sleep, and productivity, as well as stress reduction.  It is believed that pink noise triggers the hypothalamus, increasing signal speed to and from the brain.
Red Noise
     Red noise has a stronger bias toward low frequencies, with intensity decreasing at a rate of 6 dB/octave.  This results in a very deep, “bassy” sound.  It is also called brown noise, named for Brownian motion of particles.  Red is used here, as it is better aligned with the visible light analogy, as imperfect as it may be.  Sources of red noise include waterfalls, heavy rain and thunder, and some heavy equipment.
     Claimed benefits include improvement in memory and organization skills, increased attention span, and reduced reaction times.  Its bass-laden content may also be used to mask low-frequency annoyances.
Orange Noise
     Orange noise consists of a flat spectrum (i.e. white noise) with the frequencies corresponding to notes on a musical scale removed (i.e. zero intensity).  The removal of “in-tune” frequencies causes a discordant sound that is often equated to an out-of-tune orchestra.  It may also be called yellow noise.
     Sources of orange noise include the aforementioned musical instruments in need of tuning and some forms of electronic feedback.  Specific benefits are not often claimed, but orange noise can be used in studies of auditory perception and in experimental music.
Green Noise
     Green noise has been described as “bounded Brownian noise” and “vocal spectrum noise” with an energy peak at its center frequency, 500 Hz.  A more-precise definition is elusive.
     Allegedly, green noise is the sound of nature sans human influence; it is associated with gentle flows of water and wind.  It seems that internet gurus have more to say about green noise than reputable researchers, claiming that it calms anxiety, promotes relaxation, and improves sleep quality.
Blue Noise
     Blue noise is the opposite of pink noise; the intensity of its constituents increases at a rate of 3 dB/octave.  Its high-frequency dominance results in a hissing sound.
     Blue noise is typically associated with electronic equipment, as both source and application.  It is used in audio engineering to improve the quality of digital audio.  Benefits to individuals are not usually claimed; blue noise hiss tends to be unpleasant.
Violet Noise
     Also called purple noise, violet noise is the opposite of red noise; intensity increases at a rate of 6 dB/octave.  Though it does not match the profile exactly, a free-flowing water faucet approximates a violet noise source.
     Violet noise may be used to mask tinnitus.  Some claim that it can also improve attention and memory.  For many, attention will be focused on finding and silencing the source of this terribly shrill hissing noise!
Grey Noise
     Grey noise consists of all frequencies in the audible range, with intensities adjusted to achieve the perception of equal intensity at all frequencies.  This is like subjecting white noise to an inverted A-weighting scale; high and low frequencies are enhanced to compensate for the ear’s sensitivity to middle frequencies.
There are no naturally-occurring sources of grey noise; it is generated for customized tinnitus treatment, auditory testing, and similar applications.  Many substitute grey noise for white noise, finding its “softer,” balanced sound more soothing.
Black Noise
     Black noise is not a widely-used term, nor does it have a universally-accepted definition.  It is included here to bookend the comparison with visible light.  For this purpose, black noise refers to an absence of sound (i.e. silence) in the same way that “black” is an absence of visible light (i.e. darkness).
     To experience black noise, an exceptionally efficient anechoic chamber or a trip to deep space may be needed.  The experience could also be quite disconcerting.  In addition to free-floating in outer space (Where is my spaceship!?!), sounds from within the human body would be experienced in a whole new way.  Every heartbeat and vascular peristalsis, respiratory and digestive function would reveal the auditory components masked by normal life on earth.  If not for the unmitigated tinnitus, you probably could hear yourself think!
Caveats for Use of Colored Noise
     While studies have been conducted to test the effects of various noises on individuals in various situations, there is no scientific consensus.  Given the highly-variable nature of individuals’ lives and highly-subjective nature of sound exposure, a definitive guide for use cannot be provided.  In fact, the claims of benefits cited here are given to be thorough, but with no confidence in their veracity.
     One conclusion that appears to be universal is that there is no lasting detriment caused by experimenting with colored noise.  Therefore, a trial-and-error approach is a completely acceptable method of determining if one can reap benefits similar to those reported by others.  If not, the entire sound spectrum is available for experimentation; a cacophony to one is a symphony to another.  Colored noise generators are widely available as standalone devices and smartphone apps.  Extended-length files are also available online to facilitate overnight or workday trials of chosen spectra.
     Individual preferences require that great care be taken when adding colored noise to a shared space.  Whether at home, to improve sleep, or at work, to improve concentration and productivity, everyone in a shared space must approve of the added noise.  If there is no consensus, personal listening devices must be used to deliver the colored noise only to those who benefit from it.  In all cases, other auditory requirements (e.g. hearing alarms, speech, etc.) must not be compromised.
     Colored noise is a form of “acoustic perfume” used to transform a soundscape from an annoyance to an asset.  It is preferred to music in many situations, because it has less potential to create a distraction of its own.  Music played in early elevators was intended to distract passengers.  Its entertainment value made the trip seem shorter, while masking mechanical sounds of the lift equipment that could be unnerving to the uninitiated.  In an occupational setting, however, entertainment can be counterproductive.

     Part 17 is the capstone of this series, bringing together all of the information presented throughout previous installments to outline a comprehensive soundscape management program.  This includes the ways in which colored noise may – or may not – be an important component of an individual’s sound exposure.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

[Link] “Low intensity white noise improves performance in auditory working memory task: An fMRI study.”  Elza Othman, et al.  Heliyon; September 2019.
[Link] “Spectral Content (colour) of Noise Exposure Affects Work Efficiency.”  Shih-Yi Lu, Yuan-Hao Huang, and Kuei-Yi Lin.  Noise and Health; January – March 2020.
[Link] “All the Colors of Noise, Explained.”  Jacob Thrall.  August 2023.
[Link] “Music and Noise.”  The Physics Hypertextbook.
[Link] “Noise Types Uncovered Discover the Difference Between Various Sounds.”  Alecia Steen.  Prime Sound; March 4, 2024.
[Link] “The Ultimate Guide to Colored Noise.”  Abby McCoy.  Sleepopolis; November 6, 2023.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 15:  Community Noise]]>Wed, 15 May 2024 05:06:23 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-15-community-noise     The discussions of occupational soundscapes throughout this series have implied a certain perspective, either that of one exposed to noise or that of one responsible for its management and control.  In some cases, both perspectives are taken simultaneously.  Other perspectives are also possible, however.
     For anyone not engaged in the activities that generate noise of concern, the nature of a soundscape changes.  As an example, consider a professional auto race or outdoor concert.  Direct participants (e.g. racers, performers) experience occupational noise exposure, as do support personnel (e.g. course marshals, sound technicians, security).  Spectators are exposed to recreational noise; that is, the exposure is optional.
     Sound that escapes the event venue to the surrounding area becomes community noise.  The same is true of commercial and industrial facilities, where continuous operation can cause greater impacts on nearby communities than discrete events.
     Commercial and industrial facilities are among several contributors to community noise.  It also consists of recreational (e.g. music, sporting events) and transportation (e.g. aircraft, rail, and highway traffic) sources.  Agricultural, mining, and construction activities also contribute to community noise.
     In this installment of the series, community noise is treated as an extension of an occupational soundscape, specifically, a commercial or industrial noise source.  Other types of noise sources are considered only to the extent to which they are in the purview of the commercial or industrial site and its management.
     To clarify terms in use, consider the distinction between commercial and industrial sites.  Commercial sites include retail outlets, office buildings, financial institutions, and other typically “quiet” businesses.  Industrial sites include manufacturing plants, warehouses, distribution centers, and other storage areas.  This distinction is relevant to property zoning, but is less valuable to the current discussion; unless the context makes the distinction necessary, these terms may be used interchangeably.
     It is plausible that an industrial noise source interferes with nearby commercial operations, for example.  Likewise, healthcare, educational, recreational, and agricultural areas may also be impacted.  However, the primary concern of this presentation is the effect of industrial noise on residential areas.  A residential area can include various types of domiciles, including single- and multi-family dwellings, seasonal residences, and mobile homes.
     Residential areas are the primary concern of community noise because of the quality-of-life issues that it can cause, including interference with sleep.  The sanctity of one’s home is more likely to be upheld, whether in public opinion or legal proceedings, than almost any other concern that may be raised.  It is the basis for many noise ordinances, though their scope tends to include all sources of community noise.  At many sites, residential areas expand and encroach upon industrial facilities.  Pre-existing residential development is rarely helpful in defending an industrial noise source against a nuisance complaint.
     In the absence of sufficient regulation or enforcement, activists may emerge.  To avoid negative publicity and its snowballing effects, many organizations strive to maintain “good neighbor” status with surrounding communities.  This may include implementing measures to reduce noise below required levels.  This approach may preclude vigorous opposition to continued operations or expansion plans.  Once community support falters, the breadth of opposition tends to expand to other issues; maintaining a collaborative relationship with vocal advocates is an advisable strategy.

Community Noise Control
     Much of the information presented in the subseries on noise control (Parts 11 – 14) is also applicable to community noise, albeit with slight modification.  In this section, some modifications, or extensions, of the noise control presentation relevant to community noise are addressed.

Noise Control Planning (see Part 11)
     Creating a sound survey map, or noise contour map, of the entire site (e.g. facility exterior) simplifies understanding of the magnitude of noise concerns and, thus, prioritization of remediation projects.  All natural and constructed features of the property, including building structures, should be included on the map.  The nature of the surrounding areas and their boundaries should also be clearly labeled.  An example site sound map is shown in Exhibit 1.  The example is quite generic; a legend of sound levels and other information are needed for application to a real-world noise control effort.
     Details should be added to a sound survey map selectively; too much information or poor formatting can clutter a map, diminishing its utility.  Useful information may include peak and average sound levels, duration and frequency of peak levels, and relevant temporal data (e.g. time of day or year).  Methods of quantifying sound levels are discussed further in the section on noise measurement, below.

Hierarchy of Controls (see Part 11)
     The hierarchy of controls remains valid, with one exception:  PPE is not available as a last-resort protection from community noise.  Well-informed neighbors may choose to utilize HPDs to mitigate their exposure or annoyance, but they cannot be compelled to do so.  Community noise must be controlled at higher levels of the hierarchy, with elimination of the source remaining the most desirable solution.

The SPR/ETI/PAP Model (see Part 11)
     The source-path-receiver (SPR) model continues to be “the” approach to noise control.  The receiver portion of the model, however, must be deemphasized in the community noise context even more than it is for occupational noise.  Few receiver-protection options exist and their effects are minimal, reinforcing the priority of controlling noise at its source.

Source Noise Control (see Part 12)
     Significant sources of community noise include many that are often easily overlooked when conducting an occupational noise survey.  Activities that take place, and equipment that operates, on the exterior of a facility are often unnoticeable to the vast majority of workers inside.  It is precisely these noise sources, however, that are often most noticeable in surrounding areas.  The facility often provides little or no attenuation of these sounds; worse, the structure may reflect sound, exacerbating the situation.
     Outdoor equipment that contributes to community noise includes air handlers and exhaust fans, air compressors, generators, pumps and pipelines, public address (PA) systems, and much more.  Facilities with equipment operating around the clock face additional challenges; generally quieter periods of the day (e.g. nighttime) usually require lower levels of sound emission.
     Part 12:  Source Noise Control contains information that can be applied to much of this equipment.  For example, the “Hydraulic Noise” section can be applied to outdoor pipelines as well as indoor fluid-handling systems.  Helpful information on fans is provided in the “Aerodynamic Noise” section; Exhibit 2 depicts an example of how a minor change in equipment specification can have a profound impact on community noise.
     Exhibit 3 depicts another type of aerodynamic noise issue, one of noise generated without engaging in any particular activity.  In this example, noise is generated by a passive interaction with the environment.  Wind passing a smoke stack generates noise, such as a whistling sound; attaching a spiral with varying pitch modifies the airflow sufficiently to prevent noise.  Similar phenomena may be experienced with antennae, flag poles, or cables used to stabilize similar structures (i.e. guy wires); a similar solution can be implemented in each case.
     Delivery vehicles can cause significant disturbance, particularly at a high-throughput facility, such as a distribution center (DC).  In addition to 18-wheelers, box vans, or other types of vehicle that may arrive and depart a site regularly, there is often a fleet of logistics and material-handling vehicles (e.g. “yard dogs,” forklifts) that operate continuously on the premises.  Operators of these vehicle are typically evaluated for speed and accuracy, while the noise generated by their activities goes unnoticed.  Unnoticed, that is, by those within the facility; nearby residents may have a very different experience.  Vehicle and equipment maintenance, in conjunction with awareness and “quiet operation” training, are essential tools for minimizing this type of community noise contribution.
     Given the array of activities and facility and equipment variations that exists, noise sources not considered here certainly exist.  An evaluation must be broad to ensure that all noise sources for which an organization is responsible – directly or indirectly – are identified and proper controls activated.

Path Noise Control (see Part 13)
     An industrial facility is typically treated as a single source in community noise discussions, though several discrete sources may exist within it.  Facilities are typically on large plots of land, placing potential receivers of noise at significant distances from the source.  At these distances, individual noise sources are often indistinguishable, as the spectral components of each coalesce into a single perceived broadband noise.  In noise control terms, the surrounding areas are in far field, usually free field, conditions.
     Directionality of noises must be factored in to community noise analyses.  Outdoor equipment operating near the main structure of a facility exhibits increased directivity (e.g. Q = 4; DI = 6 dB).  If placed among multiple structures, directivity can increase further (e.g. Q = 8; DI = 9 dB).  Noise emanating from inside a facility can also exhibit directionality when doors are left open, for example.  Such effects are more difficult to predict; multiple scenarios should be represented in sound surveys to ensure adequate controls are developed.
     Barriers erected along property boundaries are common responses to community noise concerns.  A conspicuous example of this approach is the evermore-ubiquitous walls lining interstate highways.  While reducing the exposure of frontage properties, reverberant canyons are created, within which noise exposures and associated risks increase.  Fortunately, the scale of typical industrial sites tend to make problematic reverberant fields much less common.
     In addition to air-borne sound transmission, ground vibrations can also transmit unwanted energy to surrounding areas.  This is typically low-frequency noise caused by mining activities, heavy equipment operation, manufacturing with very large stamping presses, or similar operations.  The potential for this type of transmission is influenced by the geological composition of the site, construction techniques, and process parameters.  It must be considered carefully, as it may require a “financial solution;” a viable technical solution may not exist.

Receiver Noise Control (see Part 14)
     As mentioned above, it is unreasonable, not to mention unlawful, to expect the community-at-large to accept the use of HPDs in “everyday life” as appropriate.  Other options for receiver noise control are scarce and of limited utility.
     Activities can be scheduled for times of day – or year – that they will be least disruptive; maximum sound levels still apply.  This option is only feasible for infrequent or nonroutine activities, such as some construction or maintenance tasks.  Noise generated by frequent or continuous activities, or at high intensity, must be managed with more-sophisticated controls.

Effects of Community Noise
     The effects of occupational noise exposure, presented in Part 8, can also be caused by community noise.  However, community noise is typically experienced at lower intensity, while engaged in activities unrelated to the noise source.  A change in circumstances can have a profound effect on the perception and acceptability of sound; the experience of exposure is drastically changed.
     The types of activities typically discussed in the context of community noise reflects the emphasis on protecting residential areas from intrusive noise.  The most-prominent topic is interference with speech communication; the concepts presented in Part 9, such as masking and intelligibility, are relevant to both community and occupational noise exposures.  The change in setting, however, eliminates the use of a limited, standard vocabulary or sophisticated electronic communication system as viable solutions.
     Community noise can impact various types of speech communication, including casual conversation, professional interactions (e.g. legal or medical consultation), educational lectures, and commercial transactions (e.g. sales presentation or contract review).  The lack of visual cues available during telephone conversations and modifications of speech that occur during reproduction and transmission increase susceptibility to interference.  Similarly, TV and radio broadcasts and other forms of audio presentation of speech are subject to the same limitations.
     While the speech intelligibility concern may be reduced or eliminated when listening to music, the enjoyment of music can be degraded by the infiltration of community noise.  Playing music, or engaging in any activity that requires concentration, can be negatively impacted by unrelated noise.  The consequences of poor performance on these tasks must be considered.  A student’s inability to concentrate on homework leads to low grades; a misbalanced checkbook leads to overdrafts, missed payments, fees, or other complications.  Superficial reading of an insurance policy, mortgage refinancing agreement, or other legal document has far-reaching consequences; an incorrectly wired electrical outlet leads to shock or fire.  The dangers of distraction, spanning all potential activities of a population, are innumerable.
     Periods of relaxation and sleep are highly influential on quality-of-life assessments.  Relaxation activities include some of those mentioned above; many activities typically associated with “quiet” may also be included, such as reading, journaling, painting, meditation, yoga, nature walks, and others.  Soaking up the sun while sipping a mai tai is the quintessential relaxation activity.  Excessive noise is disturbing and can render true relaxation unattainable.
     When noise disturbances extend into periods reserved for sleep, consequences escalate rapidly.  Interference with other activities causes annoyance, but frequent sleep disturbance can result in serious health issues.  Even when a person becomes accustomed to noise, levels of stress hormones increase and sleep is less restorative.  Noise exposure causes increases in heart rate and blood pressure (BP), counteracting the usual drop in BP one experiences during sleep.  The combination of normal physiological responses to noise can result in cardiovascular disease and has been linked to increased rates of diabetes.
     These long-term effects, like noise-induced hearing loss (NIHL), are not obvious right away.  However, frequent symptoms of sleep deprivation, such as fatigue or lethargy, difficulty concentrating, or irritability may be warning signs that a serious chronic problem exists or could develop without appropriate interventions.
     Prolonged, unmitigated noise that causes any of the above impacts to those in the vicinity of a source facility is likely to degrade the operating organization’s standing in the community.  Declines in public opinion of the facility operator are commensurate with the duration or severity of the nuisance it creates.  The frequency of corresponding complaints, legal and political actions, and public outcries are likewise correlated.
     Though industrial noise has been shown in a number of studies to be a lesser problem than transportation noise, a fixed site provides a single, identifiable target for backlash.  Assigning culpability for highway traffic noise, for example, is not so simple; addressing it requires action by various agencies and political figures at multiple levels of government.  When the nuisance is created by aircraft, or airport operations, the difficulty is magnified many-fold, as it must involve the federal government in addition to all other entities.  For these reasons, industrial sites may have to endure greater scrutiny than their “offenses” warrant until scientific exoneration redirects attention to its rightful recipients.

Quantifying Community Noise – Measurement
     When citing community noise levels, exactly what noise has been measured must be specified, lest the values be misleading or confusing.  An ambient noise measurement accounts for all sources, including the source of interest, all forms of transportation, all other activities, and natural sounds, such as birdsong or rustling of leaves.  Specific noise is that attributed to the source under scrutiny and residual noise is that which remains when the specific noise is absent.
            Stated another way, specific noise – the noise of concern – is the difference between ambient noise and residual noise.  A specific noise may be defined by measuring the ambient and the residual, using “Sound Math” (Part 4):
     Ambient Noise  =  Residual Noise  +  Specific Noise.
     The term “background noise” may be used to refer to the level of sound when a specific noise is not audible, though it may be present; it could also refer to an exposure index, such as L90 (see Part 6).  The use of terms varies by author; readers are cautioned to verify definitions when consulting multiple sources.  The definitions provided above are deemed to be least ambiguous; the terms are used in this manner throughout this presentation.
     Community noise is typically defined in terms of equivalent continuous sound levels (see Part 6).  Several can be cited, including:
  • Lddaytime equivalent continuous sound level (dBA); the average sound level during the period from 07:00 to 22:00 (7 AM to 10 PM).
  • Lnnighttime equivalent continuous sound level (dBA); the average sound level during the period from 22:00 to 07:00 (10 PM to 7 AM).
  • Ldnday-night average sound level, or DNL (dBA); the average sound level during a 24-hr period.  A 10-dBA penalty is applied to nighttime values to reflect the increased intrusiveness of noise at night:
DNL is often the first, sometimes the only, community noise metric cited when assessing acceptability of, for example, development or expansion plans.
In the absence of a dominant noise source, such as an industrial site, highway, or airport, an estimate of DNL, based on population, can be calculated:  Ldn = 23 + 10 log (PD) (dBA), where PD is the population density (people/sq km) of the area; this estimate can be used where PD > 200 people/sq km.
  • Leevening equivalent continuous sound level (dBA); the average sound level during the period from 19:00 to 22:00 (7 PM to 10 PM).  When Le is used, the definition of daytime hours, for calculation of Ld, is changed to the period from 07:00 to 19:00 (7 AM to 7 PM).
  • Ldencommunity noise equivalent level, or CNEL (dBA); the average sound level during a 24-hr period.  A 5-dBA penalty is applied to evening values (“relaxation time”) and a 10-dBA penalty is applied to nighttime values (“sleep time”):
  • Normalized Ldn is derived by applying correction factors to measured Ldn values.  Descriptions of corrections and the magnitude of each are given in Exhibit 4.  The corrections cited were originally established to correlate actual community response in various circumstances to anticipated rates of complaints, “community action,” etc.  Increments of 5 dB are used, as the original authors deemed this to be the limit of accuracy.  An additional +12 dB correction is included in ANSI S12.9 for “highly impulsive sounds,” such as gunfire, jack-hammering, pile-driving, or riveting.
  • Rating level (Lr) is similar to normalized Ldn; penalties are assessed to adjust an A-weighted continuous equivalent sound level to quantify the annoyance caused by a sound.  It takes the form Lr = LAeq + KI + KT + KR + KS, where KI is an impulse penalty, KT is a tonality penalty, KR is a time-of-day penalty, and KS is a “source and situation” penalty.
The procedure for determining a rating level is defined by international standard (ISO 1996), but the penalties vary by national jurisdiction.  Readers interested in the rating level method should consult the ISO standard and local regulations.

     When conducting a sound survey, several variables must be considered; documenting conditions in which measurements are taken is critical to understanding the results.  It should be obvious that modification of source parameters can impact measurements significantly.  These include equipment operating at reduced vs. full capacity, a facility that is partially vs. fully staffed, and any other circumstances that affect sound generation or activity levels.
     Placement of measurement equipment must also be carefully considered to ensure valid data are obtained.  Microphones are typically placed 1.2 – 1.8 m above the ground, unless topography dictates otherwise (e.g. line of sight to source lost).  Measurements should not be taken over paved areas or near large objects, such as buildings, unless the objective is to document the effects these features have on measurements.  To avoid excessive influence on measurements, microphones should be placed at least 7.5 m (19 m recommended) from large reflective surfaces and at least 1.5 m from small objects (e.g. trees, poles, etc.)  Even the presence of a person near a sound meter can affect measurements; remote monitoring options should be considered whenever possible.
     Seasonal variation in measurements can be caused by changes in plant growth (e.g. tree canopy) and insect and wildlife activity (e.g. buzzing swarms, honking flocks of geese, croaking frogs, etc.)  Meteorological conditions fluctuate more rapidly, potentially causing significant variation in sound measurements.  The influence of these fluctuations is explored further in the “Calculation” section below.
     It is common practice to begin a sound survey in locations that exhibit high potential for complaints, such as boundaries between the source site and residential areas.  Identifying these locations on a site map with additional notes describing factors that influence measurements provides a rapid reference that is useful throughout a noise investigation and beyond.
     It may be necessary to repeat measurements at times when different conditions exist, or conduct long-term monitoring, to capture as much of the variability as possible.  Doing so facilitates identification of the worst case, which may be that the solution chosen under one set of conditions exacerbates a noise concern under another.  Without thorough documentation of measurement conditions, sound level values recorded contain little insight; identifying and resolving this type of issue becomes impossible.

     Other measures of sound exposure, such as noise dose (see Part 6), loudness and noisiness (see Part 7), and speech intelligibility (see Part 9) remain valid for community noise exposure.  However, it is more common to apply these concepts to individuals; as such, these measures may only enter a discussion of community noise as support for a nuisance complaint that has escalated to legal action where professional analysts are employed to investigate the validity of claims or potential resolutions.
     One collective metric that can be used to assess community noise is the total weighted population (TWP) effected.  TWP uses weighting factors associated with DNL ranges to assess the magnitude of annoyance caused by a noise source.  It is calculated as follows:  TWP = ∑i (Wi x Pi), where Wi is the weighting factor for a specific range of DNL, tabulated in Exhibit 5, and Pi is the number of people subjected to DNLs in that range.
     To compare the community impact of disparate noise sources, a noise impact index (NII) can be calculated for each:  NII = TWP/∑i Pi, where TWP and Pi are defined above.  Either TWP or NII could be used to prioritize allocation of limited resources needed to pursue noise reduction initiatives.

Quantifying Community Noise – Calculation
     Predictive calculations of sound levels can be performed in the planning phase of a greenfield or brownfield development, expansion project, or other change in operations that are anticipated to significantly affect the community soundscape.  A series of calculations is needed to account for all relevant variables.  Fortunately, each computation is fairly simple, though the number of variables and look-up tables involved may seem daunting at first glance.  The configuration of calculations varies slightly among reference sources; the form presented here attempts to simplify the progression where possible.
     Anticipated sound level at any receiver location is calculated, per octave band, as follows:  Lp = Lw – Atotal (dB), where Lp is the SPL at the defined location (dB), Lw is the source PWL (dB), and Atotal is the total attenuation (dB).
     Total attenuation accounts for environmental factors that influence sound propagation, as well as existing noise control measures:  Atotal = Adiv + Aair + Aenv + Amisc, where Adiv is attenuation resulting from geometrical divergence, Aair is air absorption, Aenv is attenuation due to environmental effects, and Amisc is attenuation attributed to all other causes.
     Geometrical divergence reduces sound levels according to the inverse square law (see Part 3); this attenuation is calculated as Adiv = 20 log (r) + C (dB), where r is the distance between source and receiver and C = 10.9 when r is measured in meters (C = 0.6 when r is measured in feet; however, use of metric dimensions is advised to maintain consistency with remaining calculations).
     Air absorption, or atmospheric attenuation, depends, in order of decreasing influence, on relative humidity, frequency, and temperature.  This attenuation is calculated as Aair = α’ r/1000 (dB), where α’ is the air attenuation coefficient (dB/km) at standard pressure (1 atm).  Values of α’ are tabulated in Exhibit 6; coefficients corresponding to intermediate parameter values can be found by interpolation.
     Environmental attenuation effects include the influences of ground effects, wind speed and direction, and temperature gradients.  Attenuation due to ground effects is determined by one of two methods, depending on whether propagation is short- or long-range.  Both methods require identification of ground conditions between source and receiver as hard, soft, very soft, or mixed.
     Hard ground includes concrete, asphalt, highly-compacted or tamped soil, bodies of water, and other low-porosity, highly-reflective surfaces.  Soft ground includes areas covered with grass, shrubs, or other vegetation, and porous ground suitable for such growth.  Very soft ground includes fresh snow (i.e. not compacted), ground covered in pine needles or other loose material, and water-saturated soil.  Mixed ground has areas of both hard and soft ground between source and receiver.

     Short-range propagation is defined as that up to 100 meters (r < 100 m); the diagram in Exhibit 7 depicts the direct and reflected sound propagation paths between source and receiver.  To determine short-range attenuation, locate the section of Exhibit 8 for the ground condition present, then, within it, the attenuation value(s) corresponding to source and receiver parameters.  At angles of incidence greater than 20°, reflection from soft ground increases such that it can be treated as hard ground.   Attenuation of mixed ground is determined by interpolation between the hard and soft ground values according to the proportion of each.  Negative attenuation values indicate that reflections increase sound levels at the receiver.

     To determine ground-effect attenuation for long-range propagation (r > 100 m), there are three zones between source and receiver to be considered, as shown in Exhibit 9.  The source zone length is 30 times the source height (hs) and the receiver zone length is 30 times the receiver height (hr), each with a maximum of r, the source-to-receiver distance.  When 30 hs + 30 hr > r, a middle zone lies between.
     Each zone is assigned a ground factor, G, based on the ground conditions in that zone.  For hard ground, G = 0, for soft ground, G = 1, and for mixed ground, G is equal to the proportion of the zone consisting of soft ground.  For very soft ground, G = 1 can be used, though attenuation values will be underestimated, particularly in the lowest frequencies.
     Long-range ground-effect attenuation (Agr) is the sum of attenuations in the three zones (two, if no middle zone exists):  Agr = As + Ar + Am.  Zone attenuations are found using Exhibit 10; factors a, b, c, and d, used to calculate source and receiver zone attenuations, are tabulated in the lower section.  To calculate middle zone attenuation, use factor e = 1 – [30 (hs + hr)/r].
     Attenuation due to wind and temperature gradients are difficult to predict precisely.  Nonetheless, it is useful to have a general understanding of the effects varying conditions can cause.
     Sound propagation is affected by air temperature.  Normal daytime conditions include a negative temperature gradient (temperature drops with altitude), or a “lapse,” creating a shadow zone at ground level, as depicted in Exhibit 11A.  Sound levels in the shadow zone can be 10 – 20 dB lower than predicted by ground-effect attenuation.
     At night, normal conditions include a temperature “inversion,” or positive temperature gradient, causing sound to be projected toward the ground.  In this condition, depicted in Exhibit 11B, environmental factors provide little to no attenuation at distances up to several hundred meters; attenuation may even become negative.
     Wind creates similarly dichotomous attenuation effects.  Upwind of a source, a shadow zone is created, as shown in Exhibit 12ADownwind of a source, sound can be intensified; the degree depends largely on wind speed.  Exhibit 12B charts potential broadband sound level changes for each of these conditions, as well as in crosswind.
     As these effects demonstrate, it is important to document the conditions under which measurements are taken or are assumed in calculations.  Downwind measurements are usually preferred to capture “worst-case” conditions.  However, prevailing conditions and locations of interest may require upwind measurement.  Again, documentation of conditions is necessary to compare results of multiple measurements and glean needed insight from recorded data.

     Miscellaneous effects include attenuation due to barriers, foliage (trees and bushes), meteorological conditions, and any other feature that previous calculations have not captured.  Attenuation attributed to barriers can be determined as discussed in Part 13, though their effectiveness at large distances is low.
     Foliage is not the attenuator that many seem to think it is; up to 100 m of dense forest is needed to provide significant broadband sound level reductions.  The table in Exhibit 13 provides octave-band attenuation rates for foliage.  Each component of Amisc may be small, but should be included in total attenuation.
     Attenuation due to precipitation and fog is negligible, but it may affect measurements in other ways.  Heavy rain, for instance, can raise the residual noise level and could potentially infiltrate microphones or equipment circuitry, creating errant data or instrument damage.

     Once Lp is calculated for each octave band, the values can be logarithmically added to obtain a broadband SPL.  Frequency weighting can also be applied to obtain an A-weighted broadband sound level (dBA).  A directivity correction can be applied to the broadband level if directionality has not been captured in spectral calculations.  An intermittency correction can also be applied, using an appropriate exchange rate (see Part 6).  An impulsivity adjustment can also be made (see Part 6) if impulsive sounds comprise a significant portion of exposure.

Standards and Regulations
     A number of organizations, across the globe, publish standards related to various noise issues, including measurement of community noise.  Among them are ISO, ANSI, US EPA, OSHA, and many other regulatory bodies and professional organizations.  Though many of these standards tend to converge over time, a thorough review is implausible.
     The good news is that all of the standards need not be known.  Determining which organizations promulgate standards relevant to a specific locale focuses initial research.  Later, research can be expanded to seek additional information, best practices, or alternative explanations that aid implementation of needed noise control measures.
     The regulatory landscape is even more difficult to navigate.  Every jurisdiction defines its own requirements; a single location could be subject to regulations established by multiple levels of government.  There is no guarantee that requirements will be consistent or compatible.  Worse, the order of preemption may be unclear.
     This portrayal is intended not to discourage, but to prepare readers for potential challenges.  To this end, a very basic guideline is offered:
  • Research statutes and regulations published by each jurisdiction (e.g. level of government) in which the subject facility and surrounding area is located.  If the affected area crosses jurisdictional boundaries, repeat the process for each jurisdiction.
  • Research preemption laws in each jurisdiction to establish priority of conflicting requirements.
  • When searching for relevant requirements, include “zoning” and other relevant keywords, in addition to “noise,” to discover useful information that might be easily overlooked.
  • If strict guidance (e.g. regulation or statute) has not been established for a site’s jurisdiction, create a standard based on nearby jurisdictions and relevant guidelines and recommendations.
  • To the extent possible, internally standardize practices and requirements when operating multiple facilities.
  • Consult the EPA’s “Model Community Noise Control Ordinance” or similar regulatory outline to predict potential future requirements.
  • If no other guidance is available, pursue the EPA-recommended 55-dBA DNL for residential areas; this is the value deemed protective of community well-being with an “adequate margin of safety.”  Slightly higher DNL may be acceptable for nonresidential areas.
  • Conduct the most-thorough sound survey possible; better data yield better results.
  • Proactively engage with regulators and community representatives to prevent contentious situations and maintain good standing.
  • Pursue continuous improvement in all areas, including measurement, noise control, and community engagement.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

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[Link] “Review of 60 U.S. Environmental Community Noise Ordinances.”  John Eichwald, Padmaja Vempaty, and Yulia Carroll.  The Hearing Journal; July 2021.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Technical Guide for:  Noise Control – Engineering Controls, Work Practices, & Administrative Controls.”  Georgia Tech; May 2023.
[Link] 40 CFR Part 211 – Product Noise Labeling. EPA.
[Link] US Code Title 42:  The Public Health and Welfare; Chapter 65 – Noise Control.
[Link] US Code Title 42:  The Public Health and Welfare; Chapter 85, Subchapter IV – Noise Pollution

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 14:  Receiver Noise Control]]>Wed, 01 May 2024 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-14-receiver-noise-control     The third noise control objective (see Part 11) is to “protect receivers from sound immission.”  This is somewhat misleading, as receivers, or “listeners,” are actually the first priority in the workplace.  However, modification of sound exposure at the receiver is the last resort to ensure protection from hearing loss and other effects (see Part 8).  If source and path control efforts are insufficient, receiver noise control becomes the critical “last stand” against damaging effects of sound exposure.
     Receiver noise control is not comprised exclusively of the use of hearing protection devices (HPDs), but HPDs often dominate the discussion.  Here, the discussion includes the types of HPD available, rating systems, selection criteria, and challenges in implementation.  Though options may be limited, engineering and administrative controls are also discussed.
Engineering and Administrative Controls
     The objective of noise controls, in one formulation, is to maintain an eight-hour time-weighted average sound exposure below the action level (i.e. TWA8hr < 85 dBA; see Part 6).  For jobs in consistent high-intensity sound, an individual’s exposure must be lower during parts of the workday to achieve this average level.  Three key methods of reducing sound exposure are (1) rotation, (2) relocation, and (3) rest periods.
     Job or task rotation alternates periods of high-intensity sound exposure with quiet periods, reducing the total exposure.  This method can increase the number of people exposed to high-intensity sound if a task must be performed continuously.  However, it may be more effective to maintain safe exposure levels for a larger number of people (though moderately higher for some) than to implement other strategies.
     Relocation of a source or receiver can reduce exposure by increasing the distance between the two.  Work unrelated to the noise source should be separated from it whenever possible.  Further gains are attainable if either is placed in a more favorable environment, such as a room lined with absorbent material.
     Rest periods are scheduled within a work cycle when alternative assignments are not available.  Modification of work cycles was discussed in the “Thermal Work Environments” series; it is not uncommon for thermal and sound exposures to require simultaneous monitoring.
     Rest periods may be spent in a “refuge room” or other quiet space.  If feasible, permanent relocation to an area of low intensity is preferred.  This option was presented in “Methods of Isolation” in Part 13, where the blurred line between path and receiver noise control was mentioned.  It also blurs the line between engineering and administrative controls.  Design and construction of an effective enclosure requires engineering.  Scheduling time in the relative safety provided by such an enclosure and ensuring its proper utilization requires the behavioral management of administration.

Hearing Protection Devices
     Hearing protection devices (HPDs) comprise a subset of personal protective equipment (PPE), which inhabits the lowest position on the hierarchy of controls.  HPDs also straddle a blurred line, similar to enclosures; selecting an appropriate HPD is, in large part, an engineering exercise, while ensuring its proper use is an administrative task.
     Various styles of HPD are available; the compatibility, or suitability, of each in a specific soundscape and for a particular user (i.e. receiver) must be assessed to ensure protection is achieved and maintained.  The three key categories of HPD to be considered are passive types:  (1) earplugs, (2) earmuffs, and (3) canal caps.  Advanced HPDs, including active types that incorporate electronics, are also explored below.

Earplugs (a.k.a. Ear Insert Devices)
     An earplug is inserted directly into a user’s ear canal.  The earplug is held in place, and its seal maintained, by compression of the plug (i.e. interference fit).  There are several styles of earplug in common use; some of the most common are described below.
     Roll-down foam earplugs are ubiquitous due to their high attenuation potential, near-universal fit (“one size fits most”), and low cost.  They must be manipulated by the user and held in place until a seal forms in the ear canal.  Push-to-fit foam earplugs are compressed upon insertion.  This type of plug can be deployed quickly, but typically provides less attenuation than a properly inserted roll-down type.
     To achieve maximum attenuation with a roll-down foam earplug, it must be fully inserted into the ear canal.  The procedure used to attain full insertion is shown in Exhibit 1.  Panel A shows how to compress an earplug by rolling it between fingers and thumb to a diameter less than that of the ear canal.  The plug must be rolled, not crushed, to size, to prevent creases or folds in the foam.  Irregularities in the rolled foam causes uneven expansion that can result in an incomplete seal of the ear canal.
     To facilitate insertion of the rolled plug, reach over the head with opposite hand, pulling the pinna upward and outward as shown in panel B.  Doing so slightly opens and aligns the entry to the ear canal, allowing the plug to be inserted quickly and easily.
     Once the plug has been inserted, release the pinna.  Hold the plug in place, as shown in panel C, until it has expanded.  When the plug has expanded, it will hold itself in place, maintaining the acoustical seal in the ear canal.
     Before entering a loud environment, where attenuation will be evident, proper insertion of the plug can be verified visually.  Examples of proper and insufficient insertion depths are shown in Exhibit 2.  If the foam can easily be grasped, it is probably not inserted to sufficient depth; removing a roll-down earplug usually requires some effort until experience overcomes the awkward manipulation it may require.
     While some foam plugs are single-use disposables, others can be used several times.  If plugs are reused, care must be taken to prevent contamination and damage that can be detrimental to ear health or reduce effectiveness.  Examples of roll-down and push-to-fit foam earplugs are shown in Exhibit 3.
     Premolded earplugs are also available in multiple configurations, including different numbers of sealing flanges.  Another type has a smooth body (i.e. no flanges) and an adjustable core to expand the plug after insertion for a better seal.  The most common styles of premolded earplugs, made of silicone or a similar material, are shown in Exhibit 4.
     Premolded earplugs offer some advantages over foam types.  First, insertion is fast and simple; no special technique is required to seal the ear canal.  Useful life is extended by precluding the need to touch the sealing portion of the earplug, a common source of contamination; the plugs can also be washed if needed.  Both the speed of insertion and reduced contamination risk are particularly helpful when a user has to insert and remove earplugs frequently, such as when transitioning between loud and quiet areas throughout a workday.  It is not one-sided, however; premolded plugs typically provide less attenuation than roll-down foam and are more likely to require repositioning when worn for long periods.
     Earplugs can also be custom-molded to fit a specific user.  Performance of these plugs is highly variable, depending on the quality of the material, skill of the technician, and the use and care of the earplugs.  Most industrial applications do not warrant the expense, lead time, and complexity of custom-molded HPDs.

Earmuffs (a.k.a. “Mickey Mouse Ears”)
     Earmuffs consist of two circumaural earcups connected by a headband or attached to a hardhat, such as the examples shown in Exhibit 5.  A third configuration, where a soft strap over the head supports the earcups while a bar behind the head provides sealing force, is typically used for communication headsets.
     An earmuff creates acoustical seals outside the ears, where its cushions contact the user’s head.  The cushions that surround the ears are typically filled with foam or gel to provide both comfort and conformity to the user’s head.
     The use of earmuffs comes with its own challenges.  Perhaps the most difficult to overcome is the potential for interference with or by other PPE, such as safety glasses, face shields, bulky garments, hats and helmets, etc.  Safety glasses, or eyeglasses, can cause air leaks where the bows pass under the earmuff cushions; bulbous earcups can limit a user’s range of motion or interfere with the proper fit of other gear.  Air leaks can also be caused by wearing earmuffs over the user’s hair.  If the earcups are too small to surround a user’s ears, pinching can cause significant discomfort in addition to air leaks.
     Discomfort often leads to disuse, exposing the unprotected worker to a high risk for hearing loss.  It can also cause frequent repositioning, which can become annoying to the user, lowering both productivity and morale.  Each time an earmuff is repositioned, the acoustical seals are broken, increasing the user’s sound exposure.  Attenuation potential is severely diminished, as there is one optimum (i.e. maximum attenuation) position and any number of positions that reduce the earmuff’s effectiveness.
     Proper care of passive earmuffs is straightforward.  Clean dust and debris from earmuffs, with particular attention to the cushions and interior of the earcups; clean cushions improve both comfort and performance.  Useful life of earmuffs can be extended through careful storage and handling.  Cushions should be in a completely relaxed state when not in use; that is, there should be no compression or distortion.  Likewise, the headband should not be deformed or the earcups damaged.  Replace any components that become deformed or otherwise damaged; cushions should be replaced periodically to ensure the pliability necessary to maintain acoustic seals.

Earplugs + Earmuffs (a.k.a. Dual Protection)
     It is possible to use earplugs and earmuffs simultaneously to increase attenuation and may be advisable in some high-intensity environments (e.g. TWA8hr > 105 dB).  It is important to note, however, that the full attenuation potential of both devices cannot be achieved simultaneously.  As a guideline, users can expect the combination to provide approximately 5 dB attenuation in excess of the higher-performing device (i.e. muff or plug) used alone, whereas the sum of the attenuations achieved by each device independently would be much higher.

Canal Caps (a.k.a. Semi-Insert Devices)
     As the name implies, semi-insert devices “cap” the ear canal without entering it fully; examples of this type of device are shown in Exhibit 6Canal caps rest in the concha, the transition between the pinna and the ear canal.  Sealing force is applied by a flexible band connecting two caps.  The band is often molded plastic, but metal versions may also be found.  Canal caps are very convenient for a user entering and exiting loud areas frequently; as they can be deployed rapidly with little adjustment required.  Upon removal, hanging the band around the user’s neck keeps the HPD convenient and relatively clean.
     Canal caps may not be the best choice for extended use; the pressure applied by the band to maintain an acoustic seal can become rather uncomfortable.  Also, this type is no more convenient than other device styles when not being removed frequently.

Helmets (a.k.a. “Brain Buckets”)
     Helmets are not common in industrial settings, but may be appropriate in some circumstances.  When helmets are used, hearing protection is often a secondary function; impact protection is typically the primary objective.  However, a helmet that is designed for hearing protection can offer a significant advantage over other types of HPDs.
     By enveloping the user’s entire skull, a helmet limits the transmission of sound by bone and tissue conduction (BC), increasing the attenuation potential of the HPD.  The pathways available to sound when an earplug or earmuff is used are depicted in Exhibit 7, panels A and B respectively; the block diagram in panel C applies to all types of HPD.  The larger structure of a helmet could increase oscillation effects; tradeoffs must be evaluated to ensure desired performance can be achieved with any HPD specified.
     The curves in Exhibit 8 compare the attenuation potentials of earplugs and earmuffs, employed independently and in conjunction, to that of helmets (“head covered”).  At higher frequencies, the attenuation provided by dual protection can be limited by the BC pathways in the user’s head.  Wearing a helmet increases the attenuation potential, though BC pathways are not completely eliminated.  It is important to note that Exhibit 8 presents representative data, but it cannot be relied on for any specific device or combination of HPDs.

Nonlinear and Level-Dependent Devices (a.k.a. Augmented HPDs)
     Nonlinear devices provide frequency-dependent attenuation.  Higher frequencies are subject to significant attenuation, while little occurs at lower frequencies (e.g. < 1500 Hz).
     Level-dependent devices attenuate high-intensity sounds without effecting low-intensity sound.  This type of device is desirable where both communication ability and protection from transient sounds are needed.  The label differentiates them from the passive devices discussed above, which are level-independent, providing equal attenuation at all sound levels, though they are nonlinear.
     Nonlinearity and level-dependence may be combined in a single device.  Augmented HPDs are usually found in specialized applications, whereas most occupational soundscapes rely on traditional, passive HPDs.  However, when a specialized device provides the best match to the needs of a user, it is worth pursuing, even if it is uncommon.

Active Noise Reduction (a.k.a. ANR)
     Active Noise Reduction (ANR) utilizes one or more microphones on the exterior of the device and a speaker in the interior to detect and cancel impinging sound.  ANR devices are most effective for low frequencies (e.g. < 500 Hz).  At high frequencies, ANR provides little to no advantage over a passive device, as seen in the comparison curves in Exhibit 9.  At mid-range frequencies (e.g. 500 – 2000 Hz), passive devices often outperform those with ANR.  The comparison in Exhibit 10 shows that dual protection with passive devices actually outperforms ANR across the entire audiometric frequency range (63 – 8000 Hz).
     Development of ANR devices may improve their performance relative to passive HPDs.  Until such time that ANR devices are clearly superior, however, the additional cost and required care and maintenance will remain strong deterrents to their widespread adoption in commercial settings.  Applications for entertainment purposes, in contrast, have already seen significant growth that is likely to continue for some time.

Hearing Aids (a.k.a. Hearing Augmentation Devices)
     Hearing aids are not HPDs!  Their use creates additional challenges to selecting appropriate HPDs, customizations, and work practices.  Simply disabling these devices and accepting an elevated threshold without analysis is not an acceptable practice.  While consulting an audiologist or physician is always advisable, it is essential for those that require hearing augmentation.  Partner with an auditory health professional to protect these individuals from further hearing loss, ensure that they can communicate effectively, and fully integrate them into the workforce.

Challenges of HPD Use
     Some of the challenges encountered when using HPDs have been mentioned in the discussion of specific device types.  The potential for interference with other types of PPE or range of motion, particularly in confined spaces, must always be considered.  Dealing with existing hearing loss can be especially challenging when hearing aids are used.
     All users must be trained in the proper fitment of HPDs to achieve required attenuation.  With adequate training, it can still be difficult to ensure that all are using HPDs properly.  Attained attenuation tends to be highly variable, subject to individual practices and motivations; this situation is discussed further in the section on HPD ratings, below.
     Apathy is a formidable adversary; it seems that no amount of training and information can convince everyone that HPDs are both necessary and worthwhile.  Those with existing hearing loss may be particularly susceptible, thinking it’s “too late.”  HPD use may exacerbate the experience of tinnitus (see Part 8), potentially contributing to their reluctance.  When discomfort causes misuse or disuse, alternative HPDs should be considered.
     Some may have a sense of invincibility, particularly the young and naturally resilient.  A lack of immediate and obvious symptoms can contribute to its perpetuation.  A user can acclimate to the physical sensations resulting from HPD use, but not to intense sounds.  Unlike heat and cold (see Thermal Work Environments series), the human body offers no physiological adaptation to noise.
     Resistance to proper HPD use may also be seen as an act of rebellion against authority.  Explaining how HPD use in the best interest of the sound-exposed individual may be futile when a person’s resistance is unrelated to the policy to be enacted.  Refusal to safeguard one’s own well-being often becomes a disciplinary matter to be handled in the same manner as other behavioral or performance issues.  Unfortunately, this may be necessary to prevent dangerous behavior from becoming a social contagion.

     Occasionally, HPDs are suspected of causing an ear infection, though it rarely occurs.  However, use of HPDs could complicate an ear condition, be it infection, irritation, excessive cerumen, etc.  Contamination of HPDs is always a concern; regular cleaning and/or replacement is required.  Use of HPDs and potential contaminants should be discussed with one’s physician during diagnosis and treatment of any aural condition; complete information is needed to choose the best course of action.

     The effects of HPD use are important considerations in the design of communication systems (see Part 10).  The extent of the effect on speech intelligibility (see Part 9) varies; it is dependent on the levels of received speech and ambient sound and a receiver’s hearing loss.  The curves in Exhibit 11 compare intelligibility with and without earplugs at various noise levels.  At low levels of noise, the curves show HPD use to be a hindrance to speech intelligibility; this is unlikely to surprise anyone.  As noise levels increase, a transition takes place; HPD use begins to aid intelligibility.  This is explained, in part, by the concept of ear overload.  In a loud environment, speech levels are raised to compensate for the ambient sound level.  Increased vocal effort may affect the clarity of spoken words, while high sound levels cause distortion in a receiver’s ear.  Use of an HPD lowers both speech and noise, maintaining a constant S/N (see Part 9), below the level at which distortion in the ear interferes with speech communication.
     Existing hearing loss influences recommendations for HPD use with respect to speech intelligibility.  The logic of such recommendations is pictorialized in Exhibits 12, 13, and 14, representing quiet, moderate noise, and intense noise environments, respectively.  In each, representative plots of ambient noise, speech level, and hearing thresholds identify the audible and inaudible components of speech.  To apply to a specific environment and receiver population, new plots must be generated from relevant data on the environment and HPD options under consideration; recommendations generated must be validated by a qualified physician.  Study of the sample plots and the summary table in Exhibit 15 is sufficient to understand the challenge that existing hearing loss adds to the provisioning of HPDs.
     The effect of HPD use on other types of auditory signals must also be considered.  If an HPD provides high attenuation at the frequency of an alarm tone or other critical signal, the HPD and/or signal may require adjustment to ensure effective communication.
     HPDs change the perception of sound and can make localization difficult.  The effect is more profound when using earmuffs, but can also be significant when earplugs are used.  When rapid identification of an auditory signal’s source or direction is important, it may be necessary to couple it with a secondary signal, such as a visual indicator, to compensate for impairment of localization ability caused by HPD use.
     Discussions of HPD use often fail to recognize that there is also a lower limit to desirable sound levels; more is not always better when it comes to attenuation.  In quiet or moderately loud environments, sound can be attenuated below levels required for situational awareness, speech communication, or signal response.  This condition is called overprotection and can be dangerous.  It occurs when HPD attenuation is greater than necessary to ensure safety and sufficiently high to interfere with the performance of regular duties.  A summary of assessments of HPD adequacy is provided in Exhibit 16.
     Overprotection interferes with verbal communication and can cause warning signals to be missed.  Normal inattention, or one’s focus on a specific task, can raise effective thresholds for auditory signals by 6 – 9 dB, increasing the risk of communication failure by overprotection.

     Perhaps the greatest challenge in provisioning HPDs is the prediction of attenuation and, by extension, the adequacy or sufficiency of devices.  Several rating systems have been developed to facilitate this task; some of the most relevant to occupational settings are introduced in the next section.

HPD Attenuation Ratings
Real-Ear Attenuation at Threshold and Assumed Protection Values
     HPD ratings are typically based on real-ear attenuation at threshold (REAT) measurements.  The REAT measurement procedure, in brief, is as follows:
  • 10 listeners with normal hearing are tested in a diffuse sound field.
  • Pulsed test signals consist of 1/3 octave bands of noise, centered at 125, 250, 500, 1000, 2000, 3150, 4000, 6300, and 8000 Hz.
  • Each listener is subjected to three pairs of tests – one with ears open and one with ears occluded (i.e. HPD in use) – in each frequency band.
  • REAT is the difference in hearing thresholds exhibited in a pair of tests.
  • Average and standard deviation are calculated with n = 30 (i.e. as if 30 subjects were tested).
     Assumed protection values (APVs) are used to determine HPD ratings.  An APV is the mean REAT minus a multiple of the standard deviation of the test data.  The multiple of standard deviation to be subtracted from REAT is defined by the rating system used; multiples of one and two are common.

Noise Reduction Rating
     Of the HPD attenuation ratings available, the one that must be understood and applied is the Noise Reduction Rating (NRR).  NRR was adopted by the Environmental Protection Agency (EPA) and codified in 40 CFR Part 211 – Product Noise LabelingSubpart B of the regulation pertains to HPDs, including test methods, labeling, and other requirements.  NRR can be calculated by the following formula:
where LAf is the A-weighted octave-band level, at frequency f, of an assumed pink noise of 107.9 dBC overall level and APVf98 is the mean attenuation minus 2 standard deviations at frequency f.
     The NRR formula contains a large number of “moving parts;” an example calculation is shown, in tabular form, in Exhibit 17 to facilitate understanding.  Broken into several steps, each with a brief instruction, this format is a more-digestible presentation of the NRR calculation procedure.
     The methods of assessing HPD adequacy using NRR, as defined by OSHA, can be summarized in the following two simple expressions.
  • NRR Method (NIOSH Method #2):
estimated exposure, protected (dBA) = sound level or TWA, unprotected (dBC) – NRR.
  • Adjusted NRR Method (NIOSH Method #3):
estimated exposure, protected (dBA) = sound level or TWA, unprotected (dBA) – [NRR – 7].
     A sample HPD label, as required by EPA, is shown in Exhibit 18.  The NRR is prominently displayed, allowing rapid comparison of devices and estimation of adequacy using one of the methods described above.
     Derating is a common method of approximating real-world performance of HPDs.  OSHA derates HPDs by 50% when comparing their effectiveness to that of engineering controls.  If A-weighted sound measurements are used, a 7-dB adjustment is applied to NRR prior to the 50% derating.  NIOSH recommends derating devices as follows:
  • Earmuffs:  NRR – 25%.
  • Formable earplugs:  NRR – 50%.
  • Other earplugs (premolded, banded, etc.):  NRR – 70%.
If A-weighted sound measurements are used, an additional 7-dB derating is applied.
     Empirical data, represented in Exhibit 19, demonstrate the need for a derating scheme when HPDs ratings are determined by laboratory testing, especially when devices are fitted by experienced experimenters.
Noise Reduction Statistic for Use with A-Weighting
     The Noise Reduction Statistic for Use with A-Weighting (NRSA) is important to consider, as it may supplant NRR in future EPA regulations.  Advantages of NRSA, include:
  • A two-number rating system provides a range of attenuation values likely to be achieved by 80% (NRSA,80) and 20% (NRSA,20) of users.  A single-number rating (NRSA,50) can also be defined for the median attenuation.
  • A-weighted sound data are used directly; C-weighted measurements are not required.
  • Derating is not required to reflect real-world performance, as testing accounts for more variables, including subject-fitting of devices.
The calculation of NRSA,Q is summarized in Exhibit 20.  Details of noise databases are available from NIOSH.  A spreadsheet and Matlab code can be acquired from the developers of the NRSA rating system (Gauger and Berger, 2004) to facilitate calculations.
Noise Reduction Statistic, Graphical
     Gauger and Berger also proposed the Noise Reduction Statistic, Graphical (NRSG) for inclusion on secondary device labels.  NRSG displays curves representing 80th and 20th percentile performance of an HPD in varying noise (C – A) conditions.  To use NRSG plots, examples of which are shown in Exhibit 21, locate the C – A value of the subject noise on the horizontal axis and project this value upward to the curves.  At the intersection with each curve, project to the vertical axis where the low and high protection values are read.  Subtracting these values from the A-weighted sound level yields the range of protected sound levels (A’) anticipated for the device.
     NRSG curves that are close together indicate less inter-user variability, an indication of a device’s ease of use.  For example, the narrow gap between “Earmuff #4” curves suggests that fitting of the device is highly repeatable.  In contrast, the wide range of “Foam plug #1” values indicates that fitment is strongly user-dependent.  The slope of the curves indicates how protective a device is in low-frequency and high-frequency sound.  The negative slope of the “Earmuff #4” curves indicates that the device is significantly less effective when low-frequency energy dominates the soundscape than when high-frequency sound is dominant.

Octave-Band Method
     The Octave-Band (OB) Method is considered the “gold standard” of HPD performance calculations.  Also known as NIOSH Method #1 or “long method, its calculation is similar to NRR.  However, it must be calculated for a specific soundscape, whereas NRR is based on a generalized sound profile (pink noise at 107.9 dB).  This eliminates the spectral uncertainty inherent in NRR and, thus, the 3-dB compensatory adjustment.  A sample calculation, using the OB Method, is shown in Exhibit 22; like the NRR example above (Exhibit 17), it is shown in tabular form with a brief description of each step of the procedure.
Class Systems
     Some HPD ratings do not report attenuation directly.  Instead, classifications are assigned that correspond to sound levels in which their use is recommended.  This method has the advantage of preventing misinterpretations of single-number ratings as precise or guaranteed attenuation values.  However, the lack of detailed information can make decisions difficult when in regard to “special” circumstances, such as hearing aid use.

Fit-Testing and HPD Ratings
     The effect of fit-testing on HPD ratings has been referenced without explicit discussion.  It is the reason for the 2-standard deviation reduction in average attenuation used in the NRR calculation.  Experimenter-fit devices exhibit much less variability in performance than when the same devices are deployed in real-world applications (i.e. “field fit”).  Though performance in service always exhibits greater variability than that revealed in laboratory testing, user-fitting of devices narrows the gap.
     The ideal approach to provisioning HPDs is to test individual users, with self-fitted devices, in the sound spectra to which they are routinely subjected.  This approach allows the greatest level of customization to accommodate individual differences and needs, but is time- and data-intensive.  The further an organization diverges from individualized provisioning of HPD, the more reliant it must be on subjective feedback and audiometric testing.

     Additional HPD rating systems are available; Exhibit 23 summarizes those presented here, as well as a few others.  Still more systems have been developed, should none of these meet specific needs.  Consult the list of references or other sources to learn more about these and other rating systems.
Summary of HPD Selection Criteria
     References to selection criteria have been made throughout this presentation.  The following list is a summary of criteria to be considered when provisioning HPDs to an organization’s workforce; others may be added as needed.
  • Level and spectral makeup of noise
  • Time spent in noise
  • Transitions between loud and quiet environments
  • Other PPE, workwear, etc. to be used simultaneously
  • Verbal and nonverbal communication requirements
  • Attitudes toward hearing protection
  • Training required for effective use
  • Comfort
  • Durability
  • Existing hearing loss
  • Risk of overprotection
  • Exposure to ototoxic chemicals
  • Risk of contamination

     Source and path noise control can be thought of as applications of physics, geometry, and economics to noise problems.  The human element complicates receiver noise control far more than the addition of a single variable.  Human variables include attitudes and motivations, training effectiveness, individual sensitivities and pathologies, social influences, device fitting techniques, and potentially many more.
     Engineering and administrative controls are often limited in the receiver noise control context, but must be enthusiastically pursued.  In extreme conditions, a combination of controls and HPDs provides the only chance of operating within safe limits of noise exposure.  The preceding discussion is only an introduction; additional research may be necessary to identify and implement adequate protections.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

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[Link] “What is a Personal Attenuation Rating (PAR)?”  E. H. Berger.  3M Occupational Health & Environmental Safety Division; April 2, 2010.
[Link] “Hearing Protection Fit Testing — An Introductory Guide.”  UK Hearing Conservation Association; February 3, 2022.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] 40 CFR Part 211 -- Product Noise Labeling. EPA.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 13:  Path Noise Control]]>Wed, 17 Apr 2024 07:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-13-path-noise-control     The discussion of noise control continues, pursuing the second priority (see Part 11):  “Abate transmission of sound along its path.”  Although the singular form is used in the priority statement, sound from a single source may travel along several paths, each requiring its own analysis and solutions.  This fact complicates noise control efforts and reinforces the pursuit of source noise control (see Part 12) as the first priority.
     For sound sources that cannot be sufficiently tamed, a number of potential path treatments are available.  The pursuit of path noise control begins with identification of transmission paths.  Assessment of each path’s contribution to, or potential for mitigation of, the soundscape informs the selection of appropriate measures to be implemented.  Finally, implementation and verification complete the process.
     The “blurred lines” in the hierarchy of controls were discussed in “Part 11:  Concepts in Noise Control” [20Mar2024].  What was not explicitly mentioned, but may have been noticed, in Part 12 is that there is also some “blurriness” between source and path noise control.  For instance, when discussing fan noise, treatments of ductwork were presented as source control because the sound had not yet become air-borne.  The transmission of sound, or vibration, along the duct also qualifies these treatments as path control.  However, the earlier the intervention, the better; thus the inclusion in the source control discussion.

Sound Fields
     Air-borne sound transmission paths are comprised of different regions, within which the behavior of sound differs.  These regions, called sound fields, are depicted in Exhibit 1, plotted as SPL vs distance from source.  The near field, as the name suggests, extends from the source to the distance at which the sound begins to act as a single, coherent, broadband noise.  Within the near field, SPL measurements vary with position, due to phase differences of the generated sound waves.  To minimize near field effects on SPL measurements, readings should be taken at a distance from the source greater than the wavelength at the frequency of interest.  For instance, to measure SPLs at typical audiometric frequencies (500, 1k, 2k, 4k, 8k Hz; see Part 5) at a single location, the sound level meter should be more than 0.686 m (2.25 ft) from the source [λ(500 Hz) = 0.686 m].
     Beyond the distance at which sound waves coalesce to propagate as a single sound is the far field region.  Within the far field is the region called the free field (or direct field), in which sound waves travel without interruption.  That is, the sound waves encounter no reflective or absorbent surfaces in the free field.  In many industrial settings, or other dense environments, the free field may extend for only a very short distance.
     Beyond the free field, reflections add to free field sound to create the reverberant field.  The transition from free field to reverberant field conditions varies by frequency and the characteristics of various materials upon which the sound waves impinge.

Directivity Factor and Index
     The preceding presentation of sound fields is applicable to transmission in any direction from a source.  However, the location of the source, relative to reflective surfaces, can change the nature of the sound field significantly.  The influence of reflective surfaces near a sound source is called its directivity factor (Q).  The formal definition of Q is somewhat wordy and esoteric; a more practical description is attempted here.
     As we know, sound is transmitted omnidirectionally.  When a source is located near a reflective surface, transmission in the direction of that surface is reduced, causing the sound to be more directional in nature.  The more reflective surfaces in close proximity to a source, the more directional the sound transmission becomes.  Numerically, the directivity factor is the ratio of the directional sound intensity to the nondirectional sound intensity; Q = Id/In.  The nondirectional sound intensity, In, is the intensity of sound that would exist at the same point if the reflective surfaces were not present.
     To visualize directivity in an occupational soundscape context, imagine the potential placements of equipment (i.e. sound sources) within a room.  If a source is suspended in the center of the room (e.g. a fan), the sound it generates propagates in all directions without interruption.  In this case, no reflective surfaces influence transmission of the sound; therefore, Id = In and Q = 1.
     A more common installation is the placement of a machine on the floor.  If located in the center of the room, only the floor effects directionality, limiting sound to hemispherical propagation.  In this case, Id = 2 In and Q = 2.  Extending this to placements adjacent to a wall and in a corner yields Q = 4 and Q = 8, respectively.  Stated another way, Q is the inverse of the fraction of the omnidirectional sound transmission sphere that remains in free field conditions, as depicted in the center column of Exhibit 2.
     Practical application of knowledge of directionality often manifests in use of the directivity index (DI).  The directivity index represents the increase in SPL expected, given a value of the directivity factor, Q, and is calculated as DI = 10 log Q (dB).  Exhibit 2 provides a summary of Q and DI values and the shape of the sound field resulting from each sound source placement discussed.
     The directivity factor only accounts for the influence of major structures, i.e. floor and walls in rooms assumed to be rectangular.  Irregular shapes, other objects, and material properties are not considered.  Therefore, Q and DI values can only estimate the true characteristics of sound transmission.  However, these estimates typically suffice for evaluation of occupational soundscapes, particularly in preliminary assessments made for planning purposes.

Transmission Paths
     Sound is transmitted directly through air; it can also be first transmitted as vibration through solid materials.  A sound may be transmitted in several media simultaneously.  Examples of this “multimodal” transmission, from several sources, are depicted in Exhibit 3air-borne paths on the left and structure-borne paths on the right.  Though the root cause of a sound should be identified whenever possible, it need not be known to treat its transmission paths.  However, the spectral composition of the sound and characteristics of the room to be quieted are required to develop effective treatments.
     The multitude of possible transmission paths from a single sound source is represented by the example shown in Exhibit 4.  Some issues depicted in the example of a centrifugal fan and connected ductwork were addressed in Part 12; this installment provides guidance on other matters, such as flanking transmission, acoustic leaks, and variations in material properties.
     Flanking transmission occurs when sound travels around a barrier, rather than through it, taking the path of least resistance.  The example depicted in Exhibit 4 – that of a wall ending at a false, or suspended, ceiling, rather than extending to the top of the structure – is a common form of construction.  It is an “out of sight, out of mind” error; though a large area is available for air-borne sound transmission, it is not visible under normal circumstances.  Flanking is, therefore, often neglected in noise control efforts, yielding unsatisfactory results, particularly at sites where an SPL differential greater than 35 dB is needed.
     The guiding principle of acoustic leakage is “if air can pass, sound can pass.”  Leaks are common at the periphery of windows, doors, and access panels; they can also occur at any joint between materials.  When nonstructural partitions are built in a large space, attention must be paid to connections to structural members and other partitions to prevent leaks.  Many times, the flanking described above and leaks share a root cause – insufficient sealing of nonstructural partitions to structural members.  Constructing a wall can visually separate two areas while they remain acoustically linked.
     Sealing acoustic leaks is often straightforward.  Cracks and joints can be caulked; larger gaps can be filled with absorbent material.  Installing compression seals or acoustic gaskets can effectively reduce leakage around doors and access panels.
     A sound wave that impinges on a barrier divides its energy three unequal ways, as depicted in Exhibit 5.  A portion of the incident sound energy is reflected by the barrier.  The remainder continues through, where it is further reduced by absorption within the barrier material.  The final remainder is transmitted, or reradiated, from the opposite side of the barrier.  The difference between the incident sound energy and the transmitted sound energy is the transmission loss (TL) of the barrier.  TL is frequency-dependent and is an important property of noise control constructions.  Analysis and treatment of transmission paths, properties of noise control materials and their use in combination are discussed further in the following sections.
Barriers and Partial Enclosures
     A barrier is any wall or other physical obstruction to line-of-sight transmission.  A partial enclosure is a set of barriers that is open in one or more directions (e.g. sides or top, if floor-mounted).  Noise control may be the express purpose for constructing a barrier, or it may be one of several objectives.  Additional objectives could include protection from moving parts, contamination control, aesthetic improvement, and others.  Additional objectives may influence design decisions regarding barrier size, placement, materials, etc., but they must not be allowed to compromise proper management of the soundscape.
     As shown in Exhibit 5, the first influence a barrier exerts on a sound wave is to reflect a portion of its energy.  These reflections establish or intensify the reverberant field, shown in Exhibit 1.  Rigid materials with hard, nonporous surfaces are the most effective sound reflectors.  Sealing surface porosity can increase a barrier’s reflectivity.  Depending on the location of the barrier relative to sources and receivers, this could be beneficial to noise control or detrimental to soundscape management.  A common example is a block wall that is painted for aesthetic purposes or to facilitate cleaning.  Increased reverberation results, potentially hindering speech communication or increasing overall SPL in the area, requiring other measures to counteract the effect.

     Sound waves that are not reflected traverse the barrier, losing energy to absorption.  Effective reflectors tend to be poor absorbers and vice versa.  Therefore, a highly-reflective barrier of homogeneous construction typically dissipates little sound energy by absorption.  To achieve significant sound level reductions, barriers are often lined with a layer of absorbent material.
     Absorbent material placed on the incident side of a reflective barrier reduces reverberation and the amount of energy available for transmission through the barrier.  If the absorbent is susceptible to abrasion, chemical attack, or other damage in the incident environment, it can be placed on the opposite side of the barrier.  This arrangement has no effect on the reverberant field on the incident side of the barrier; it reduces transmitted noise, but to a lesser degree than incident-side mounting.
     Effective absorbent materials tend to be lightweight (i.e. low density), porous, and cellular (e.g. foam) or fibrous (e.g. carpet).  Other types of absorbers may also be used to dissipate sound energy.
     Reactive absorbers, or Helmholtz resonators, were mentioned in connection to mufflers installed on internal combustion engine exhausts or pneumatic equipment in Part 12.  A Helmholtz resonator consists of a cavity connected to the surrounding atmosphere via a small, necked opening.  The mass of air in the cavity acts as a spring, oscillating in response to passing sound waves, absorbing energy.  This type of absorber is effective in a very narrow frequency range; it must be designed (i.e. “tuned”) to closely match a troublesome frequency.
     The frequency range for which a Helmholtz resonator is effective can be extended by adding absorbent material in the cavity.  While unlined resonators are typically most effective at low frequencies, absorbent materials are better absorbers of high-frequency sound energy.  Fabricated panels are commercially available to exploit the benefits of combination in a straightforward installation.
     A diaphragmatic absorber is a panel that oscillates at a frequency matching the incident sound wave or one of its harmonics.  As with the resonators discussed above, inducing oscillations in a mass transfers energy to that mass, where it is dissipated.  The sound attenuation provided by this type of absorber is difficult to predict, though it is usually most effective at lower frequencies.  It may require less effort and time to properly specify other noise control measures to target frequencies of concern.
     An absorber’s effectiveness is quantified by its sound absorption coefficient, α, the proportion of incident sound energy that is not reflected or transmitted.  To be “industrially useful,” according to NIOSH, material should have a coefficient of α > 0.60 at 500 Hz and higher frequencies.  At lower values of α, the amount of material required to achieve necessary SPL reductions could be prohibitive.
     Though theoretically impossible, highly-absorptive materials may be catalogued with α > 1.0.  Many values used to develop noise controls are estimates; actual performance may vary from that calculated or anticipated.  Awareness of potential deviations allows inclusion of a margin of error in specifications and prevention of overly-optimistic expectations.
     Absorbers may also be catalogued by their noise reduction coefficients (NRC).  An absorber’s NRC is simply the arithmetic average of its sound absorption coefficients at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05.  Owens Corning considers materials with NRC > 0.40 to be “sound absorbers;” this is a significantly lower threshold than the NIOSH guideline cited above.  NRC may be useful for initial at-a-glance comparisons of materials, but the frequency dependence of absorption requires that individual α values be analyzed to ensure appropriate specification of materials.  Coefficients of example materials are compiled in Exhibit 6 for comparison.
     An alternative specification of the absorption capability of a barrier is the sabin.  A sabin is the equivalent of the sound absorption provided by one square foot (1 sq ft) of a “perfect” absorber (i.e. α = 1.0).  It may be called a “standard” or English sabin, when using “English” units, to explicitly differentiate it from a metric sabin.  A metric sabin is equivalent to one square meter (1 sq m) of perfect absorption and is equal to 10.76 standard sabins.
     To determine the total absorption (A) provided by a panel or other treatment, multiply the area of the treatment and its sound absorption coefficient:  A = S α.  The total absorption of a composite panel, wall, etc. is found by summing the absorptions of its constituents:  A = ∑(Si αi).  Be certain that the system of units is known to all who use this information; a factor-of-ten error can be difficult to overcome!  Also, the symbols used may not be intuitive, due to inconsistency with other contexts.  However, this notation is consistent with existing literature on acoustics and noise control; to avoid further confusion, it will be maintained.

Transmission Loss
     Transmission loss (TL) can be defined a few different ways.  It was introduced conceptually in the previous section as simply “the difference between the incident sound energy and the transmitted sound energy.”  As depicted in Exhibit 5, this difference is comprised of the energy reflected by a barrier and that absorbed within it.
     The TL discussed here is actually the “apparent” transmission loss, known to some as the apparent sound reduction index.  This formulation, with “apparent” dropped for convenience, is used because it is most useful in practical applications, a category in which occupational soundscapes naturally belong.  This is so because the apparent TL allows for flanking transmission that is likely to occur outside strictly-controlled laboratory environments.  The present discussion considers stand-alone barriers and partial enclosures, where the occurrence of flanking is certain.  In “real-world” conditions, apparent TL provides a better estimate of relevant noise control metrics.
     The transmission loss of a panel is determined experimentally by placing it between two reverberant rooms and measuring the difference in SPL between the source and receiving rooms.  The panel’s TL is then calculated using the following equation:
  TL = SPLs – SPLr + 10 log (S/A) = SPLs – SPLr + 10 log S – 10 log A (dB),
where SPLs is sound pressure level in source room (dB), SPLr is sound pressure level in receiving room (dB), S is surface area of panel (sq ft or sq m), and A is total absorption in receiving room [sabins (sq ft) or metric sabins (sq m)].  Practically, it may be more useful to rearrange the expression to estimate SPL differentials that can be achieved with various barrier or enclosure configurations.
     The frequency dependence of a barrier’s transmission loss is divided into four regions, or zones, as shown in Exhibit 7.  In each zone, TL is controlled by a different characteristic of the barrier or panel.
     At very low frequencies, a panel’s stiffness controls sound transmission, with decreasing effectiveness as the frequency increases.  Once the resonance zone is reached, TL begins to appear erratic, though it trends higher with increasing frequency.  The addition of damping material to a panel smooths the TL curve in this zone.  At approximately twice the lowest resonant frequency (i.e. one octave higher), damping effects disappear and sound transmission becomes mass controlled.
     In the mass controlled zone, TL increases linearly at a rate of approximately 6 dB/octave according to the Mass Law, expressed as TL = 20 log (f M) – 47 dB, where f is the incident sound frequency (Hz) and M is the surface density of the barrier.  Surface density is the mass per unit area of incident surface (kg/sq m).  From the Mass Law expression, it can be seen that doubling the barrier thickness (doubling surface density or mass) achieves the same 6-dB increase in TL as a one-octave increase (doubling) in incident sound frequency.  Large increases in barrier thickness can quickly become cumbersome, however.
     The mass controlled zone ends at the barrier’s critical frequency (fc).  The critical frequency is the lowest at which coincidence occurs.  Coincidence, or coincidence effect, occurs when the speed of sound in air and in the barrier are equal.  Sound at the critical frequency must be at grazing incidence (θ = 90°) for coincidence to occur.  At higher frequencies, coincidence occurs at progressively lower angles of incidence (θ), where direct (i.e. perpendicular) incidence is defined as θ = 0°.
     Coincidence causes a barrier to become a more-effective transmitter of sound, as the large drop in TL, called the coincidence dip, above the critical frequency shows.  In the coincidence controlled zone, damping effects return, reducing the severity of the dip.  However, mass law performance should be considered an asymptotic limit in this zone.
     A few points to which the preceding discussion alludes are worth making explicitly, including:
  • The vertical axis (ordinate) in Exhibit 7 is labeled “sound reduction index,” an alternate term for transmission loss, as noted above.
  • There are no values of TL or frequency given in Exhibit 7 because the amount of sound reduction and the transitions between zones will vary by material and construction details of barriers.  For a homogeneous barrier, the critical frequency can be calculated:
where c is the speed of sound (m/s), t is barrier thickness (m), P is barrier mass density (kg/m^3), and E is the elastic modulus of barrier material (kg/m s^2).  Heterogeneous barriers are often constructed to join reflective and absorptive materials and to reduce coincidence effects below a level of significance.
  • There is no coincidence effect unless the air-borne wavelength (λ) is less than the structure-borne wavelength (λB) of the sound.  Coincidence occurs when sin θ = λ/λB.  For direct impingement, sin 0° = 0; coincidence does not occur.  This has little practical effect, however, as the context of discussion is a reverberant room, where sound impinges at all angles.
  • Once again, the spectral composition of incident sound is critical information needed to apply noise control measures in the appropriate zones of transmission loss.

     Transmission loss of a material may also be characterized by its sound transmission class (STC) or noise isolation class (NIC).  STC and NIC are single-number indices that represent the frequency-dependent TL and insertion loss, respectively.  These indices are analogous to the use of NRC for absorption coefficients.
     A material’s STC and NIC are determined using 1/3 octave band measurements from 125 to 4000 Hz (16 total; see Part 6).  The data are plotted and compared to standard curves while applying a set of rules that ultimately determine the ratings assigned.  Details of the procedures are not provided here because they are of limited value in the context of industrial settings.
     An STC rating is equivalent to the sound intensity reduction, in dB, that a material is expected to provide.  It is typically used to assess the prevention of transmission of intelligible speech; thus, it may be more applicable to a government building than a factory.  NIC compares sound levels with and without a panel in place.
     The standard caveat applies to STC and NIC:  theory and practice differ; variables present in application are not represented in laboratory tests.  The best results can be obtained by using constituent octave band data.  However, single-number ratings may be useful to narrow the field of candidate materials for testing; examples of material TL and STC ratings are tabulated in Exhibit 8 and Exhibit 9, respectively.
Path Length
     As mentioned above, standalone barriers and partial enclosures cannot force sound to impinge on attenuating material; flanking is assured.  To maximize the benefit provided by a barrier, the length of the flanking path between source and receiver must be considered.  Recall that doubling the distance from a source reduces sound intensity by 6 dB at the receiver.
     In Part 10, low-frequency communication signals were recommended when the direct path (i.e. line of sight) to listeners is obstructed.  This is because low frequencies experience greater diffraction around obstacles, resulting in a smaller shadow zone, as depicted in Exhibit 10.  It is the Fresnel theory of diffraction that allows us to predict sound level reductions attainable with a barrier of certain size and placement.  To do so, consider the diagram, in Exhibit 11, of Fresnel zones created by placing a barrier between a source and a receiver.  Although the description references sound flanking the top of a barrier, the method is the same when applied to flanking around the side of a tall barrier.
     The information needed to predict a barrier’s performance are:
  • A – distance from sound source to top of barrier (m).
  • B – distance from top of barrier to receiver (m).
  • D – direct path distance from source to receiver (i.e. no barrier present) (m).
  • δ – path length difference (m); δ = A + B – D.
  • N – Fresnel number (dimensionless); N = ± 2 δ/λ.  N > 0 in the shadow zone (i.e. attenuation zone) and N < -0.2 in the bright zone (i.e. free field).  Greater attenuation is achieved at higher Fresnel numbers, attained with greater path length difference and/or higher frequency.
     To estimate the attenuation provided by a barrier, the Fresnel number is used to locate a corresponding point on a standard curve.  A simplified method is to calculate it as follows:  ΔSPLf = 10 log (3 + 0.12 f δ), where
  • ΔSPLf is the barrier attenuation at frequency f (i.e. difference in SPL at receiver with and without barrier present) (dB).
  • f is the frequency of incident sound (Hz).
  • δ is the path length difference (m).
Summing the SPL differentials (see Part 4) provides an estimated overall SPL reduction due to the barrier.  The practical limit of sound level reduction by use of a barrier is approximately 20 dB; greater reductions require additional noise control measures.
     There are several design and placement decisions to be considered in order to maximize the attenuation potential of a barrier, including:
  • Place the barrier as close as is practical to the sound source or the receiver; both should be in the direct field to attain significant level reductions.
  • The barrier should be as tall as is practical, given other constraints.  Its effective height (H in Exhibit 12) is the vertical distance from the source to the top; its total height is less important.
  • Each side of the barrier should be at least twice as far from the source as is the top of the barrier (i.e. width > 4 H).
  • To prevent transmission (i.e. to “force” flanking), construct a barrier with surface density greater than 2 lb/sq ft (9.76 kg/sq m).
  • To minimize the impact of ceiling reflection, ceiling height should be at least 150% of the source-to-receiver distance (D); see Exhibit 13.
  • To achieve 10 dB or greater attenuation, the angle between the lines defining (1) incidence at the top of the barrier and (2) the direct path from the top of the barrier to the receiver should be greater than 30°.  This is the angle θ in Exhibit 12; it is also shown in Exhibit 13.
     Adding absorbent material to the ceiling, hanging baffles, or other similar treatment can also reduce reflections of sound reaching a receiver.  The more effective the barrier or enclosure, the more noticeable ceiling reflections become.
Rooms and Total Enclosures
     Rooms and total enclosures are structures that are closed in all directions.  Common usage of the terms often differentiates them by the nature of construction or intended use.  A “room” typically refers to a space occupied by humans and of robust construction, possibly utilizing structural members of a building.  “Enclosure,” in contrast, often refers to a structure surrounding equipment, of a temporary nature, constructed, for example, of reconfigurable panels.
     While the connotations of common usage may be useful shortcuts in conversation, use of the terms in this way is not strictly accurate; parameters must be established at the outset to avoid misunderstanding.  Many design decisions, operational procedures, etc. depend on proper understanding of noise control requirements; all assumptions must be validated.
     Much of the preceding discussion of partial enclosures also applies to total enclosures; e.g. the transmission loss of an enclosure is the sum of its constituent barrier TLs.  Completing an enclosure, however, adds points of analysis needed to fully characterize its performance.
Room Constant and Reverberation
     The total absorption of an enclosure can be characterized by its room constant, R.  Room constant is calculated at each frequency as
is the average absorption coefficient of all barriers comprising the enclosure, and S is the total surface area of the enclosure (sq ft or sq m).  A larger enclosure, of identical construction, has a higher room constant which corresponds to a less-intense reverberant field within the enclosure.
     Knowing the room constant allows us to estimate the location of the transition from free-field to reverberant conditions (see Exhibit 1).  The distance from the source at which this transition takes place, r’, known as the transition zone, is estimated for each frequency as
     An enclosure’s room constant is also effected by its contents, including equipment, furniture, people, and so on.  The contribution to R of a room’s contents is much more difficult to calculate, however.  A rule of thumb for industrial environments is to increase R by 25% or more.  The sound intensity calculations are relatively insensitive to this adjustment; therefore, the exact value used is noncritical.
     For other settings, such as conference rooms, performance halls, etc., tabulated data are available from various sources to aid in estimating the effect of a room’s contents on R.  A sample of the type of data available is provided in Exhibit 14.
     A related measure is a room’s reverberation time, TRReverberation time is defined as the time required for sound intensity to decrease 60 dB from its steady-state value upon silencing the source.  A room’s reverberation time is estimated as TR = 0.05 V/(S α_bar) (s), where V is the volume of the room (ft^3) and S is surface area (sq ft) or TR = 0.16 V/(S α_bar) (s), where V is the volume of the room (m^3) and S is surface area (sq m).  (S α_bar) is the total absorption of the room in sabins or metric sabins, respectively.
     The curves in Exhibit 15 represent mean reverberation times for various types of venue; actual values are highly variable.  As can be seen from the curves, longer TR is desirable for music, while speech intelligibility requires relatively short reverberation times.  The “ideal” TR in an industrial setting can be generalized “as short as possible,” as the vast majority of sound is unwanted (i.e. noise) and often at objectionable intensity.

     When a sound source is completely enclosed, the energy trapped inside causes “sound build-up,” as intensity increases until a steady-state condition is reached.  The average sound pressure level inside the enclosure (SPLE) is found as follows:  SPLE = PWL + 10 log (4/RE) (dB), where PWL is the sound power level of the enclosed source (dB) and RE is the enclosure’s room constant (sq m).  The level at the exterior of the enclosure (SPLEXT) is determined as follows:  SPLEXT =  SPLE – TLE – 6 dB, where TLE is the average transmission loss of the enclosure panels (dB).
     Sound energy is absorbed by converting it to thermal energy.  Without proper ventilation, “heat build-up” inside an enclosure can reach dangerous levels.  Performance of temperature-sensitive equipment can be altered and, in extreme cases, a fire hazard can be created.
     The amount of air flow needed to maintain a stable temperature inside an enclosure is determined as follows:  Qv = 1.76 W/ΔT, where Qv is the ventilating air flow required (ft^3/min or cfm) (assumed to be at ambient temperature), W is the amount of heat generated in the enclosure (W), and ΔT is the allowable temperature rise (°F).  Air flow through an enclosure should be as uniform as possible to prevent hot spots.  Treating the calculated value as a minimum is advisable, though a balance must be sought with the additional noise concerns associated with fans and ductwork, as discussed in Part 12.

Enclosure Access Points
     Normal operation and maintenance requires equipment to remain accessible.  Small enclosures, or covers, can be removed to provide access for maintenance tasks, but this is usually infeasible for larger enclosures.  The four key access types are doors, windows, tunnels, and utilities.
     A door allows a person to fully enter an enclosure.  An access panel may be used when all required tasks can be safely performed from the exterior (e.g. within arm’s reach, with a tool, etc.).  Access panels are treated as a type of door for this discussion for two key reasons:
  1. The methods of sealing and materials of construction, for a given enclosure, are the same.
  2. Both present the risk of being left open, severely limiting the enclosure’s noise reduction capability.
      Windows are used when visual monitoring of the interior is required, but physical access is not.  The types in use range from a single sheet of clear polycarbonate to residential-style multiple-pane windows.  The choice of window depends on the construction of the enclosure and the attenuation required.
     Beyond the cost considerations, the number and size of doors and windows in an enclosure significantly impact its ability to reduce sound levels.  As seen in the curves of Exhibit 16, an enclosure’s attenuation capability is reduced as a function of the difference between the access point and wall TLs and the proportion of the wall comprised of the access.  If multiple accesses with different TLs reside in a single wall (e.g. door and window), calculating the effective TL of the wall, as presented above, is more accurate.
     Tunnels are openings in enclosures used to pass material into or out of an enclosure, typically on a conveyor.  The effect on sound levels of a typical three-sided tunnel are shown in Exhibit 17.  The effect of fully enclosing the conveyor (i.e. adding a fourth side to tunnel, below conveyor) depends, in large part, on the type of conveyor installed.  For example, a full tunnel is likely to provide greater noise reduction when an open roller-type conveyor is used than when one with a heavy rubber belt is installed.  In any case, it is recommended that any tunnel be constructed with a length at least twice the largest cross-sectional dimension (i.e. width or height) of the opening in the enclosure.
     Additional noise reduction can be achieved by closing the tunnel opening between material transfers.  The greatest advantage is gained from this approach if closure is automated to ensure use and cycle times are long relative to transfer times.  Frequent transfers reduce the potential advantage; at or near continuous material flow, no reduction in noise exposure can be achieved by this method.
     Utility access points can easily be overlooked, as they are often located in inconspicuous areas of enclosures.  This group includes any passage through an enclosure of electrical wiring, pneumatic or hydraulic plumbing, ventilation ducts, exhaust ports, process fluids, and so on.  The necessity of these passages, combined with a lack of awareness of their impact on noise control efforts can lead to carelessness in installation.  Proper sealing is paramount; this fact must be conveyed to installers and maintainers to ensure noise-reduction targets are achieved.
     Irrespective of the cause of leaks – door gaps, construction joints, unsealed utility pass-throughs, etc. – the combined impact can be enormous.  An opening as small as 0.1% of total area can halve the noise reduction capability of a high-TL enclosure!  Leaks also cause attainable attenuation to rapidly plateau; a high-TL enclosure with leaks is no more effective than one with a modest TL rating.  Curves representing attenuation loss due to acoustic leaks are shown in Exhibit 18.
Methods of Isolation
     The discussion of barriers and enclosures has been biased toward isolation of the source, consistent with the priorities established in the source-path-receiver (SPR) model (see Part 11).  An alternative approach is to isolate the receiver(s), creating another blurred line between path and receiver noise control.
     Isolation of receivers is achieved by housing them in an enclosure – or room – when it is not possible or practical to enclose the source.  Examples include mills, foundries, and construction sites, where the equipment or work area is too large to enclose.  A receiver enclosure may be a “control room,” where equipment is remotely operated, or simply a refuge from high-intensity surroundings used when tasks permit or during break-times.
     When an enclosure is used to house personnel, the noise control challenges can become greater than when enclosing equipment.  The number and size of windows and doors is typically higher, the ventilation requirements are more stringent, numerous electrical and data cables are often required, and plumbing may be needed.  Fortunately, analysis and solution of these issues remain as presented above for equipment enclosures.

     An example noise control “journey” is presented in Exhibit 19; panel A depicts the initial condition.  Noise is a known problem; personnel are issued hearing protection devices (HPDs) as a reactive, interim solution.  In panel B, an installed partition provides some protection from direct sound exposure, but the intensity of the reverberant field requires continued HPD use.  The addition of absorptive material along the ceiling, shown in panel C, reduces the reverberant field intensity such that HPD use is no longer required for those engaged in “quiet” operations.  Finally, in panel D, the source of noise is fully enclosed, reducing noise levels sufficiently to cease HPD use for all operators.
     Key take-aways from the Exhibit 19 example, in brief:
  • HPD use is a default short-term response to noise exposure, but is the last resort as a long-term solution.  This point is discussed further in Part 14.
  • Progressing through insufficient solutions can be avoided with up-front analysis and planning.
  • None of the treatments depicted address the sound power of the source; reducing it may render less-extensive path treatments sufficient.
  • High-intensity exposure for some personnel should not be considered inevitable.  Had the source enclosure (i.e. protection of the machine operator) been considered first in the example, all personnel would have reaped maximum benefit without intervening measures unnecessarily consuming resources.
  • Though it is a hypothetical example, the type of progression presented is far too common.  The urge to “do something” simple, visible, and ineffective must be suppressed in favor of purposeful analysis and true solutions.

And So Much More
     This installment of the “Occupational Soundscapes” series pushes the limits of the “easily consumable” objective to simply be a reasonable overview or introduction to path noise control.  There are myriad recommendations for design and construction details of enclosures, material alternatives, and so on that address practical applications of noise control.  On the theoretical side, discussion of additional performance indices and material ratings have been omitted.  The goal of this, and all entries on “The Third Degree,” is to provide a foundation on which readers can build knowledge while putting the basics into practice.  If it inspires investigation beyond the limitations of this medium that lead to improved soundscapes, it will be deemed a tremendous success.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

[Link] An Introduction to Acoustics.  Robert H. Randall.  Addison-Wesley; 1951.
[Link] The Effects of Noise on Man.  Karl D. Kryter.  Academic Press; 1970.
[Link] Human Engineering Guide to Equipment Design (Revised Edition).  Harold P. Van Cott and Robert G. Kinkade (Eds).  American Institutes for Research; 1972.
[Link] Architectural Acoustics, 2ed.  K.B. Ginn.  Brüel & Kjaer; November 1978.
[Link] Industrial Noise Control Manual (Revised Edition).  National Institute for Occupational Safety and Health (NIOSH); December 1978.
[Link] Compendium of Materials for Noise Control.  National Institute for Occupational Safety and Health (NIOSH); 1980.
[Link] Handbook for Industrial Noise Control.  National Aeronautics and Space Administration; 1981.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications; 1996.
[Link] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] “Noise and Vibration.”  Evan Davies in Plant Engineer’s Reference Book, 2ed.  Dennis A. Snow, ed.  Reed Educational and Professional Publishing Ltd.; 2002.
[Link] “Noise Control Design Guide.” Owens Corning; 2004.
[Link] “Engineering Controls for Reducing Workplace Noise.”  Robert D. Bruce.  The Bridge; Fall 2007.
[Link] Engineering Noise Control – Theory and Practice, 4ed.  David A. Bies and Colin H. Hansen.  Taylor & Francis; 2009.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] “Controlling Noise at Work.”  (UK) Health and Safety Executive (L108- 3ed); 2021.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Technical Guide for:  Noise Control – Engineering Controls, Work Practices, & Administrative Controls.”  Georgia Tech; May 2023.
[Link] “Noise control.”  Wikipedia.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 12:  Source Noise Control]]>Wed, 03 Apr 2024 07:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-12-source-noise-control     Limiting the amount of sound energy emanating from any given source is the best way to protect hearing and maintain effective communication.  This is reflected in the first noise control priority statement (see Part 11):  “Prevent emission of sound by a source.”  Control of noise at its source can take many forms, depending on the nature of the generator.
     In this installment of the “Occupational Soundscapes” series, various types of sources are discussed.  Representative examples of source treatments are provided to demonstrate the types of analysis that may be required.  Multiple treatments may be needed to tame a soundscape, bringing noise to a safe, manageable level.
     Much of the discussion of noise control relates to retrofitting existing equipment and facilities; that is, reactive management of soundscapes.  Addressing sound in the design phase, to develop the quietest soundscape possible, is always preferable to retrofitting noise control measures after equipment is in service.  However, unforeseeable changes can occur within a machine’s service life, requiring reactive strategies.
     Similarly, the discussion is biased toward the lower rungs of the hierarchy of controls.  It is assumed, in many cases, that all opportunities for elimination and substitution have already been exploited and, therefore, only engineering and administrative controls and PPE require discussion.  In this discussion, examples of source elimination and substitution are presented agnostically, applicable to the design phase or to retrofit while in service.  Initial designs can limit the retrofit options available; creative adaptations may be needed.
PPE is the primary topic of a future installment and few administrative controls (i.e. behavior changes) effect the emission of noise.  This presentation, therefore, mostly involves engineering controls.

     Regular maintenance of equipment is conducted to meet productivity and reliability targets.  A thorough maintenance program also aids noise control; some activities support multiple objectives.  Examples of this type of activity include:
  • Lubricating moving parts.
  • Tensioning drive belts and chains.
  • Aligning drive components, such as gears, pulleys, or sprockets.
  • Tightening loose fasteners, particularly on guards and enclosure panels.
  • Securely latching all access panels.
  • Repairing damaged seals or other leaks in enclosures.
  • Sealing leaks in compressed air or other pressurized system.
  • Replacing components worn beyond design specifications.
  • Adjusting operating speeds and pressures to optimum levels.
  • Verifying that all specified noise control measures are installed and functioning properly.
     A useful rule of thumb for machine noise is that the level of sound emitted should remain within 2 dBA of its ideal condition.  Maintaining a stable noise level facilitates troubleshooting other issues that arise within a machine, as discussed in “Troubleshooting is a Six-Sense Activity” (4Dec2019).

     When vibration is induced by operation of a machine, it can be radiated from surfaces as sound.  In some cases, the energy can travel surprising distances before becoming air-borne; locating a source in these situations can be challenging.
     The primary source of vibration is imbalance; according to some sources, it is responsible for half or more of machinery vibration issues.  Imbalance typically refers to rotating components; however, reciprocating components generate similar cyclical forces that can manifest in vibration.  Misalignment, looseness, and collision are also significant contributors to machine vibration and noise.
     Balancing, aligning, and tightening of components are routine maintenance tasks, as discussed above.  If these actions do not prevent collisions, the design and operating parameters of equipment should be reviewed to ensure it is being operated as intended, that it has not been modified, and that it is within its expected service life.
     Some collisions are inherent to processes and equipment in use; for example, an actuator that extends and retracts to hard stops.  In this case, the collision and resulting vibration should be damped.  Common solutions include the placement of elastomer pads or dashpots at the hard stops and reducing the speed or acceleration of the actuator.
     Other collisions, or impact noise sources, include the feeding of parts into or out of a machine.  These sources include material hoppers, collection bins, and sheet stackers.  To reduce the noise generated, the height from which material falls should be minimized.  Methods of achieving minimal drop height include the use of adjustable-height equipment, fall interrupters, and transition slides; examples of these are shown in Exhibit 1.  If parts are handled manually, an administrative control can be implemented, requiring that parts be placed, rather than dropped or tossed.  Higher variability of noise levels can be expected for manual operations, with or without an administrative control in place.
     Damping at the machine level can be achieved by placing a soft layer in mounting fastener joints (i.e. a “rubber washer”) or placing the entire machine on an elastomer mat.  When a machine’s panels are significant radiators of sound, the panels can be stiffened by adding braces or curvature.  A panel’s resonant characteristics can also be changed by increasing its total mass or dividing its surface into smaller areas, as shown in Exhibit 2.  Adding damping material, such as mastic, increases the effectiveness of these methods.  The panel can also be replaced with a constrained-layer (“sandwich”) panel, a preferred approach if the operating environment is likely to damage exposed damping material.
     When damping is insufficient to prevent transmission of vibration to adjacent areas, the source must be isolatedIsolation is often achieved by using a combination of methods.  Spring-mounting of equipment is often combined with dampers for improved vibration control.  The thickness of concrete floors beneath machines that generate, or are sensitive to, vibration is often increased to improve damping characteristics.  Depending on the composition of the soil beneath the concrete slab, it may be necessary to extend pilings deep into the ground to achieve isolation.  The reinforced area is typically separated from the adjacent floor by an elastic joint.

Hydraulic Noise
     Special treatments may be needed when noise is caused by fluid flow; this type is called hydraulic or hydrodynamic noise when referring to incompressible-fluid systems (e.g. oil, water).  The primary contributor to hydraulic noise is cavitation.  Cavitation occurs when the pressure of the fluid drops sufficiently for vapor bubbles to form, only to collapse when pressure rebounds.  Cavitation can cause physical damage to a piping system in addition to noise.  Pipe “shuddering” and noise can be reduced with improved mountings, such as those shown in Exhibit 3.
     Transmission of hydraulic noise can be reduced by wrapping pipes in acoustical insulation, known as “lagging.”  Two common configurations of pipe lagging are shown in Exhibit 4.  Reductions up to 10 dBA can be achieved with a single-layer application, shown in the upper portion of the figure.  The double-layer configuration shown in the lower portion can yield sound level reductions of 15 – 20 dBA.
     Another method of limiting sound transmission in pipes involves the use of flexible connections or sections of flexible hose between runs of solid pipe.  This method is particularly helpful near direction changes that can cause turbulence in the fluid or create impingement forces that shake the pipe.  A pictorial guideline for the application of this method is provided in Exhibit 5.
     Addressing the root cause of cavitation requires the selection of appropriate system components and operating parameters.  Cavitation is induced when fluid experiences a rapid, large pressure drop, as commonly occurs in control valves.  If a valve with sufficiently low pressure drop cannot be installed, the line pressure can be reduced to minimize cavitation.  An insert that can be installed upstream to reduce valve inlet pressure is depicted in Exhibit 6.  Fluid that undergoes a series of small pressure drops is much less susceptible to cavitation than when subjected to a single large pressure drop.
     Avoiding rapid changes in flow rate also reduces noise and risk of physical damage to a piping system.  A rapid reduction in flow rate (i.e. fluid deceleration) can cause a pressure spike, shock wave, and associated noise.  This phenomenon is known as “water hammer” and can be quite destructive.  Water hammer can be prevented with gentle transitions of fluid flows or the installation of accumulators to alleviate excess pressure.  Gradual changes in cross-section and direction (e.g. large radius bends) minimize contributions to cavitation and water hammer.

Aerodynamic Noise
     When noise is generated by compressible fluid flow (e.g. air, steam, industrial gases), it is called aerodynamic noiseAerodynamic noise may be present in open (e.g. fan) or closed systems operating at high pressure (e.g. compressed air) or low (e.g. HVAC).
     The conditions and countermeasures described above for hydraulic noise are very similar to high-pressure (closed) compressible-fluid systems.  One that is common and familiar to most is a distributed compressed air system.  Leaks often form in distribution piping, wasting energy, causing equipment malfunctions, and adding substantially to noise levels.  Sealing leaks in pressurized systems is an important component of a maintenance program, as mentioned above.
     Compressed air is also exhausted to atmosphere intentionally when used to operate handheld tools, automated equipment, etc.  Whereas leaks in a distribution system may be high above personnel, where piping is routed near the rafters, equipment exhausts are typically quite near operators, requiring measures to limit sound exposure.  A dispersive, or diffuser, muffler is typically installed to reduce the emitted sound.  Slotted and porous types are common; examples are shown in Exhibit 7.  The terms muffler, silencer, and suppressor are often used interchangeably; readers will be spared a discussion of the terms’ nuance.
     If a muffler cannot be installed, due to backpressure concerns, for example, the exhaust should be routed away from personnel, exploiting the inverse square law (see Part 3), to the extent possible, to reduce exposure.  Aerodynamic noise is generated in proportion to flow rate (i.e. fluid velocity), pressure drop, and turbulence.  Operating equipment at the lowest possible pressure minimizes these contributors to aerodynamic noise.
     Though use compressed air blow-offs is increasingly discouraged, the practice remains common in many facilities.  Unrestricted exhaust of compressed air by open-ended tubing, for example, should be eliminated by installing nozzles designed to reduce noise while maintaining the working capability of the air flow.
     Reactive mufflers are best suited for “pulsing” gas flows, typical of internal combustion engine exhaust.  The internal geometry of a reactive muffler reflects sound waves to dissipate energy and create destructive interference of subsequent incoming waves.  Examples of reactive mufflers that use expansion chambers and Helmholtz resonators are depicted in Exhibit 8.
     In contrast to liquid-medium systems, abrupt changes in ductwork can be advantageous to noise control.  Bends, branches, and changes in cross-section can induce reflections and eliminate line-of-sight transmission paths, providing additional attenuation of higher frequencies.
     Another feature that can be used is a sound-absorbing plenum, such as that depicted in Exhibit 9.  Flexible joints can also be inserted in duct runs to interrupt the transmission path created by long sections of sheet metal.  These can be made of an elastic material, canvas, or similar fabric.  Despite any advantage offered in noise control, these features must be accounted for in system design calculations to ensure appropriate downstream flow characteristics.
     If additional noise control is needed, it is typically achieved using dissipative mufflers, in which absorbent material converts sound energy to thermal energy.  Ducts must be designed to accommodate the absorbent material while maintaining sufficient volume to achieve desired flow characteristics.  At a constant flow rate, reducing the cross-sectional area of a duct increases fluid velocity, potentially reducing effectiveness of the muffler.  Exhibit 10 presents possible configurations of sound absorbents in ducts based on the frequency attenuation required.

     The primary source of noise in a ventilation system is typically a distribution fan.  Since noise is emitted omnidirectionally, the fan should be isolated from both inlet and outlet ductwork, as well as its mounting surface.
     Placement of the fan relative to bends, control vanes, or other features is also important.  Operating too near a turbulence-inducing duct feature can be detrimental to performance with respect to air flow and noise generation.  Considerations for specifying a ductwork fan include:
  • A fan is quietest when running at peak efficiency; it should be sized to operate at its optimum speed when providing the required air flow.
  • Large, slow fans generate less noise than smaller, faster ones.
  • Centrifugal fans are quieter than propeller fans.  Centrifugal fans with forward-curved blades tend to operate at lower speeds, with less noise, for a given flow rate than those with other blade styles.
  • Tube-axial fans tend to be quieter than centrifugal fans, providing high-volume flow at low pressure.
  • Directional fan blades are more efficient than perpendicular (radial) blades.
Exhibit 11 provides a comparison of fan and blade types.
     Unducted fans are typically of the propeller type used for general ventilation of large areas.  In this application, too, a large, low-speed fan is preferred.  The benefits of a large, slow fan can be experienced in many factories and warehouses where they have been installed overhead.  Their existence is often imperceptible until stifling, stale air causes one to look up and realize that these effective air movers have been switched off!

     It has been noted that turbulence in fluid flows contributes to sound generation.  When turbulent flow is required for the fluid to perform its intended function, such as heat transfer, sound emitted as a consequence is called irreducible noise.  This is one of many trade-offs that may be encountered in system design.  Other examples include:
  • The quietest type of fan or pump may not be compatible with the medium, contamination, or other environmental or operational characteristic it must accommodate.
  • An increased cross-sectional area of a duct may be required to accommodate the addition of sound-absorption material; this, in turn, effects the design of the space within which it must be housed.
  • A quieter power source may not deliver an equivalent level of performance; e.g. a battery-electric tool produces less torque than a pneumatic tool, requiring a larger tool and battery to achieve acceptable performance.
  • A cost vs. performance trade-off is inherent to every design decision where configuration or feature options must be evaluated.
     The trade-offs mentioned, and many more, are defined by requirements and constraints that are unique to specific projects; there are innumerable potential combinations of noise sources and system configurations.  Any presentation of noise control options, such as that attempted in this series, must, therefore, be generalized.  Only the analysis of a specific system in a specific environment can ensure an optimal solution.

Design and Procurement
     Prevention of noise occurs, primarily, in the design phase of products and equipment.  Purchasing custom-built machinery affords a high level of involvement in its design and specification development.  Standard tools, household appliances, and other “off-the-shelf” products provide no opportunity to influence performance characteristics; purchasing these items is a “take what you get” proposition.
     Fortunately, increasing awareness of noise and hearing protection issues has prompted manufacturers to begin treating sound as an important characteristic of products.  Though not yet universal, the inclusion of sound level information in product documentation is becoming more common.  Publication of such information suggests that a manufacturer is committed to noise-reducing design practices.  When this information is not published, independent testing may be needed to select appropriate equipment to meet all performance requirements.
     A number of standards and guidelines have been promulgated, by various organizations, to facilitate design and purchasing decisions regarding noise, some more narrowly focused than others.  Among them are SAE International’s automotive-related standards; ASHRAE is the authority on HVAC noise.  A more-generalized set of standards, developed by the Acoustical Society of America (ASA), can be found in the ANSI catalog.

     The “Buy Quiet” movement began at NASA to provide guidelines for purchasing equipment that is not only quiet, but safe, efficient, and cost-effective.  Its focal point is a defined process that guides the collection of performance requirements, research of available alternatives, and verification of in-situ performance.
     The Buy Quiet Roadmap includes specific guidance on the procurement process, plus references to product noise databases, regulations, and equipment-specific standards.  The Roadmap also includes several worksheets that can be used to calculate costs, evaluate trade-offs, and document other steps of the process.  There is even a built-in tutorial to assist new users of the Roadmap.

     Opportunities to reduce noise, thereby protecting hearing and easing communication, range from obvious to easily-overlooked.  This range includes production machinery, material handling equipment, printers and copiers in offices, hand dryers in restrooms, and so much more.
     The success of a noise control program depends largely on an organization’s commitment to it.  In its noise control guide, OSHA declares that “[i]n the field of noise control, where there’s a will, there’s a way.”  Integral to this will is a continuous improvement mindset; until the risk of hearing loss, interference with communication, and other effects on performance and well-being are eliminated, noise control and reduction must remain a priority.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

[Link] Industrial Noise Control Manual (Revised Edition).  National Institute for Occupational Safety and Health (NIOSH); December 1978.
[Link] Compendium of Materials for Noise Control.  National Institute for Occupational Safety and Health (NIOSH); 1980.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications; 1996.
[Link] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[Link] “Noise and Vibration.”  Evan Davies in Plant Engineer’s Reference Book, 2ed.  Dennis A. Snow, ed.  Reed Educational and Professional Publishing Ltd.; 2002.
[Link] “Noise Control Design Guide.” Owens Corning; 2004.
[Link] “Designing Quiet Products.”  Richard H. Lyon and David L. Bowen.  The Bridge; Fall 2007.
[Link] “Engineering Controls for Reducing Workplace Noise.”  Robert D. Bruce.  The Bridge; Fall 2007.
[Link] Engineering Noise Control – Theory and Practice, 4ed.  David A. Bies and Colin H. Hansen.  Taylor & Francis; 2009.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] “Voluntary National and International Noise Standards for Products and Machines.”  Robert D. Hellweg Jr.  The Bridge; Summer 2021.
[Link] “Resources for Noise Control Engineering.”  George C. Maling Jr.  The Bridge; Summer 2021.
[Link] “Controlling Noise at Work.”  (UK) Health and Safety Executive (L108- 3ed); 2021.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Hierarchy of Controls.”  NIOSH; January 17, 2023.
[Link] “Noise control.”  Wikipedia.
[Link] “Technical Guide for:  Noise Control – Engineering Controls, Work Practices, & Administrative Controls.”  Georgia Tech; May 2023.
[Link] Handbook for Industrial Noise Control.  National Aeronautics and Space Administration; 1981.
[Link] “Beyond Blades: Types of Centrifugal Fans and Their Unique Applications.”  The Mechanical Engineer; May 8, 2021.
[Link] “Comparing Axial Fans and Centrifugal Fans.”  Ryan Smoot.  Digikey.com; March 22, 2022.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 11:  Concepts in Noise Control]]>Wed, 20 Mar 2024 07:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-11-concepts-in-noise-control     “Noise Control” is an umbrella term used to describe any action taken to reduce communication interference or hearing loss risk due to noise exposure.  There is a wide variety of options available to pursue these objectives, including modifications to equipment, facilities, and work practices.  Behavior changes and use of personal protective equipment (PPE) are also encompassed by the term when they occur in response to noise exposure.
     This installment of the “Occupational Soundscapes” series discusses planning, prioritization, and validation of noise control measures.  It also introduces fundamental principles that guide improvement efforts.  Finally, a model is presented that describes the nature of a soundscape, completing the structure within which noise control objectives are pursued.
Noise Control Planning
     Like any improvement effort, implementation of noise control measures are more successful when thoroughly planned in advance.  Doing so requires detailed information on the composition (octave bands) and intensity of the soundscape.  Results of a sound survey should be mapped on a facility layout (see “Commercial Cartography – Vol. III:  Facility Layout or Floor Plan” [6Nov2019]).
     A visual representation facilitates evaluation of potential solutions, particularly when multiple areas or people are affected by a single change.  The use of isolines or color-shading can further elucidate the challenge presented or inspire creative solutions that resolve multiple exposure issues simultaneously.  A simplified sound survey map example is shown in Exhibit 1.
     The sound survey map should also display the target level, or criterion, for each area, such as “TWA8hr ≤ 85 dBA,” to clearly identify the magnitude of improvement needed.  Adding the SPL to a hazard map or body map for an area (see “Commercial Cartography – Vol. V:  Hazard Mapping” [12Jan2022]) can reduce complacency in the use of hearing protection and the search for better noise control measures.
     The next step is to determine the precise source of each sound; those of greatest intensity are typically sought first.  Identifying “machine x” as a noise source is only a preliminary result; the root cause of the noise must be identified.  That is, the specific component or characteristic of “machine x” that generates the offending sound must be identified.
     When noise control is considered during the design phase of a product, machine, or facility (i.e. direct measurements cannot be made) – the ideal time – it may be necessary to estimate each contribution to the proposed soundscapeLikely root causes are determined by applying knowledge of the behavior of equipment and structures with regards to noise.  Proactive, preventive measures are lower cost and, typically, more effective than reactive mitigation of noise.
     For each source identified, potential solutions are devised and evaluated.  To prioritize implementation of noise control measures, several aspects of each must be considered, including:
  • intensity of exposure
  • duration of exposure
  • number of people exposed
  • costs and benefits of implementation
  • service life of equipment, process, etc.
  • additional risk factors of exposed individuals or environment (e.g. medical conditions, chemical exposure, etc.)
  • other issues created or resolved by implementation
     Once priorities have been set, the plan begins to take a recognizable form.  Timelines and resource allocations are generated to guide the progression of implementations.  The project manager monitors compliance to schedule and budget, making adjustments when necessary to ensure successful implementation of all required noise control measures.
     The final component of the plan is verification and validation.  The soundscape must be reassessed after implementation is complete to verify that objectives have been met.  For a new installation, this also validates assumptions made in the design phase regarding sources of noise and appropriate countermeasures.  If objectives have not been met, the process must be repeated to identify and correct errors in assessment or implementation.
     Reassessment of the soundscape should include subjective evaluations in addition to SPL measurements.  While lower SPLs reduce the overall risk of hearing loss, the resulting sound composition may remain troublesome.  Though difficult to predict, reducing A-weighted SPLs may not reduce the loudness of sound; that is, there may be no subjective improvement in the sound exposure.  The resultant could also be deemed more annoying than the untreated soundscape.  This can occur, for example, when high frequencies are unmasked, making them more noticeable, by the differential frequency attenuation provided by a noise control measure.
     The preceding was merely an overview of the planning process; the following sections provide additional information used to plan an effective noise control program.  In particular, prioritization is greatly influenced by the options outlined below.

Hierarchy of Controls
     The hierarchy of controls, shown in Exhibit 2, is a very important concept, visited repeatedly in “The Third Degree.”  Here, its direct application to noise control will be explored.
     Atop the hierarchy, elimination is the most effective method of noise control.  Opportunities for elimination of noise sources, however, are not common; those that are available are often of minimal consequence.  Examples that can be found include:
  • A circulation fan with metal cage and blades damaged to the point that its contribution to noise is far greater than that to air flow.
  • A refrigerator in a break room or office.
  • Ancillary equipment that remains powered on when not in use, whether for convenience (e.g. startup time savings) or through complacency.
     Next in the hierarchy is substitutionSubstitution opportunities are more common in some environments, but may not be obvious.  “Out-of-the-box” thinking is helpful here and may yield additional non-noise-related benefits.  Example substitutions include:
  • Replace brushed motors with brushless motors.
  • Replace pneumatics with hydraulics or electric drives.
  • Replace a hammer with a press (e.g. to install a bearing in a housing).
Equipment and components thereof, materials, processes, and tools all provide opportunities for substitution, though each must be considered carefully before implementation.
     The remainder of the hierarchy is more familiar to many, as most effort is exerted in these areas in a reactive mode.  Technological barriers can prevent elimination or substitution in some cases.  Beyond that, the choice of a noise control measure is often dictated by other limitations, such as the availability of financial and technical resources.  This is simply a caveat that the “best” solutions are not always prioritized for implementation, though it should remain the goal.
     Physical changes to tasks performed, equipment used, or the operating environment are called engineering controls.  The physical nature of this type of control embeds them in the process, making them less susceptible to behavioral influences.  That is, defeating an engineering control is more often a “crime” of commission (e.g. sabotage) than of omission (e.g. failing to follow procedure).  Many engineering controls require periodic verification and maintenance, however, to ensure consistent, reliable operation.  Examples include:
  • Balancing rotating components of machinery.
  • Installing mufflers on exhaust ports of pneumatic tools.
  • Installing barriers and absorption material between machine and operator.
Engineering controls are “passive” interventions from the standpoint of listeners; no action is necessary to engage them.  In fact, those that benefit from these controls are often unaware of their existence or purpose.
     Administrative controls, in contrast, are overt influences on individuals’ behavior aimed at reducing exposure to noise.  Often, the only physical artifact of an administrative control is a document describing the desired behavior, process, or policy.  Administrative control examples include:
  • A job rotation schedule, alternating assignments in “noisy” and “quiet” environments.
  • Requiring remote operation of equipment under normal conditions (i.e. production as opposed to maintenance or emergency situation).
  • Scheduling tasks to be performed during “low noise” periods.
Because administrative controls require behavior modification (i.e. active engagement), training and supervision are required to ensure effectiveness.
     When none of the above measures are sufficient to remove the risk of hearing loss, personal protective equipment (PPE) must be used.  To state it explicitly, PPE is always a last resort or interim countermeasure used while better noise control measures are developed.  Relevant PPE includes several types of earplugs and earmuffs.
     There is some “blurring of lines” within the hierarchy of controls.  Some examples that demonstrate this include:
  • Substitution of machine components (e.g. belts for gears) is a physical modification and, therefore, an engineering control.  It is also an elimination, followed by the introduction of an alternative noise source.
  • Proper operation or maintenance of an engineering control may require instructions, an administrative control, to ensure reliability.
  • Provision of PPE requires supervision, an administrative control, to ensure proper and consistent use.  Selecting a hearing protection device (HPD) requires aligning the physical change to the working conditions, suggesting that PPE is also an engineering control if properly evaluated.
This “blurriness” is mentioned only to prevent alarm or concern when a noise control measure does not fit neatly into one category.  Each control is typically classified by its “strongest” component.  The hierarchy simply provides an additional point of comparison when evaluating noise control options.
     Many more examples of noise control measures, spanning the hierarchy, are provided in upcoming installments of the series.  The structure of these installments is described in the final section.

Principles of Noise Control
     The fundamental categories of noise control techniques, called the “four principles,” are:
  • sound insulation
  • sound absorption
  • vibration damping
  • vibration isolation
Each of these requires a physical change to the environment – they describe engineering controls.  If these were the only mechanisms available, the definition of “noise control” would be much narrower than that offered in the introduction.  Preventing the generation of sound is always preferred; when that is not possible, the four principles comprise the first line of defense, whether used individually or in conjunction.  The exclusionary moniker notwithstanding, developing a successful noise control program requires understanding these basic methods.
     Sound insulation refers to barriers that reflect air-borne sound, reducing its transmission.  These barriers can be existing structures, modified to improve insulating characteristics, or free-standing partitions constructed exclusively for noise control purposes.
     Sound absorption is achieved by placing porous materials in the path of air-borne sound.  Sound energy is converted to thermal energy within the absorbing material, preventing transmission and reflection.
     Vibration damping converts structure-borne vibratory energy to thermal energy (“absorbs vibrations”), reducing the amount available to be emitted as sound.  This method is typically associated with the attachment of absorbent materials to thin panels to reduce resonance.
     Vibration isolation prevents transmission of structure-borne vibratory energy by physically separating the source from receivers.  A gap or joint composed of elastic material can eliminate the transmission path, preventing reradiation of energy as sound.
     While these basic methods of noise control can be highly effective, none are perfect.  The methods are employed to sustain sound levels below that which causes hearing loss, annoyance, communication interference, or other interruption to normal function or productivity.  Examples of the four principles in practice will be presented in upcoming installments of the series.

     A soundscape can be defined by the three types of components that comprise it:  the sources of sound, the paths on which sound travels, and the receivers that are exposed to the sound.  This definition is known as the source-path-receiver (SPR) model.  The model name also indicates the priority of noise control development – first, treat the source, then the path, and finally, the receiver.  This is simply another way of stating that prevention of sound generation is preferred to other measures.
     The ETI formulation stands for emission, transmission, and immission.  These labels describe the “activity” of sound energy at each component of the SPR model; a source emits a sound that is transmitted along a path until it immits, or impinges upon, a receiver or listener.  At each stage, the goal is to prevent, to the extent possible, the sound energy activity.
     Another alternative formulation is PAP, for prevention-abatement-protection.  These labels represent the objectives of noise control measures at each component of the SPR model.
     The SPR model is widely accepted and is routinely referenced in the literature; it is “the” formulation for noise control.  Alternative formulations offer readers additional memory triggers to reinforce understanding of this fundamental model of noise control.  Use of any formulation alone, or mixing of terms, will prompt appropriate developments for each of the three components.
     The links among the alternative formulations of the noise control model is summarized in the following three statements of objectives, in order of priority:
  • Prevent emission of sound by a source.
  • Abate transmission of sound along its path.
  • Protect receivers from sound immission.

     The components of the SPR model are explored in greater detail in forthcoming installments of the series.  Each of the next three is dedicated to discussing noise control measures for one component of the model.  Examples provided demonstrate the application of priorities established by the SPR model and the hierarchy of controls.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

[Link] Industrial Noise Control Manual (Revised Edition).  National Institute for Occupational Safety and Health (NIOSH); December 1978.
[Link] Compendium of Materials for Noise Control.  National Institute for Occupational Safety and Health (NIOSH); 1980.
[Link] “Why can a decrease in dB(A) produce an increase in loudness?”  Rhona Hellman and Eberhard Zwicker.  The Journal of the Acoustical Society of America; November 1987.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications; 1996.
[Link] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[Link] “Criteria for a Recommended Standard - Occupational Noise Exposure, Revised Criteria 1998.”  Publication No. 98-126, NIOSH, June 1998.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] “Noise and Vibration.”  Evan Davies in Plant Engineer’s Reference Book, 2ed.  Dennis A. Snow, ed.  Reed Educational and Professional Publishing Ltd.; 2002.
[Link] “Noise Control Design Guide.” Owens Corning; 2004.
[Link] “Administrative controls for reducing worker noise exposures.”  E.R. Bauer and D.R. Babich.  Transactions of the Society for Mining, Metallurgy, and Exploration; December 2005.
[Link] “Noise as a Technological and Policy Challenge.”  William W. Lang and George C. Maling Jr.  The Bridge; Fall 2007.
[Link] “Perception-Based Engineering:  Integrating Human Response into Product and System Design.”  Patricia Davis.  The Bridge; Fall 2007.
[Link] “Engineering Controls for Reducing Workplace Noise.”  Robert D. Bruce.  The Bridge; Fall 2007.
[Link] Engineering Noise Control – Theory and Practice, 4ed.  David A. Bies and Colin H. Hansen.  Taylor & Francis; 2009.
[Link] “Noise Control Engineering and Education.”  Adnan Akay.  The Bridge; Summer 2021.
[Link] “Resources for Noise Control Engineering.”  George C. Maling Jr.  The Bridge; Summer 2021.
[Link] “Controlling Noise at Work.”  (UK) Health and Safety Executive (L108- 3ed); 2021.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Hierarchy of Controls.”  NIOSH; January 17, 2023.
[Link] “Noise control.”  Wikipedia.
[Link] “Noise - Measurement of Workplace Noise.”  Canadian Centre for Occupational Health and Safety (CCOHS); October 30, 2020.
[Link] “Technical Guide for:  Noise Control – Engineering Controls, Work Practices, & Administrative Controls.”  Georgia Tech; May 2023.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 10:  Communication Systems]]>Wed, 06 Mar 2024 08:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-10-communication-systems     When it comes to communication and communication systems, the definition of “effective” or “satisfactory” can vary significantly.  It depends on the level of compatibility of all components of the system and the potential consequences of a breakdown.  The need for higher performance when conducting safety-critical operations is implicit and, for many, held in the subconscious.  To develop a successful communication system, this requirement should be brought into conscious dialogue, made explicit.
     To support successful communication system design, this installment of the “Occupational Soundscapes” series provides a rapid-fire presentation of system components and recommendations.  Methods for determining the performance required, which affects how a system is designed and operated, are also discussed.
Signal Detection Theory
     For minimally-challenging soundscapes or simple communications, the concepts presented in Part 9 may provide all the information necessary to choose appropriate signals and design an effective system.  As complexity of communications increases or the consequences of miscommunication become more severe, an additional level of analysis may be needed.
     A framework for additional analysis is provided by Signal Detection Theory (SDT).  SDT expands the concept of signal-to-noise ratio (S/N) by assessing the impacts of variance and the application of a decision criterion on the rate of correct interpretation of signals.  A decision criterion establishes the level of certainty, or confidence, that a signal has been received needed to report its detection.
     The choice of criterion influences the perceived effectiveness of a communication system, as shown in Exhibit 1.  The curves correspond to three basic qualitative criteria that can be summarized as follows:
     (a) “Don’t miss.”        (b) “Do your best.”        (c) “Be sure.”
Progressing through these criteria, from left to right in the chart, reduces the proportion of signals reported at a given level because the “standard” for detection is raised.
     To choose the most-appropriate criterion, the consequences of failures and the probabilities of occurrence must be assessed and compared.  Two types of failure are possible:
  1. A signal is not reported, though present – a “miss” (Type II error).
  2. A signal is reported, though none is present – a “false alarm” (Type I error).
[Type I and Type II errors are discussed in “The War on Error – Vol. V:  Get Some R&R – Attributes” (26Aug2020); the signal in that context is a product defect.]
The “don’t miss” criterion results in the highest rate of “hits” (correct detection of signals) and the highest rate of false alarms.  The “be sure” criterion reduces false alarms to the minimum rate, but maximizes the miss rate.  The “do your best” criterion results in intermediate rates of all metrics – hits, misses, false alarms, and correct rejections.
     Data for correct and erroneous signal detection rates can be summarized in a “stimulus-response matrix,” such as that in Exhibit 2.  The matrix can act as a record of empirical data, represented in each cell by a letter and used to calculate system performance metrics.  It can also be used to present predicted or targeted hit rates, etc. in the design phase of a system.  These are referenced in the format P(x|y), read “the probability of ‘x’ response given ‘y’ condition.”  The probability of a hit (“yes” response in presence of signal+noise) is notated P(Y|S+N) and so on.
     When performance data are available from a system in use, the proportion of each stimulus-response pair can be calculated as follows:
     P(Y|N) = A/(A+C);                 P(Y|S+N) = B/(B+D);
     P(N|N) = C/(A+C);                 P(N|S+N) = D/(B+D).
Also, P(Y|N) + P(N|N) = 1.0 and P(Y|S+N) + P(N|S+N) = 1.0 irrespective of the decision criterion chosen.

     Thus far, consequences of failure – miss or false alarm – have been referenced generically, as they can take many forms.  Quality spill, machine breakdown, property damage, and personal injury, among others with a wide range of severities, are possible consequences of signal detection, or communication, failure.
     To facilitate design decisions, foreseeable consequences of each stimulus-response pair should be assigned monetary values (cost or benefit).  Placing this information in the corresponding cells of the stimulus-response matrix creates a payoff table that can be used to calculate the financial impact of a communication system’s performance.  This information could also be displayed in a tree format, such as that shown in Exhibit 3, where costs (or benefits) are represented by the notation C(x|y), read “the consequence of ‘x’ response’ given ‘y’ condition.”  Costs are represented by negative values and benefits by positive values.  [See “Making Decisions – Vol. IX:  Decision Trees” (23Feb2022) for a thorough presentation of payoff tables and decision trees.]
     While S/N considers average or instantaneous levels of signal and noise, SDT considers the variability of signal and noise levels.  Variability and its effects on signal detection performance is visualized by plotting the probability distributions of signal and noise levels, as shown in Exhibit 4.  The distributions are assumed normal and equal, with that of the noise centered at μ1 and signal+noise at μ2.
     The decision criterion is represented by a vertical line, treated as a “slider.”  Sliding the line to the left approaches the “don’t miss” criterion and to the right approaches the “be sure” criterion.  A central position is akin to the “do your best” criterion (see Exhibit 1).  The probability of each stimulus-response pair is defined by the area under one of the curves relative to the criterion line:
     P(Y|N) = area under the noise curve to the right of the criterion line.
     P(Y|S+N) = area under the signal+noise curve to the right of the criterion line.
     P(N|N) = area under the noise curve to the left of the criterion line.
     P(N|S+N) = area under the signal+noise curve to the left of the criterion line.
These probabilities are represented by the shaded areas in Exhibit 5; the colors used correspond with those used in Exhibit 2, visually linking the two presentation formats.
     The representation of the discriminability index, d’ (sometimes called the sensitivity index) in Exhibit 4 could be misleading.  The distance between the means of the noise and signal+noise distributions is equivalent to the S/N, while d’ accounts for the variance of each:
     d’ = (μS+N - μN)/σ ,
where, for clarity, μS+N and μN have replaced μ2 and μ1 as the means of the signal+noise and noise distributions, respectively, and σ is the standard deviation of the distributions.
     When d’ = 0, the probability of a hit and that of a false alarm are equal:  P(Y|S+N) = P(Y|N).  As discriminability of signals improves (d’ increases), the hit rate increases relative to the false alarm rate.  This performance “shift” can be seen in the Receiver Operating Characteristic (ROC) curves in Exhibit 6.
     While the ROC curves of Exhibit 6 aid in comprehension, interpolation of intermediate values can be difficult.  An alternative method is provided by d’ tables, such as that compiled by P.B. Elliott (see references).  Modern technology provides a faster, more-precise determination of d’ with attractive visuals – and it’s freely available.  Sample output of an online tool can be seen in Exhibit 7; the upper panel presents an equal-variance example.  A crosshair identifies the point on the ROC curve corresponding to the specified criterion level.
     The tool can also be used to determine d’ when the variance of the distributions differ, as shown in the lower panel of Exhibit 7.  To do this manually, the denominator of the equal-distribution d’ calculation (σ) is replaced by the square root of the average variance (√[(σS+N^2 + σN^2)/2]).  The difference in variance is reflected in the shape of the associated ROC curve.
     System performance requirements can be defined using any of the metrics discussed – discriminability index (d’), articulation index (AI), signal-to-noise ratio (S/N), Speech Interference Level (SIL), etc.  Whenever feasible, use of multiple indices should be considered.  Favorable results on multiple assessments can increase confidence in system performance in a range of use cases that may be encountered or anticipated.

System Components
     When discussing systems, focusing on tangible items is a common trap.  However, a communication system is much more than a collection of sound system hardware; in fact, some communication systems utilize no such hardware whatsoever.  The following list is offered to expand one’s thinking about the constituent content of communication systems, though it should not be treated as comprehensive or limiting.
  • Microphones
  • Loudspeakers
  • Headsets
  • Amplifiers
  • Filters
  • Radios
  • Speakers (“talkers”)
  • Listeners
  • Vocabulary (verbal and nonverbal)
  • Noise sources
  • Signal characteristics
  • Context of message
  • Decision criterion
  • Facility characteristics
  • Room characteristics
  • Hand & body motions
     Several components listed could be expanded into multiple entries reflecting specific characteristics.  Relevant hardware specifications, for example, could be listed separately.  Vocabulary could be split into entries defining the language used or specific words and phrases used in speech communication.  Nonverbal communication also uses a vocabulary of specified signals, each with well-defined meanings.  Signal characteristics could be replaced by frequency, intensity, duration, and so on.

     What follows is a series of recommendations for communication system design.  Little explanation is offered here; the background information needed can be found elsewhere in this series and linked references.  No set of recommendations can address all possible scenarios; the objective here is to provide a reasonable starting point for development of systems to be customized for their intended applications.
  • Employ auditory signals when the message is simple, short, and “disposable” or a warning; when the listener is visually overloaded or must remain mobile.
  • Use tones instead of speech when sufficient information can be conveyed by such a signal.
  • For warning signals, use frequencies of 150 – 1000 Hz.
  • For other (e.g. “routine”) signals, use frequencies of 1000 – 4000 Hz with prominent harmonics; avoid high frequencies.
  • Include prominent frequency components <1500 Hz when hearing loss or HPD use is a factor.
  • When localization ability is critical, include prominent frequencies >3000 Hz for intensity-difference detection and <1500 Hz for phase-difference detection.
  • For listeners at long distances (>1000 ft), use frequencies <1000 Hz at high intensities.
  • Use frequencies <500 Hz when line of sight between speaker and listener is obstructed.
  • Frequency-discriminated signals should use a maximum of five frequencies; four or fewer is preferred.
  • Match the type of alarm to signal requirements.  See Exhibit 8 for a comparison of alarm types.
  • Consider use of communication aids or modifications to the system in ambient noise >50 dB.
  • Set signal level halfway between masked threshold and 110 dB.
  • Target S/Ns >15 dB for reliable detection and <30 dB to minimize annoyance and startle.
  • At levels above 110 dB, substitute visual for auditory signals, or add redundant visual signals.  When designing visual signals, be sure to account for visual-field narrowing and changes in color perception that high-intensity noise can induce.
  • Intensity-discriminated signals should use a maximum of four levels.

  • Use signals of duration >300 ms; if shorter duration is necessary, increased intensity is required to maintain detectability.  For equal detectability, the product of duration and intensity is constant.
  • Limit signal duration to a few seconds, unless explicit acknowledgement is required, for which a manual reset is required.
  • Duration-discriminated signals should use only two durations.

  • Modulated signals are preferred to steady-state signals; temporal variation of 1 – 3 times/s or 1 – 8 beeps/s is recommended.  Frequency shifting can also be used.
  • Complex tones are preferred to pure tones.
  • Use multichannel presentation to increase detectability/intelligibility (e.g. tones, speech, visual, tactile).
  • Reduce S/N in lower-intensity noise to minimize startle and annoyance, particularly when performing high-concentration tasks.
  • Limit duration of high-concentration and critical vigilance tasks to <30 min with rest periods or alternate assignments between.
  • Limit the complexity of tasks or number of input channels to monitor.
  • Match signal presentation to its urgency and context (intensity, repeat rate, tone vs. speech, etc.)
  • Signals differentiated by combinations of frequency and intensity should be limited to eight combinations; fewer is better.

  • In-person (“face-to-face”) communication is preferred to reproduced speech.
  • Listener should face speaker when practicable.
  • Begin messages with the context to increase clarity.
  • Limit vocabulary as much as possible.
  • Use common words and phrases.
  • Use multisyllabic words.
  • Provide feedback to speaker; e.g. repeat the message received as confirmation.
  • Use reply tones, in lieu of speech, in high-intensity noise.
  • Limit speech rate to 30 phonemes/s; 15 – 20 phonemes/s is preferable (phonemes are basic speech sounds, or “building blocks” of speech).
  • Use digits to “spell out” numbers.
  • Use a phonetic alphabet (“word spelling”) to increase intelligibility.  See Exhibit 9 for the standard phonetic alphabet used in military, aviation, and other sensitive applications.
  • A noise-cancelling microphone at the lips is preferred to a contact (“throat”) microphone when both speaker and listener are in noise.
  • Headsets are preferred to loudspeakers when high ambient noise levels require HPD use, different messages must be delivered to different listeners, or reverberation limits intelligibility of loudspeakers.
  • Use distributed loudspeakers of lower power to limit reverberation.
  • Operate earphones out of phase, or delay transmission to one earphone by ~500 μs.
  • When the speaker is in quiet and the listener in noise, peak-clipping provides higher intelligibility.
  • Digital systems require a sampling rate greater than twice the highest frequency in the signal to maintain intelligibility.
  • Use automatic gain control (AGC) when signals’ dynamic range falls below 20 – 30 dB.  Dynamic range describes the difference between the strongest clear signal and weakest discernible signal.  Exhibit 10 shows how masking effects the dynamic range of a signal.
  • Use sound insulation and sound-absorbing materials to reduce reverberation and minimize the interference of noise on communication.

     The recommendations provided above are generalizations.  Every environment is unique and may require deviation from a “standard” setup or a compromise solution to address conflicting recommendations (e.g. minimum S/N vs. maximum SPL).  If this list allows system design to begin or inspires relevant questions that lead to improved system performance, it has served its purpose.
     Recommendations for equipment, in particular, have been purposefully limited.  The range of options available is too broad to generalize coherently.  Review of equipment specifications and predictions of the performance of various combinations of components must take place during system development for a specific application environment to have merit.
     Upcoming installments of the “Occupational Soundscapes” series will provide additional information that can be used to improve communication system performance.  Noise-control techniques and use of hearing protection are relevant to both major themes of this series – hearing conservation and communication.  To these topics, the series now turns.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] The Effects of Noise on Man.  Karl D. Kryter.  Academic Press; 1970.
[Link] Human Engineering Guide to Equipment Design (Revised Edition).  Harold P. Van Cott and Robert G. Kinkade (Eds).  American Institutes for Research; 1972.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc.; 2004.
[Link] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[Link] Engineering Noise Control – Theory and Practice, 4ed.  David A. Bies and Colin H. Hansen.  Taylor & Francis; 2009.
[Link] “Protection and Enhancement of Hearing in Noise.”  John G. Casali and Samir N. Y. Gerges.  Reviews of Human Factors and Ergonomics; April 2006.
[Link] “A d' Primer.”  Eshed Margalit.
[Link] “Signal-to-noise ratio.”  Wikipedia.
[Link] “Extra-Auditory Effects of Noise as a Health Hazard.”  Joseph R. Anticaglia and Alexander Cohen.  American Industrial Hygiene Association Journal; May-June 1970.
[Link] “Tables of d'.”  P.B. Elliott.  The University of Michigan Research Institute; October 1959.
[Link]  “Signal Detection Theory.”  David Heeger.  New York University; 1997.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[World Hearing Day]]>Wed, 21 Feb 2024 08:00:00 GMThttp://jaywinksolutions.com/thethirddegree/world-hearing-day     Every year, on March 3, the World Health Organization (WHO) partners with healthcare and community organizations to observe World Hearing Day.  Each year, events are held around the world, with a common theme, to promote ear and hearing health and broaden awareness of related issues.
     The title of the 2024 program is “Changing Mindsets” and the unifying theme for this year’s events is “Let’s make ear and hearing care a reality for all!”  As much a rallying cry as a theme, World Hearing Day organizers strive to eliminate the stigma often associated with hearing issues and to expand global access to information, monitoring, and treatment.
     World Hearing Day 2024 partners and advocates are encouraged to develop programs and host events that further the following objectives:
  • Counter the common misperceptions and stigmatizing mindsets related to ear and hearing problems in communities and among health care providers.”
  • Provide accurate and evidence-based information to change public perceptions of hearing loss.”
  • Call on countries and civil society to address misperceptions and stigmatizing mindsets related to hearing loss, as a crucial step towards ensuring equitable access to ear and hearing care.”

     To grasp the enormity of the issues of hearing loss and ear health and their global impact, consider the following (from WHO):
  • Globally, unaddressed hearing loss costs ~$980B (USD) annually.  Of that, more than $310B is health-related cost, and over $180B is attributed to productivity losses.
  • In 2019, 1.5 billion people (~1 in 5) suffered from hearing loss; nearly 430 million of these cases were moderate or worse.  The total number is predicted to rise to 2.5 billion (~1 in 4) by 2050.
  • In the Americas Region alone, 217 million people have existing hearing loss, and another 196 million people in the European Region.
  • Worldwide, more than 80% of hearing and ear care needs are unmet.
  • Recreational sound places 50% of 12 – 35-year-olds at risk for hearing loss.
  • Approximately 16% of hearing loss in adults is attributed to occupational noise exposure.
     World Hearing Day events and promotions may be generalized to present the broadest array of information to the widest audience possible.  Many default to addressing noise-induced hearing loss (NIHL) in adults or presbycusis, simply because these are the most-salient issues.  However, there are several other issues that span a person’s “life course.”  Initiatives related to those issues, whether or not they become the focus of a special event, are worthy of note; these include:
  • Immunization programs to prevent infections that can effect hearing and ear health.
  • Education and strategy development to manage exposures to ototoxic chemicals and medications.
  • Hearing aid fitment education and training.
  • Establishing “safe listening” practices for personal electronic devices and recreational activities.
  • Design guidance for entertainment venues to enhance “safe listening.”
  • Promoting lifelong hearing monitoring and protection, from infancy onward.
  • Defining the People-Centered Care public health strategy, summarized by the H.E.A.R.I.N.G. acronym:
     Events planned in the USA in observance of World Hearing Day 2024 include:

Related Websites
World Hearing Day – the official website
World Health Organization (WHO)
CDC/NIOSH – US Affiliate
World Hearing Day – Wikipedia (includes information on past observances)
Hearing Cooperative Research Center
Dangerous Decibels
Hearing Industries Association

     The upcoming World Hearing Day serves to reinforce the notion that all causes of hearing loss and all stages of life require attention.  Though “The Third Degree” focuses on aspects of commercial enterprises, it is also a welcome opportunity to remind readers that our interests, our lives, and our impacts are not limited to professional endeavors.  Taking a holistic approach – to any health topic – can improve the quality, and perhaps the quantity, of life for ourselves and others.

     For additional guidance or assistance with Safety and Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

[Link] “Integrated People-Centred Ear and Hearing Care:  Policy Brief.”  World Health Organization; 2021.
[Link] “World Report on Hearing.”  World Health Organization; 2021.
[Link] “Safe listening devices and systems: a WHO-ITU standard.”  World Health Organization and International Telecommunication Union; 2019.
[Link] “WHO global standard for safe listening venues and events.”  World Health Organization; 2022.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 9:  Concepts in Communication]]>Wed, 07 Feb 2024 08:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-9-concepts-in-communication     One of the most important aspects of soundscape management is the maintenance of communication capabilities.  Achieving stable communications is particularly challenging, as communication both contributes to and competes with the soundscape in which it takes place.  Types of communication necessary may include verbal and nonverbal, two-way or broadcast, face-to-face or remote, emergency and routine.
      Effective communication requires that a message’s content, delivery mechanism, sound characteristics, and receiver are compatible.  To design an effective communication system, due consideration must be given to the sender (e.g. speaker), receiver (listener), and everything in between.
     This installment of the “Occupational Soundscapes” series explores characteristics of and interactions among ambient sound, messages or signals, and auditory capabilities to provide the conceptual background needed to establish communication system requirements.  There is an emphasis on speech communication, given its prevalence and challenges in workplaces.
Auditory Fitness for Duty
     Auditory Fitness for Duty (AFFD) standards are often associated with military service and related occupations.  It is an assessment of one’s auditory capabilities with respect to safe and efficient performance of one’s duties.  AFFD is being supplanted in military applications, but continues to serve as a cautionary tale.
     Even without details of the test and scoring procedures, serious flaws can be seen in the AFFD recommendation chart, shown in in Exhibit 1.  Examples include:
  • With a score of 11, the recommendation is based exclusively on length of service.  An individual could be discharged despite identical performance to another retained without restriction.
  • With a score as low as 4, an 18-year veteran is recommended for retention, while a 17-year veteran is recommended for reassignment.  With less than 15 years of service, one’s career is in jeopardy.
  • With a “perfect” score (13), only 8+-year veterans are recommended for retention.
     Experience tends to improve test scores.  Some improvement can be attributed to genuine increases in task performance via learning curves.  However, experience with a test procedure can also inflate scores in some cases.  In this scenario, true performance of the less-experienced test-taker is actually higher.  Unless length of service can be shown to improve task performance beyond that demonstrated by the test, it should not be a factor in the recommendation.  Overreliance on arbitrary criteria can lead to bizarre and dangerous results.
     The concept of functional hearing is more useful in determinations of fitness for duty.  It refers to auditory capability sufficient to maintain situational awareness and speech communication and perform other tasks that require audition.  An assessment of functional hearing requires testing or monitoring of task performance of an individual in the setting of concern.  Results obtained in a laboratory setting or using special controls may not be representative of performance in the task environment.  However, other data, such as audiometric and soundfield measurements are complementary and may have diagnostic value.
     Whether subject to an AFFD protocol or less-formal evaluation, an organization must ensure that “hearing-critical” tasks are assigned only to those with the auditory capabilities necessary to perform them successfully.  A hearing-critical task is one with the following three characteristics:
  • Successful performance of the task is a required component of the job.
  • No compensatory substitutes (e.g. experience, visual cues) can overcome auditory incapacity.
  • Failure to perform the task successfully creates a hazard for oneself or others.
Implicit in the terms functional hearing and fitness for duty is a higher standard for hearing-critical tasks than for more-mundane or routine tasks.
Signal-to-Noise Ratio
     Signal-to Noise Ratio (S/N or SNR) is a fundamental concept in communication system design.  The series, thus far, has focused on the noise, but effective communication requires attention also be paid to the signal and a key relationship between the two.
     Conceptually, S/N is the detectability of a signal in the presence of noise.  Mathematically, it is the ratio of signal power to noise power:  S/N = Wsig/Wnoise.  Following the convention of sound levels (see Part 3), S/N is typically expressed in decibels (dB) using pressure values:  S/N = 10 log (Psig/Pnoise) dB.  Fortunately, measurements are typically recorded in decibels, yielding the simple expression
          S/N = (SPLsig – SPLnoise) dB .
     Positive S/N values (ratios > 1.0) identify signals whose intensity exceeds that of the accompanying noise.  For example, an S/N of 10 dB indicates that the signal is 10 dB “louder” than the noise.  Conversely, negative S/N values (ratios < 1.0) identify signals of lower intensity than the noise.  An S/N of 0 dB (ratio = 1.0) indicates a signal and noise of equal intensity.
     One possible framework for the use of S/N is to treat the intensity of noise, with any controls (explored further in future installments) active, as an independent variable and signal intensity as a dependent variable.  Target S/N is the parameter used to determine the appropriate intensity of a given signal.  This simple relation yields a series of parallel lines, as shown in Exhibit 2.
     S/N is a simple but important concept, relevant to all forms of communication.  As such, it can be cited in reference to verbal (i.e. speech) and nonverbal communication, with or without the use of electronic equipment (e.g. telephone, radio, amplifier, loudspeaker, etc.)  When referencing speech communication, S/N may be called the speech-to-noise ratio.  The adjustment in terminology serves only to specify the type of signal under scrutiny; definitions and application do not change.

     Masking is a phenomenon that causes a signal or message to be more difficult to hear or decipher in the presence of other sounds.  Standard audiometric tests determine one’s absolute threshold – the lowest intensity at which a sound is audible in quiet.  The lowest intensity at which a sound is audible in the presence of other sounds is called the masked threshold.  The difference between the two – that is, the magnitude of the increase in hearing threshold – is the amount, or level, of masking caused by extraneous sound.
     “Extraneous sound” is often referred to as “noise” to simplify the presentation.  However, the common definition of noise – “unwanted sound” – may not be fully applicable.  In fact, several coincident sounds may be necessary, or “wanted,” such as warning signals or other feedback sounds.  In this context, a modified definition of “noise” is helpful:  “any sound other than the sound of current interest.”  This definition accounts for individual analysis of multiple sounds that cannot or should not be eliminated from the soundscape.  The “other” sound is also called the masking sound or masker.
     Masking of a signal can occur in several ways.  The most prevalent is direct masking, which occurs when the signal and masker have similar frequencies.  The area of the cochlea needed to process the signal (see Part 2) is preoccupied with the masker, possessing no capability for “attention shift.”  The signal cannot, therefore, be perceived as a distinct input.
     Whether pure tone, narrowband, or broadband, a masker’s influence extends beyond its constituent frequencies.  Frequencies lower than the masker are masked to some degree, but to a much lesser extent than frequencies higher than the masker.  This phenomenon is called the upward spread of masking and is the reason that high frequencies are more susceptible to masking than are low frequencies.
     Example masking curves, for a range of pure tone frequencies, are shown in Exhibit 3; the value on each curve is the level of the masking tone (frequency shown at top of each plot) above its threshold.  Several characteristics of the masking phenomenon can be seen in these plots of masking vs. frequency, including:
  • The upward spread of masking is seen in the rapid rise to maximum masking below the masking tone frequency and slow decay at higher frequencies.
  • Local minima at the masking tone frequency represent increased sensitivity due to the creation of beats (see Part 7).
  • Additional local minima occur at integral multiples of the masking tone frequency.  These are the harmonics for which sensitivity is greater than adjacent frequencies.
     The comparative curves shown in Exhibit 4 demonstrate that a band of noise is more effective as a masker than a pure tone.  Key takeaways include:
  • A band of noise raises thresholds near its center frequency much higher than does a pure tone of that frequency.
  • Effects of beating and harmonics are not significant when the masker is a band of noise.
  • At higher frequencies (> ~1000 Hz in this data set), the pure tone is a more-effective masker than narrowband noise.  The frequency at which this “crossover” occurs is dependent upon the degree of distortion in the listener’s ear.
     A high-frequency band of noise, at high intensity (> 80 dB), can mask pure tones at low frequency.  This phenomenon is known as remote masking.  It is believed to be a result of low-frequency distortion caused by overstressing the auditory system.  This effect can be reducing by filtering.
     Interaural masking occurs when one ear receives the signal while the other receives the noise.  A masking sound at a level at least 50 dB greater than the signal is needed for significant masking of this type to occur.
     Central masking occurs when sound incident on one ear raises the threshold of the opposite ear.  It is believed to be negligible and, accordingly, receives little attention.
     Adding noise is counterintuitive, but can provide a benefit in certain conditions.  If signal and noise are received in one ear, presenting the other with a 100-Hz-wide band of noise, unrelated to the signal or noise in the first ear, provides ~1 dB “release from masking.”  A release from masking is a lowering of masked threshold.
     Once signal intensity exceeds that of its masker by a few decibels, it seems as loud as it would in the absence of the masker.  Loudness of a signal increases more rapidly above its masked threshold in noise than it does in quiet.  These points are demonstrated by the converging curves in Exhibit 5.
     The preceding discussion of masking focused on various effects of frequency on the audibility of signals, but there is also a temporal component.  Forward and backward masking refer to an increase in the threshold of a signal caused by a sound occurring before or after it, respectively.
     Forward masking – when a signal follows a masker – is somewhat intuitive.  The cochlea must be “freed” from its prior stimulation in order to process the next.  Though brief, this refractory period should not be ignored.
     Backward masking – when a signal precedes a masker – is much more difficult to comprehend.  It involves complex interactions in the auditory system, the exploration of which is beyond the scope of this series.  For purposes of this discussion, it is accepted as a genuine phenomenon supported by research detailed in cited references.
     A graphical representation of forward and backward masking is provided in Exhibit 6.  The break in the graph represents a 5-ms-duration “probe tone” (signal).  Backward masking of the tone is presented, in “negative time,” to the left of the break and forward masking to the right.  The smaller threshold shift experienced with dichotic presentation (signal in one ear, masker in the other) further demonstrates the advantage of binaural listening.
     When the sequence of auditory inputs is important, sufficient delay must exist between signals to allow the listener to determine which occurred first.  With a 2 – 3 ms delay, two distinct signals can be recognized, but the sequence is indeterminable.  A 10 – 20 ms delay is required to correctly identify the sequence of two sounds received.
     While pure tones can be generated for use as warnings and other auditory signals, they are not the norm in naturally-occurring soundscapes or occupational settings.  Bands of noise are more-common competitors for listeners’ “auditory attention.”  The most complex, and often most important, signals are contained in speech communication.  Speech is subject to S/N and masking concerns, as are pure tones and other signals, but experiences additional challenges; these are explored in the following sections.
Audibility and Intelligibility
     For many sounds, such as pure tones or narrowband sounds used as warning signals, mere audibility is sufficient to serve its intended purpose.  If there is a relatively large number of signals to be monitored, rapid discrimination among them becomes more challenging.  When speech communication is needed, there is a much higher bar to be cleared; in addition to being audible and discriminable, speech must also be intelligible to serve its purpose.
     The expansion of telephony from commercial enterprises to personal use and its subsequent proliferation provided great impetus for the study of speech intelligibility.  Over the past century, several test procedures, media sets, and evaluation schemes have been developed to quantify performance of communication technologies.  Though the study of intelligibility originated in telephony, face-to-face communication is subject to similar challenges and can be assessed in similar fashion.  Use of an electronic or other intermediary device may improve or degrade intelligibility, but it does not alter the requirements for effective communication.
     Some methods of assessment and scoring are rather sophisticated and complex.  Reproduction of lengthy procedures is not warranted; readers are encouraged to consult cited references or other sources for additional detail.  In lieu of comprehensive instructions, some prominent indices are introduced to provide conceptual understanding of intelligibility testing and scoring.  Conceptual understanding is sufficient to recognize the influence of a soundscape on communication system design choices and vice versa.  For those that choose to perform calculations, dedicated software and formatted spreadsheets are available to assist in this effort from sources such as the Acoustical Society of America (ASA).
     Articulation Index (AI) is the benchmark to which other intelligibility indices are typically compared.  Calculation of AI is a laborious process, requiring a series of data plots and correction factorings.  This follows the choice of method to be used, based on the data available or precision desired.  Its complex calculation process and the limited value of additional precision in most occupational settings prompts a focus on alternative methods to estimate AI.
     AI ranges from 0 to 1.0, expressing the proportion of a speech signal that is audible or “available to” a listener.  The portion of a speech signal that is available to a listener is that which contributes to the listener’s understanding of the message.  The relationship of AI to the proportion of signals correctly understood is not a 1:1 correlation, however.  As seen in Exhibit 7, an AI of 0.5 can yield comprehension rates at or near 100%, provided the signal content (vocabulary) is sufficiently limited or additional cues are provided.  The high rate of sentence comprehension is afforded by contextual clues inherent in extended messages, even when unfamiliar to the listener (i.e. first presentation).  Performance for all media sets shown in Exhibit 7 exceeds 50% comprehension by significant margins at AI = 0.5.
     The “overperformance” of speech comprehension, relative to AI, is attributed to the amazing powers of the human brain.  With knowledge of the language in use, the brain can extrapolate small portions of the message that were not received clearly.  This is not faultless, of course, or comprehension scores would consistently be 100%.  In casual conversation, where the consequences of misunderstanding are minimal, these extrapolations can lead to rather humorous exchanges.  In consequential communications, however, messages should be crafted such that any extrapolations necessary have a high probability of correctness.
     To give AI values intuitive meaning, a qualitative guideline is often used.  A typical example is as follows:
  • AI < 0.3:  generally unsatisfactory except in very limited circumstances, such as small vocabulary, highly-skilled listener, etc.
  • 0.3 < AI < 0.5:  generally acceptable performance.
  • 0.5 < AI < 0.7:  good; satisfactory performance.
  • AI > 0.7:  very good to excellent performance.
Users may choose to modify the qualitative assessment guideline to reflect the realities of specific applications.  For example, use of an unlimited vocabulary by unskilled listeners may shift acceptability up the AI scale.
     Speech Interference Level (SIL) is less precise than an AI calculation; it is used to predict intelligibility of speech in face-to-face communications.  SIL is the maximum noise level in which a listener correctly understands 75% of phonetically balanced (PB) words or ~98% of sentences; this comprehension rate is equivalent to AI ≈ 0.5.  PB words are those included in a test set such that various speech sounds occur in the same proportion as “normal” speech.
     Mathematically, SIL is the arithmetic average of ambient SPLs in the octave bands 600 – 1200 Hz, 1200 – 2400 Hz, and 2400 – 4800 Hz.  SIL varies by the speaker’s vocal effort and distance from listener; several combinations of these variables are tabulated in Exhibit 8.
     Preferred Speech Interference Level (PSIL) is used to predict the likely level of difficulty using speech to communicate in various circumstances.  PSIL is the arithmetic average of ambient SPLs in the octave bands with center frequencies of 500, 1000, and 2000 Hz.  Exhibit 9 provides a graphical reference relating PSIL, distance between speaker and listener, and vocal effort to anticipated speech communication difficulty.  It also includes a convenient cross-reference to SIL, A- and C-weighted SPLs, and perceived noisiness values as alternative metrics.  Estimates of AI at each level of vocal effort are also tabulated, providing additional predictive insight.  The chart indicates where noise-reduction efforts may need to be focused or communication system upgrades implemented.
     Speech Intelligibility Index (SII) is the most sophisticated index commonly available.  It has been adopted in the ANSI S3.5-1997 (R2020) standard, outlining four calculation methods.  The details of SII calculations will not be reproduced here; readers are referred to the ANSI standard, available software, and other resources for that information. 
     Interpretation of SII and AI values are comparable; both range from 0 to 1.0, though SII is often cited as a percentage.  Both indices are “outperformed” by speech comprehension over much of this range.  Using comparable test sets, SII and AI results are approximately equal.  For example, the ~98% comprehension rate of sentences at AI = 0.5 is duplicated at SII = 0.5.  This can be seen in Exhibit 10, as well as the comprehension rates as a function of SII for other test sets.  As seen in Exhibit 7 for AI, Exhibit 10 shows that simpler vocabulary and additional clues provided by sentences improves comprehension at lower SII values.
     In lieu of intensive calculations, visual estimation procedures have been developed.  Killion and Mueller’s revised “count-the-dots” method incorporates research on the importance of frequencies outside the 500 – 4000 Hz range that is often the focus of speech communication studies.  It has also been adjusted to correlate with SII calculations (1/3 octave importance function) and is now titled “The SII-Based Method for Estimating the Articulation Index.”
     The procedure is as follows:
  • Perform audiometric tests using frequencies from ~200 Hz to 8000 Hz.
  • Plot the results on the audiogram form shown in Exhibit 11.
  • Count the number of dots below the threshold line; these are called the “audible dots.”
The number of audible dots approximates the AI of speech at 60 dB SPL.  For example, if 45 of the 100 dots on the audiogram lie below the threshold line, AI ≈ 0.45.  This method provides an indication of where communication difficulties may arise while being far simpler to execute than a precise AI or SII calculation.
     It should be clear by now that intensive calculations are often unwarranted overkill in occupational settings.  Variability is introduced by changes in personnel and daily operations; estimates may be the only data available in a reasonable timeframe.  AI and similar indices are typically used as indicators, where approximations and trends are more useful than precise values.  This in no way diminishes the importance of understanding the concept of intelligibility and how it influences communication system design; it is merely an acknowledgment that a more-practical approach is needed to accommodate resource limitations that exist in most workplaces.
Factors Related to Intelligibility
     The previous sections provided background information related to challenges involved in communicating in occupational soundscapes.  The presentation now turns to examples that connect these concepts to practical application in system design.
     The general “rule” for signal-to-noise ratio is higher is better; however, there are limitations.  In general, those with existing hearing loss (HL) require higher S/N to match the comprehension rates of those with normal hearing.  In “low-noise” situations, however, higher signal intensity may be unnecessary and can become annoying or otherwise detrimental.  For example, high-intensity sound induces distortion in the ear, decreasing intelligibility for all listeners. 
     SII and other indices were developed for normal hearing.  The influence of HL on intelligibility could vary greatly, depending on the nature and severity of hearing loss, the makeup and intensity of the soundscape, and characteristics of the speech signals.
     The vocabulary used in speech communication can have a profound impact on its effectiveness (see Exhibit 7 and Exhibit 10).  Variables that influence vocabulary effectiveness include the number of words in use (i.e. standardized or free-form), the number of syllables in each word, the uniqueness of words used (e.g. rhymes), and the context in which they are spoken.
     Similar words can be difficult to differentiate in random noise due to “consonant confusion.”  The “confusion tree” in Exhibit 12 shows the S/Ns at which various consonant sounds become indistinguishable.  Two adjacent lines indicate that, at S/Ns below their level of convergence, the corresponding consonant sounds are easily confused.  Filtering the speech signal alters the confusion tree; all components of a communication system must be considered in conjunction to achieve desired results.
     Dialects add an interesting variable to speech communications.  Imagine a meeting with one attendee from each of the following cities:  Boston, Houston, London, Dublin, Sydney, and Mumbai.  All are fluent in English, the native language of each.  Each speaks the language differently, however, stressing different speech sounds, pronouncing words differently, and defining words differently.  Add to this scenario high-intensity noise, poor reproduction of vocal inputs to an electronic communication device, and speakers of English as a second language (ESL) and the value of a limited, standardized vocabulary becomes self-evident.
     Communication at large gatherings can be difficult.  While “listening” to one voice, other voices in the vicinity create masking noise.  The effect on intelligibility is shown in Exhibit 13 for a voice of interest held constant at a level of 94 dB.  With one masking voice, “selective attention” facilitates relatively high comprehension rates – nearly 80% at S/N = 0 (vertical dashed line).  Additional voices degrade comprehension at significantly higher rates.  The data on masking voices provides empirical evidence of the productivity-crushing effects of sidebar conversations and unmoderated “debates” in meetings (see “Meetings:  Now Available in Productive Format!” [18Dec2019]).
     When a speaker must increase vocal effort to be heard above noise, intelligibility can suffer.  Increasing vocal effort to shouting levels (> ~80 dB) can result in 20% lower comprehension rates at constant S/N = 0.  At lower S/N, shouting degrades comprehension more rapidly despite starting at a lower baseline rate.  The decline in comprehension rates when low vocal effort (< ~50 dB) is used is essentially a mirror image.
     Acoustic properties of a room in which communication takes place can exacerbate other difficulties.  Reverberant properties can cause echoes or hamper the dissipation of sound energy required to “free” a listener’s auditory system to process a new signal.
     Face-to-face communication can enhance intelligibility relative to the same message recorded or transmitted electronically.  Vocal inflections are undistorted by reproduction and may aid comprehension of the message.  In addition, visual cues are readily available, such as facial expressions or “body language.”  The additional signals, in some cases, can convey more information than the message itself, particularly among highly-familiar or highly-skilled communicators.  The ability to see a speaker’s lips, even if the listener is not a skilled lip-reader, has been found to improve intelligibility significantly in negative-S/N conditions.
     Much of the research conducted on speech communication, hearing, and related topics has involved only men.  Differences between male and female speech and hearing are believed to be significant, but the details are not well-established.  This serves as yet another reminder that every environment is unique, requiring validation of systems within each.
     Vast amounts of research have been conducted on the influences of noise on communication, particularly speech communication.  Sharing details of this research yields diminishing returns as explorations become more peripheral or less practical to employ in an occupational setting.  The preceding presentation is akin to a high-speed flyover of the subject matter, highlighting only the most-relevant and practically-applicable information.  However, readers are encouraged to explore the literature on this interesting and valuable subject.  “The Third Degree,” meanwhile, will proceed to a presentation of recommendations for design of effective communication systems.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications; 1996.
[Link] The Effects of Noise on Man.  Karl D. Kryter.  Academic Press; 1970.
[Link] Human Engineering Guide to Equipment Design (Revised Edition).  Harold P. Van Cott and Robert G. Kinkade (Eds).  American Institutes for Research; 1972.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc.; 2004.
[Link] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[Link] Engineering Noise Control – Theory and Practice, 4ed.  David A. Bies and Colin H. Hansen.  Taylor & Francis; 2009.
[Link] An Introduction to Acoustics.  Robert H. Randall.  Addison-Wesley; 1951.
[Link] “Protection and Enhancement of Hearing in Noise.”  John G. Casali and Samir N. Y. Gerges.  Reviews of Human Factors and Ergonomics; April 2006.
[Link]  “On the Masking Pattern of a Simple Auditory Stimulus.”  James P. Egan and Harold W. Hake.  The Journal of the Acoustical Society of America; September 1950.
[Link] “Methods for the Calculation and Use of the Articulation Index.”  Karl D. Kryter.  The Journal of the Acoustical Society of America; November 1962 and Errata [Link].
[Link] “Pediatric Audiology:  A Review.”  Ryan B. Gregg, Lori S. Wiorek, and Joan C. Arvedson.  Pediatrics in Review, July 2004.
[Link] “Signal-to-noise ratio.”  Wikipedia.
[Link] “An Easy Method for Calculating the Articulation Index.”  H. Gustav Mueller and Mead C. Killion.  The Hearing Journal; September 1990.
[Link] “Twenty years later: A NEW Count-The-Dots method.”  Mead C. Killion and H. Gustav Mueller.  The Hearing Journal; January 2010.
[Link] “The Speech Intelligibility Index: What is it and what's it good for?”  Benjamin Hornsby.  The Hearing Journal; October 2004.
[Link] “SII Predictions of Aided Speech Recognition.”  Susan Scollie.  The Hearing Journal; September 2004.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 8:  Effects of Exposure]]>Wed, 24 Jan 2024 07:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-8-effects-of-exposure     Some effects of exposure to sound with certain characteristics have been mentioned in previous installments of this series.  Given the importance of understanding the potential consequences of failing to manage soundscapes effectively, compiling these in one place is advantageous.  The effects of exposure to challenging soundscapes provide the “why” that motivates efforts to manage them.
     In this installment, both auditory and extra-auditory effects are explored.  Auditory effects may be more intuitive, as direct impacts to hearing are highly relatable.  Extra-auditory effects, in contrast, often lack an obvious link between sound and the effects experienced by those exposed.  Recognizing this link is key to effective facility and workforce management.
     The term “auditory effect” is used herein to refer to physiological changes that directly impact an individual’s ability to perceive (“hear”), identify, differentiate, locate, or interpret various sounds.  These changes take place within the auditory system, anywhere from the outer ear to the brain.
     Extra-auditory effects are those that occur outside the auditory system; they can be physiological or psychological in nature.  The term “nonauditory effect” is also commonly used; the two are used interchangeably in this discussion.

Auditory Effects
     Several references to auditory effects have been made as the primary focus of this series.  However, this should not lead readers to conclude that it is only these that are important.  It is their relevance to previous discussions, prevalence in occupational settings, and relatability that encourage frequent mention.
     In this section, brief descriptions of the most common auditory effects are provided.  Those previously mentioned are included here to provide a single resource for this type of information.
Noise-Induced Hearing Loss (NIHL):  results from damage to the inner ear, specifically, “over-bending” the hair cells in the cochlea.  Trauma suffered by the tympanic membrane (i.e. “ruptured eardrum”) can also cause hearing loss.  NIHL refers to both temporary and permanent threshold shifts (TTS, PTS).  Refer to Part 2 for a presentation of the parts of the ear and Part 5 for a discussion of threshold shifts.  Experiencing a TTS is sometimes called “auditory fatigue,” but the term’s ambiguity limits its acceptance and value as a descriptor.  The greatest NIHL typically occurs in the first 10 – 15 years of exposure.  After this, its impact declines as other forms of hearing loss become more influential to overall auditory health.
Tinnitus:  ringing in the ear(s); the perception, usually, of a high-pitched sound that is not externally generated (i.e. not “received” by the “listener”).  Typically, tinnitus accompanies hearing loss and is often the warning sign that prompts individuals out of their complacency about sound exposures.
Hyperacusis:  extreme sensitivity to sound, often occurring in the aftermath of a traumatic “noise event.”  Hyperacusis can be difficult to diagnose and manage, as those that suffer from it often generate “normal” audiograms.  That is, the increased sensitivity to sound exposure is not detected by standard audiometric testing.
Recruitment:  narrowing of the audible range of SPLs that may accompany hearing loss.  A person with recruitment has a raised hearing threshold and increased sensitivity to higher-intensity sound.
Diplacusis:  asymmetrical hearing loss, typically caused by differential exposure.  For example, a machine operator whose task posture causes his/her left ear to “face” the noise source is susceptible to greater NIHL in the left ear than in the right.  As this condition worsens, localization of a sound source (see Part 6) becomes increasingly difficult.
Acoustic trauma:  sudden damage to the auditory system caused by an explosion or similarly-extreme release of energy in excess of 130 dBC.  Acoustic trauma is associated with transient sounds, whereas NIHL is associated with continuous and intermittent sounds (see Part 6 for descriptions of sound types).
Speech discrimination:  the ability to differentiate speech sounds is effected by total hearing loss and the nature of one’s soundscape.  Consonant sounds (e.g. “b” vs. “d;” see Part 5) are effected most and background noise is particularly problematic.
     There are two types of auditory system damage to consider in relation to auditory effects:  mechanical injury and metabolic injury.  Mechanical injury correlates with peak pressure levels (SPLs); the most severe conditions exist with transient sounds.  Metabolic injury correlates with the duration of exposures and corresponding recovery periods.  Both types must be considered to effectively manage a soundscape.
     The preceding presentation is merely an overview of the most-commonly cited auditory effects of sound exposure.  Sound level measurements, exposure indices (e.g. 8-hr TWA), and frequency weighting (see Part 6) are used to quantify the potential for NIHL.  Individual sensitivities will always differ, however; this variability must not be ignored.

Extra-Auditory (Nonauditory) Effects
     Previous references to extra-auditory effects have been made without clarifying this distinction; for example, annoyance was discussed in Part 7.  Annoyance is a subjective experience and a psychological response to sound that may occur at relatively low intensity levels.  This exemplifies an important characteristic of nonauditory effects of sound:  they are not correlated with auditory system damage and often occur at energy levels much lower than required to cause a threshold shift.
     Several physiological and psychological effects of sound exposure are introduced in this section.  Thorough analysis of these effects requires medical and/or psychological expertise far beyond the scope of this series.  Fortunately, for most practitioners, awareness and superficial knowledge of these effects suffice; detailed research is left to those with the interest and capacity to conduct it independently.
     Sound exposure has been identified as a significant stressor.  Readers are likely aware of health concerns related to excessive stress, such as hypertension and other cardiovascular conditions.  It has also been linked to issues in the digestive system, such as ulcers, and sleep disturbance, which can lead to a further decline in health.
     A natural response to sound exposure – namely, shouting to communicate – can also cause an indirect health effect.  Throat pain, lesions, and hoarseness can result from exerting the vocal energy required to be heard in a noisy environment.
     Behavioral changes have also been linked to sound exposure.  Absenteeism and disciplinary action have been correlated to levels of sound exposure in workplaces.  Depression and social isolation are experienced within the work environment and outside it, particularly when substantial permanent threshold shifts (PTS) have occurred.  Cognitive decline and dementia can also be accelerated.
     Productivity, quality, and safety performance also suffer in noisy environments.  Increases in falls and other mishaps, including traffic accidents, have been correlated with sound exposure.  The combined effect of distraction by sound and difficulty hearing warning signals or other auditory feedback contribute to the occurrence of various types of accidents and errors.  Vigilance tasks, where changes in conditions are to be closely monitored, exhibit significant declines in performance in this type of environment.  Learning, typically used to improve performance on various metrics, can also be impaired by the soundscape.
     Although the nonauditory effects have not been explored in great detail, the presentation should, nonetheless, make clear that a multitude of negative impacts can result from an uncontrolled or poorly-managed soundscape.  The connections between sound exposure and nonauditory effects are not as intuitive as those to auditory effects and could, therefore, easily be overlooked.  Mere awareness of the potential to cause or contribute to these ailments could be the key to successful management.

Interactions and Synergies
     Sound exposures and NIHL do not occur independently of other conditions.  Other characteristics of the surrounding environment and of the exposed individual can amplify, accelerate, or otherwise modify the effects of sound exposure.  Individual sensitivity to any single factor, or combination of factors, is highly variable and extremely difficult to quantify or predict.  For this reason, awareness, rather than deep knowledge, remains the goal of this presentation.
     Environmental factors to be considered in conjunction with the soundscape include:
·     Ototoxic substances (solvents, heavy metals, etc.) affect the cochlea.
·     Neurotoxic substances affect the central nervous system.
·     Vibration.
·     Thermal conditions (see Thermal Work Environments series).
·     Verbal and nonverbal communication requirements.
     Individual factors to be considered in conjunction with the soundscape and environmental factors include:
·     All types of hearing loss (NIHL, presbycusis, sociocusis, etc.).
·     Overall health and fitness (cardiovascular health, for example, seems to exhibit a circular influence – that is, a decline in CV health increases sensitivity to sound exposures and sound exposures increase CV risk).
·     Illness, infection, or chronic disease (e.g. diabetes).
·     Diet and exercise.
·     Smoking or other tobacco use.
·     Alcohol consumption.
·     Use of OTC or other medications, drugs.
Specific concerns about any of these factors should be referred to medical professionals.  Professional medical advice may be needed to provide appropriate protections for individuals working in challenging soundscapes.

      Thus far, the focus has been on implications of a “noisy” environment.  The effects of a “quiet” environment should also be considered.  Very low levels of continuous sound could exacerbate startle reactions (see Part 7), as changes in sound are more noticeable.
      A very quiet environment can be relaxing, causing drowsiness, increasing error rates, and reducing productivity.  To offset this, some introduce random noise or music to the environment.  Either of these can be problematic and, in general, are not recommended.  If either is used, careful evaluation of the task, environment, communication requirements, and effect on others must be conducted to ensure that problems are not created unnecessarily.  Expending the analysis effort on other aspects of job design will typically yield more favorable results.  For example, adaptive workstations, such as those that accommodate both standing and seated postures, assignment rotations, or task modification to reduce monotony may yield better results than the introduction of an artificial soundscape.

Summary and Conclusion
     There are many potential effects of sound exposure.  Some are physiological and quantifiable, while others are psychological and sometimes perplexing.  These effects are often interconnected, exhibiting complex relationships that inhibit thorough comprehension.  This in no way diminishes their applicability to workforce and facility management, as superficial knowledge is often sufficient to implement appropriate protections.
     Examples of the effects of sound exposure at various intensity levels are summarized in the concluding exhibits.  In Exhibit 1, effects on conversation and perception are presented, while Exhibit 2 presents additional physiological and psychological responses to sounds throughout the audible range.  The levels at which the onset of certain health effects typically occur are provided in Exhibit 3.  Finally, conditions affecting the relationship between signal presence and human perception of sound are explored in Exhibit 4.
     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications; 1996.
[Link] The Effects of Noise on Man.  Karl D. Kryter.  Academic Press; 1970.
[Link] “Protection and Enhancement of Hearing in Noise.”  John G. Casali and Samir N. Y. Gerges.  Reviews of Human Factors and Ergonomics; April 2006.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc.; 2004.
[Link] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[Link] “Risk Observatory Thematic Report - Noise in Figures.”  Elke Schneider, Pascal Paoli, and Emmanuelle Brun.  Luxembourg Office for Official Publications of the European Communities; December 2005.
[Link] “Noise exposure as related to productivity, disciplinary actions, absenteeism, and accidents among textile workers.”  Madbuli H. Noweir.  Journal of Safety Research; Winter 1984.
[Link] “Extra-Auditory Effects of Noise as a Health Hazard.”  Joseph R. Anticaglia and Alexander Cohen.  American Industrial Hygiene Association Journal; May-June 1970.
[Link] “Noise Exposures:  Effects on Hearing and Prevention of Noise Induced Hearing Loss.”  Sally L. Lusk.  American Association of Occupational Health Nurses Journal; August 1997.
[Link] “Hazardous Exposure to Intermittent and Steady-State Noise.”  K.D. Kryter, W. Dixon Ward, James D. Miller, and Donald H. Elderedge.  Journal of the Acoustical Society of America; March 1966.
[Link] “Occupational Noise-Induced Hearing Loss.”  Raul Mirza, Bruce Kirchner, Robert A. Dobie, and James Crawford.  Journal of Occupational and Environmental Medicine; September 2018.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 7:  Perceptions]]>Wed, 10 Jan 2024 07:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-7-perceptions     The previous installment (Part 6) of the series dealt with objective measures of sound exposure.  Objective measurements, however, do not fully describe one’s experience of the surrounding soundscape.  The subjectivity of human perceptions of sound plays a vital role in effective noise control and communication system design.
     This installment of the “Occupational Soundscapes” series explores aspects of the human experience of sound that SPLs and TWAs alone do not explain.  These include the concepts of “loudness” and “noisiness” – terms that reflect the subjective nature of sound.
     “Loudness” is the term used to describe the subjective perception of a sound’s intensity or pressure.  Sometimes called a “loudness index,” loudness is quantified in sones, a linear scale defined as follows:
  • One sone is the loudness of a 1000-Hz pure tone at a sound pressure level (SPL) of 40 dB (the “reference tone”).
  • A sound of any frequency and SPL perceived to be of equal loudness to the reference tone is 1 sone.
  • A sound “twice as loud” as the reference tone is 2 sones, a sound three time as loud is 3 sones, and so on.
  • A sound that is “half as loud” as the reference tone is 0.5 sone, a sound one-quarter as loud is 0.25 sone, and so on.
  • The threshold of hearing at 1000 Hz is 0 sones.
     The loudness of a sound has a corresponding “loudness level.”  The loudness level is analogous to the power, intensity, and pressure levels introduced in Part 3, though it is not derived in the same way.  Loudness level is also defined for a reference tone in units of phons.  A sound’s loudness level, in phons, is numerically equal to the SPL, in decibels (dB) of a 1000-Hz tone of equal loudness.  The reference tone (1000 Hz, 40 dB) has a loudness level of 40 phons by definition.  The loudness level is conceived as a perceived or “equivalent” SPL, as it ascribes an equivalent experience to a subject sound and the reference tone.
     Contours of equal loudness and loudness level are shown in Exhibit 1 for pure tones; Exhibit 2 presents curves for equal loudness levels of octave bands.  Alternative representations, using 1/3 octave band curves, are shown in Exhibit 3 and Exhibit 4.  These may facilitate estimation of perceived loudness in some applications, depending on the data available.  Equations have been developed to convert between perceived loudness/level and SPL.  Such calculations will not be presented here, however, as their value is dubious; the precision of a calculated value can be misleading.  For example, citing an SPL can obscure the fact that is was “converted” from subjective assessments, implying that knowledge of the sound is more “concrete” than it really is.
     To further illustrate the potential for confusion, consider the inverse square law example presented in Part 3.  There, it was shown that doubling the distance from a sound source reduced the sound’s intensity by a factor of 4 and its intensity level by 6 dB.  This equates to a reduction in sound pressure by a factor of 2, achieving an SPL reduction of 6 dB.  However, this is not the same as the change in loudness level.  A change of approximately 10 dB is needed to perceive a change in loudness by a factor of 2, as shown by the relationship of sones to phons (i.e. an increase of 10 phons doubles the sones) depicted in Exhibits 1 – 4.
     The curves represent the average responses of research subjects.  Human subjectivity, as always, renders the curves approximations or estimates of individuals’ actual experience.  Nonetheless, these estimates are useful references for communication system and noise control designers.
     The terms perceived noisiness and annoyance are often used interchangeably to describe the extent to which a sound is unwanted, unacceptable, or bothersome.  Analogous to the sone for loudness, noisiness has been assigned the unit of noy, where a 2-noy sound is twice as noisy as a 1-noy sound, a 3-noy sound is three times as noisy, and so on.  The perceived noise level (PNL) is “translated” into units of PNdB according to the following:  PNdB = 40 + 10 log2 (noy).  There is also a conversion scale on the right side of Exhibit 5, which provides equal-noisiness contours.  These curves were originally developed by assessing the sound of aircraft flyovers; as such, they may not be broadly applicable to occupational settings.  The caveats offered in regards to loudness are also applicable to noisiness.
     The concept of noisiness and techniques for its assessment have been developed to an extent far beyond the scope of this series.  In lieu of mastering the nuances of “synthetic” measurement units, it may be of greater practical value to bear in mind the characteristics of sound that contribute to annoyance.  Five parameters have been identified as significant factors in noisiness assessments:
  • Spectral composition and levels – this relates to frequency weighting scales (e.g. dBA, dBC) and equal-noisiness curves.
  • Spectral complexity – this refers to concentrations of energy within a broadband sound spectrum.
  • Duration of exposure.
  • Time of increasing levels prior to reaching maximum (continuous sounds).
  • Level increase within a 0.5-second interval (impulsive sounds).
Awareness of these factors facilitates effective soundscape management, even if perceived noisiness (PNdB or other unit) numbers are unknown.
     The relationship between frequency and the perception of pitch, first mentioned in Part 2, has been formulated in a fashion similar to that between SPL and loudness.  Again, a reference tone of 1000 Hz at 40 dB SPL is used; it is assigned a value of 1000 mels.  A sound perceived to be twice the pitch of the reference tone is 2000 mels, and one-half the pitch of the reference tone is 500 mels.
     The sensitivity of human perception to changes in sound also varies with its frequency.  Curves showing the variation in sensitivity to changes in frequency are shown in Exhibit 6 and to changes in SPL in Exhibit 7.  High sensitivity to pressure and frequency changes in the 2 – 5 kHz range contribute to speech intelligibility.
     The Doppler Effect is a frequency-related phenomenon.  It is experienced when a sound source and listener are in relative motion (either or both can be moving).  As the distance between source and listener increases, the pitch of the sound decreases, and vice versa.  In this case, the frequency of the emitted sound does not change; it is the relative motion that causes a change in perception.
     Other frequency-related phenomena include perceived frequency shifts due to extended exposure or high intensity, consonance and dissonance, and beatingFrequencies deemed “pleasant” in combination are said to be consonant, while those found objectionable are called dissonant.  This concept is applicable to annoyance and musical preferences, for example.
     Beating occurs when two sounds with similar frequencies are coincident, causing periodic reinforcement and cancellation.  The occurrence of beats can be used to identify mismatched operating speeds of equipment.  When two identical fans, for example, are synchronized, beating ceases.

     A “binaural effect” allows humans to locate the source of a sound, or its directionality; this is called localization.  A sound may reach each ear at different times (“phase difference”) or at different intensity.  These differences are caused by the human body itself; this effect is known as the Head-Related Transfer Function (HRTF) among other names.  By analyzing the type and magnitude of perceived differences, the brain can determine the direction in which the source lies.

Other Perceptual Phenomena
     A sudden unexpected sound, particularly one of high intensity, can cause a startle reaction.  The startle itself may not be dangerous, but induced stress could cause uncontrolled movements or distraction that jeopardizes safety or task performance.  A sudden change in sound can produce a similar effect.
     High-intensity sounds (> 130 dB SPL), whether continuous, intermittent, or transient, can exceed the threshold of pain.  In addition to extreme discomfort and distraction in the short term, permanent hearing loss is likely to also occur.
     Another phenomenon of sound involves a lack of perception.  Ultrasonic and infrasonic “sounds” are those with frequencies above and below, respectively, the audible range.  Exposure to significant energy at these frequencies can cause discomfort or illness.  The inability to recognize the exposure often leaves victims baffled, though, in some cases, vibrational sensations may aid diagnosis of the discomfort.
     The concepts of masking and audibility vs. intelligibility are also perceptual in nature.  However, their relevance to communication warrants deferring detailed discussion to a future installment focused on the design of effective communication signals and systems.

In Conclusion
     The preceding discussion of auditory perception phenomena is merely a cursory introduction to the topic.  Expectations for direct application of the information presented are much lower than for other installments of the series.  The subjectivity of loudness, pitch, etc. requires substantial research to tailor a soundscape to a specific group of people.  Doing so proactively, while ideal, is beyond the capability of most commercial operations.
     Nonetheless, familiarity with perceptual variations is valuable to anyone that designs, maintains, or uses a communication system in an imperfect environment.  Even shallow knowledge of these concepts can aid in troubleshooting existing or potential performance issues.  With this knowledge, a focused project can be undertaken, requiring a manageable number of sound measurements and experiments to be conducted.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

[Link] Engineering Noise Control – Theory and Practice, 4ed.  David A. Bies and Colin H. Hansen.  Taylor & Francis; 2009.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications, 1996.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] An Introduction to Acoustics.  Robert H. Randall.  Addison-Wesley; 1951.
[Link] The Effects of Noise on Man.  Karl D. Kryter.  Academic Press; 1970.
[Link] “Handbook for Acoustic Ecology.”  Barry Truax, Ed.  Cambridge Street Publishing, 1999.
[Link] “Protection and Enhancement of Hearing in Noise.”  John G. Casali and Samir N. Y. Gerges.  Reviews of Human Factors and Ergonomics; April 2006.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Year-End Reset]]>Wed, 27 Dec 2023 07:00:00 GMThttp://jaywinksolutions.com/thethirddegree/year-end-reset     Though spring is traditionally associated with cleaning and refreshing one’s surroundings, doing so during the year-end transition can provide significant advantages.  A range of possibilities exist in our physical, digital, and mental spaces to reduce clutter and stress while increasing value and productivity.
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     While personal obligations may be plentiful due to holiday festivities, shopping, and other related activities, for many, the pace of our professional lives slows during this time.  Many non-retail businesses slow down, or even close, for a period at the end of the year.  This less-chaotic time provides opportunities to complete tasks that are often neglected during more-hectic periods.  The more frequently these tasks are performed, the easier they become.  A virtuous cycle is created as the tasks become less intrusive and, therefore, more likely to be accommodated in a busy schedule.
     A year-end reset consists of three activity types:  cleaning, organizing, and planning.  Performing each activity in all of our physical, digital, and mental spaces creates a healthier, more productive environment in which to work and live.  If the calendar year and fiscal year do not coincide, it may be appropriate to perform the reset activities at the end of the fiscal year; there is no “bad time” to do so.

Physical Spaces
     The physical spaces in which we live, work, and play often become cluttered and disorganized.  Spaces where these parts of our lives intersect are particularly susceptible.  Those that have adopted 5S at work often find the method useful at home, as well.  It is usually informal, lacking a documented standard, but application of the concepts is evident.  [See “Safety. And 5S.” (22Feb2023) for more information.]
     An entire home may benefit from a reset, but, typically, a few areas have greater impacts than the rest.  If working from home, the home office is an obvious target.  The state of one’s kitchen may play a large role; an organized kitchen can reduce the burden of preparing meals, lowering overall stress.  A tidy entryway can ensure that car keys and an umbrella are easily located, facilitating a smooth start to the morning commute.
     One’s office, workbench, toolbox, garden shed, garage, gym locker, or any other space can benefit from a reset.  Physical spaces that are not in fixed locations should also be reset occasionally, such as one’s car, wallet, briefcase, or handbag.  Anywhere that physical objects are used or stored are candidates for a reset.

Digital Spaces
     The number of digital spaces we inhabit grows effortlessly.  If not careful, we can find ourselves on many email lists and signed in to several online services without even realizing it.  This, in addition to all of the legitimate uses for which needs change, is why digital spaces should be reset.  Cleaning up one’s digital presence is also an important protection against various types of fraud.
     Digital spaces include any device with onboard memory, internet connectivity, WiFi, Bluetooth, or any other communication technology.  Also included are any online services, such as email, social media, and streaming services.  Applying 5S principles to digital spaces is highly recommended; additional steps are also advised, including:
  • Back up data to prevent loss or corruption.
  • Update software – operating system, apps, etc.
  • Activate device and account security measures – antivirus software, strong passwords, etc.
     Achieving “inbox zero” has become a popular objective; one way of achieving this is to organize messages in folders.  A digital space reset should include purging obsolete folders, whether archiving in a project file, for example, or simply deleting those that are no longer needed.
     Mobile devices are prone to acquiring unique collections of clutter.  Unneeded photos and videos, obsolete text messages and voicemails, unsupported or unused apps, and other flotsam hinder device performance and user productivity.

Mental Spaces
     One’s mental spaces are the most complex and least influenced by others’ attempts to help.  Mental spaces house knowledge, thoughts, beliefs, fears and insecurities, desires and goals – the entire conscious and unconscious mind.  Suggested subjects for reflection include:
  • Lessons learned.  These need not be limited to professional endeavors; personal revelations and improved hobbyist techniques, for example, are also worthy of recognition.
  • Progress toward objectives.  Assess professional, personal, financial, and fitness goals, among others, periodically.
  • Self-satisfaction survey.  Survey your own satisfaction with various components of your life, without regard for the opinions of others.  Considering career, education, residence, lifestyle, etc. can clarify goals and provide motivation to make changes necessary to achieve them.
  • Relationships.  Perhaps the trickiest subject to broach, even in an internal dialogue, is the quality of one’s relationships and the impacts they have on one’s life.  Whether personal or professional, casual or close, relationships play a huge role in a person’s well-being.  If these seem out of balance, a decision may be needed to confront a lingering issue, end an unhealthy relationship, or deepen a desirable one.

     Some of the activities described overlap.  Execution of a year-end reset is quite personal, even in physical spaces; the highly-individualized nature of the activity renders an attempt to provide an exhaustive set of examples futile.  Therefore, many examples were eschewed in favor of encouraging readers to explore their own spaces with an open mind.  The hope is that a less-prescriptive treatment of the topic leads to better outcomes for individuals than could be predicted or illustrated by examples.

     To make suggestions for a more-effective reset, feel free to leave a comment below.  For additional guidance or assistance with Operations challenges, leave a comment, contact JayWink Solutions, or schedule an appointment.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Thermal Work Environments – Part 10:  The Search for a Universal Index]]>Wed, 13 Dec 2023 07:00:00 GMThttp://jaywinksolutions.com/thethirddegree/thermal-work-environments-part-10-the-search-for-a-universal-index     Over the past century, many researchers have attempted to quantify the physiological impact of high- and low-temperature environments (see Part 4 and Part 8, respectively).  As knowledge of human biometeorology increased and computational tools became more powerful, the models used became much more sophisticated.  However, models are typically focused on one type of environment – hot or cold – requiring use of multiple indices to accommodate varying conditions.  Other shortcomings in thermal index formulations further limit their utility in highly-variable conditions.
     In this installment of the “Thermal Work Environments” series, indices presented earlier in the series are evaluated and compared.  Additional indices are also considered for recommendation in workplaces.  Finally, a universal index, valid across the foreseeable range of human environmental exposure, is presented.
Classification and Evaluation of Thermal Indices
     The classification and evaluation schemes summarized here are the work of C. R. de Freitas and E. A. Grigorieva (2015, 2017).  de Freitas and Grigorieva scoured scientific literature to catalog more than 160 thermal indices.  Characteristics of each index were used to sort them into eight categories, or classes, identified by letters A through H.  The following descriptions are used to differentiate the classes:
simulation device for integrated measurement;
  A:  single-parameter index;
  B:  index based on algebraic or statistical model;
  C:  proxy thermal strain index;
  D:  proxy thermal stress index;
  E:  energy balance strain index;
  F:  energy balance stress index;
  G:  special-purpose index.

     Each index was then scored on a 5-point scale for six evaluation criteria.  The criteria and scoring methods are summarized below.
     Comprehensiveness indicates the number of relevant variables accounted for in the index.  Any factor contributing to thermal stress or strain is a relevant variable, including air temperature, humidity, wind speed, insolation or other radiation exposure, metabolic rate, clothing and protective gear in use, etc.  Comprehensiveness is scored as follows:
   Each relevant variable = +1.
   Maximum score = 5.
     Scope indicates the range of conditions for which the index is valid.  Scope is scored as follows:
   A narrow range of conditions is covered = 1.
   A broad range of conditions is covered = 3.
   Both cool and cold conditions are covered = 4.
   Both hot and cold conditions are covered = 5.
     Sophistication indicates the theoretical soundness or empirical support of the index.  Sophistication is scored according to the classification scheme, described above, as follows:
   Classes A and B = 2.
   Class C = 3.
   Classes D and E = 4.
   Classes F and G = 5.
   Class H is scored according to the methods used (i.e. classes A to G, above).
     Transparency indicates the clarity and justification of the rationale underpinning the index.  Transparency is scored as follows:
   None of the terms used are justified = 0.
   Justifications for terms used are poor or weak = 1 or 2.
   Terms used are justified in most cases = 3.
   Terms used are justified = 5.
     Usability indicates the ease with which an index can be implemented and interpreted.  Evaluation is based on the presence or absence of three characteristics:
  1. Computational procedures are straightforward.
  2. Only ‘standard’ data are required.
  3. Outputs are easily interpreted.
Usability is scored as follows:
   The index exhibits none of the three characteristics = 0.
   The index exhibits only one of the three characteristics = 1.
   The index exhibits two of the three characteristics = 3.
   The index exhibits all three of the characteristics = 5.
     Validity indicates the degree to which the index value accurately reflects the physiological impact, or human experience, of environmental conditions.  Validity is scored as follows:
   The index has not been validated = 0.
   A rational index that has not been validated = 2.
   The index has been compared to one fully-validated index = 3.
   The index has been compared to multiple fully-validated indices = 4.
   The index is derived from or tested with empirical data = 5.
     The six evaluation criteria are deemed to be of equal importance and are equally weighted in the final score.  The total score for each index, therefore, is simply the sum of the six criteria scores.  Using the de Freitas and Grigorieva scheme, scores can range from 4 to 30; indices with higher scores are expected to be more useful.
Heat and Cold Indices Revisited
     The merits of various indices used in hot (Part 4) and cold (Part 8) environments have been discussed.  The de Freitas and Grigorieva framework provides a consistent method of evaluation and comparison.  The summary table in Exhibit 1 presents the scores for heat and cold indices previously discussed.
     All of the heat indices listed in the summary table are direct indices, as discussed in Part 4.  However, identifying the “best” index for use in a hot environment is not as simple as finding the highest total score.  Although the scoring method weights each criterion equally, specific circumstances may influence practitioners’ preferences.  Also, scores may be disputed or obsolete, as the following examples demonstrate.
     Heat Index is given a comprehensiveness score of 5, though it could be argued that holding constant all but temperature (Tdb) and humidity (RH) reduces this to 2.  Updating its usability score of 3 may also be warranted.  De Freitas and Grigorieva (2017) state that “it is reasonable to argue that once user-friendly routines are made available (on a website, say) to run the calculations…, the usability would achieve a top score” in reference to another index.  This logic also applies to Heat Index, for which online calculators have long been available.
     Accepting both arguments changes the Heat Index score from 23 to 22.  While not a large change in total score, maintaining second rank on this list, the constituent scores give a different impression of this index.  Similarity of Heat Index and humidex may also inspire questions about the discrepancies in scores on these two criteria.
     Thermal Work Limit (TWL) provides another cautionary example.  It attained the highest score of the heat indices examined, scoring the maximum on four of the six criteria.  However, its narrow scope of applicability [36 – 40° C (96.8 - 104° F)] and data requirements (e.g. body dimensions) may preclude its use in some situations.
     Ranked third on our list by total score (20), Wet Bulb Globe Temperature (WBGT) is worthy of continued attention.  It is the basis on which ACGIH has set threshold limit values (TLVs) for heat stress.  The availability of instrumentation, simple calculations and estimation procedures seems to warrant a usability score of 5; instead, it has been scored a 3.  It should also be noted that WBGT’s comprehensiveness score of 3 applies to outdoor environments.  For indoor environments, dry bulb temperature (Tdb) is omitted, reducing the score to 2.

     Among the cold indices examined, The New Improved Wind Chill Index (Twc) and required clothing insulation (IREQ) tied with a high score of 26.  The tie-breaker, in occupational settings and other practical applications, is ease of use.  The simplicity of using Twc (usability = 5), whether calculated or estimated from a table, outweighs its deficiency in comprehensiveness by making consistent use more likely.
     The examples provided are not intended to be exhaustive or the conclusions to be absolute.  Reasonable people can disagree on individual scores or the method of scoring, particularly when reviewing a list as extensive as that compiled by de Freitas and Grigorieva.  It is important to recognize this potential for disagreement, understand its implications, and move past it to implement appropriate tools for one’s own situation.
Highest-Scoring Indices by Class
     To explore additional candidates for use in occupational settings, a list of the highest-scoring indices in each classification was compiled.  A summary table is provided in Exhibit 2, listing indices that tied for high score in each class.  This subset was then subjected to analysis, similar to that described above, to identify indices that exhibit sufficient potential for supplanting a familiar index to warrant in-depth investigation.  Admittedly superficial, the large number of indices necessitates an abbreviated analysis.  A description of this review, further condensed, follows.
     The leading index in Class A is Thermo-Integrator, despite only scoring 18 points.  Though fully validated (validity score = 5), its other scores are uninspiring; further research was eschewed.
     Class B maxed out at 14 points, where air temperature (Ta) and wet bulb temperature (Twb) tied.  Both are components of WBGT; use of either independently would be injudicious.
     The standout in Class C is Resultant Temperature (RT) or Net Effective Temperature (NET) with a total score of 23.  Its maximum scores for scope and usability make it an attractive option for consideration.  Unfortunately, an English-language version of the defining paper could not be found, ending the inquiry.
     Two indices tied for the top spot in Class D, with 24 points each.  The Index of Physiological Effect (IPhysE or Ep) assesses the level of strain on a points scale, while the Predicted Four-hour Sweat Rate (P4SR) uses liters of sweat produced as index values.  The impracticality of utilizing such indices in occupational settings was discussed in Part 4.
     Class E is topped by two indices with total scores of 25.  The defining paper for Classification of Weather in Moments (CWM) could not be found in English.  The description provided by de Freitas and Grigorieva of the output of this index is “weather types,” dampening enthusiasm for a continued search.  Effective Temperature (ET) is not valid in cold temperatures.
     There is a three-way tie, at 28, at the top of Class F.  Unfortunately, the three leading indices are impractical for use in occupational settings.  Body-atmosphere Energy Exchange Index (BIODEX) requires core temperature monitoring, while Skin Temperature Energy Balance Index (STEBIDEX) requires skin temperature monitoring.  The Subjective Temperature Index (STI) causes concern with its name alone.  It also requires “nonstandard” data and the maximum valid temperature is 40° C (104° F), which excludes settings that are in great need of effective monitoring and controls.
     Atop Class G is a four-way tie at 28 points.  Of the four high-scorers, only the Standard Effective Temperature for Outdoors (OUT_SET*) outputs an equivalent temperature, making it the most-easily understood index.  Its use of “nonstandard” data reduces its usability score and, thus, its practicality for use in occupational settings.  Its focus on outdoor environmental conditions also limits its applicability.
     Bucking the upward trend in high scores, Class H falls back to 26 with a tie between two indices with questionable value in our chosen context.  The Acclimitization Thermal Strain Index (ATSI) is focused on the physiological adaptations required for travel; investigation of the Bioclimatic Distance Index (BDI) was thwarted by another language barrier.  Maximum valid temperatures for both indices are also rather low.  A lack of viable candidates in the special-purpose category is not surprising.

     The search for a thermal index to solve all our problems, based on high scores in the de Freitas and Grigorieva framework, has been rife with disappointment.  The context of use in occupational settings, in all potential permutations, has heightened the challenge.  Changing focus slightly reveals a new subset of indices; these candidates are reviewed next.

Other Candidate Indices
     Widespread familiarity and accessibility of Heat Index (HI) and Wind Chill Temperature (Twc) charts and calculators and the resultant levels of effectiveness they have achieved sets a high bar for any potential replacement.  High total scores in the de Freitas and Grigorieva scheme is an ineffective metric for identifying potential replacements for the HI/Twc duo, as shown in the previous section.  Instead, focus must be narrowed to the characteristics that determine the feasibility of an index in the context of concern – any occupational setting.  In this context, two scoring categories stand out.
     To be considered universal, an index must be valid throughout the range of conditions that may be encountered.  In the de Freitas and Grigorieva scheme, this equates to a scope score of 5.  A single index should eliminate the gap in valid temperatures between Twc [-40 – 10° C (-40 – 50° F)] and HI [20 – 60° C (68 – 140° F)].
     To ensure consistent, reliable application in occupational settings, an index must be simple to implement.  Here, this equates to a usability score of 5.
     Sorting the list according to these criteria yields the set of indices shown in the summary table in Exhibit 3 (previously-reviewed indices are excluded).  These indices were subjected to analysis similar to that described above; a brief review follows.
     The first index listed, Effective Temperature (ETM) includes only two variables in its calculation.  Other indices, including the HI/Twc duo, are often criticized for a lack of comprehensiveness; it would be difficult to argue that popularizing a “new” index with the same shortcoming is worthwhile.
     The next seven indices on the list scored 0 for validity.  Without empirical support, none of these indices is likely to gain traction as a potential replacement for HI or Twc.  Without some level of validation, use of these indices is also too risky when worker well-being is at stake.
     The Class G indices in this list show promise, though most score low on validity; additional work is needed for them to be viable alternatives.  The exception is the Universal Thermal Climate Index (UTCI), which warrants a closer look.  An overview of UTCI is provided below.

The Universal Thermal Climate Index
     Development of the Universal Thermal Climate Index (UTCI) was initiated by the International Society of Biometeorology (ISB) and the European Union COST (Cooperation in Science and Technology) program’s Action 730.  Objectives of the index development project, which involved scientists from 23 countries, included:
  • An index based on advanced thermophysiological models, incorporating all modes of heat transfer between the human body and the surrounding environment.
  • An index capable of predicting whole-body effects (e.g. heat stroke) and local effects (e.g. frostbite).
  • An index valid for all conditions and all “time and spatial scales.”
  • An “apparent temperature” index, where the output is the air temperature of a reference environment that causes the same physiological response as the input conditions.
  • An index that requires minimal computational capacity, allowing rapid, widespread application.
      A visualization of the UTCI calculation procedure is provided in Exhibit 4.  The generic mathematical formulation of the calculation is as follows:
where Ta is air temperature, Tr is radiant temperature, va is wind speed, and pa is vapor pressure (humidity).  Mean radiant temperature (Tmrt or MRT) is typically used for Tr and can be calculated as follows:
where Tg is globe temperature (°C), ε is emissivity of the globe, and D is the diameter of the globe (m).  For a standard globe, such as that typically used for WBGT measurements, where D = 0.150 m and ε = 0.95,
This is an approximation of a more-rigorous calculation accounting for direct, indirect, and reflected solar radiation and infrared radiation from the sky and surroundings.
     Meteorological data typically include wind speeds measured at a height of 10 m (33 ft).  Heat balance models often use a height of 1.1 m (3.6 ft) as the average height for human exposure to wind; other heights are needed in the detailed calculations of the clothing model (e.g. head, upper leg, etc.), discussed below.  Meteorological wind speeds can be converted to an appropriate height for specific calculations according to the following:
where va is wind speed (m/s) at height Z (m), vZr is wind speed (m/s) at height Zr (m) (reference height for meteorological measurements), and Z0 is the “roughness length” at ground level, often assumed to be 0.01 m (0.4 in), representing short grass or a street.
     In addition to the environmental variables, UTCI also incorporates the metabolic rate of heat production and a detailed clothing model.  The clothing model is used to determine whole-body and local insulation values for the head, torso, lower arms, hands, upper and lower legs, and feet.  The vapor resistance of garments and air layers are also determined in the model.  Calculators available for practical application of UTCI do not require direct input of clothing characteristics; the model incorporates typical clothing appropriate for the environment.
     An online calculator is provided at utci.org with a simple interface.  If an offline option is desired, an executable file (“source code”) can also be downloaded from the site.  Repeated calculations in the program’s DOS interface can be tedious, however.
     For more information on the UTCI calculation process, there is a poster, also available on utci.org, that summarizes the operational procedure.  See Exhibit 5 for a preview of the UTCI summary poster.
     Like Heat Index and Wind Chill Index, an output of the UTCI model is a color-coded chart of thermal stress.  A simplified version appears in the visualization in Exhibit 4 and the summary poster in Exhibit 5.  The chart in Exhibit 6 provides additional details of typical physiological responses corresponding to various index temperatures within each stress category.  The “thermal comfort zone” is shown as the upper portion of the “no thermal stress” category, while there is no “slight heat stress” category defined.  The remaining categories – moderate, strong, very strong, and extreme – are mirrored for heat and cold stress.
     Returning to the de Freitas and Grigorieva scoring scheme, UTCI received a total score of 27, scoring the maximum for comprehensiveness, scope, sophistication, and transparency.  In this case, the scope score (first filtering criterion) was certainly deserved, with a cited valid range of -90 – 60° C (-130 – 140° F).
     The second filtering criterion, usability, scored only 3.  However, the rationale for increasing this to 5, acknowledging the availability of simple tools and calculators, once again applies.  This is the reason for its inclusion in the list despite its nominally deficient score.
     In the final category, validity, UTCI scored 4, reflecting the development team’s work comparing UTCI to several other indices.  This brings the revised total score to 29, placing UTCI alone atop the index-scoring hierarchy.  Scores alone, however, will not propel any index to a position of prominence in meteorological or industrial hygiene domains.  If it is to unseat the incumbent HI/Twc duo, there is much for UTCI’s advocates to accomplish.  Continued development of the index and its underlying models and assumptions are important.  Perhaps more difficult will be the education and persuasion of the general public and practitioners in several fields, cultures, and languages, whose motivations and capacities for change differ greatly.

     For additional guidance or assistance with management of thermal environments, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Thermal Work Environments” entries on “The Third Degree,” see Part 1:  An Introduction to Biometeorology and Job Design (17May2023).

[Link] “A comprehensive catalogue and classification of human thermal climate indices.”  C.R. de Freitas and E.A Grigorieva.  International Journal of Biometeorology; January 2015.
[Link] “A comparison and appraisal of a comprehensive range of human thermal climate indices.”  C.R. de Freitas and E.A Grigorieva.  International Journal of Biometeorology; March 2017.
[Link] “The Perceived Temperature:  The Method of the Deutscher Wetterdienst for the Assessment of Cold Stress and Heat Load for the Human Body.”  G. Jendritzky, et al. International Society of Biometeorology; 2000.
[Link] “A Universal Scale of Apparent Temperature.”  Robert G. Steadman.  Journal of Applied Meteorology and Climatology; December 1984.
[Link] “The Acclimatization Thermal Strain Index (ATSI): A preliminary study of the methodology applied to climatic conditions of the Russian Far East.”  C.R. de Freitas and E.A Grigorieva.  International Journal of Biometeorology; March 2009.
[Link] “New Indices to Assess Thermal Risks Outdoors.”  Krzystof Blazejczyk.  Environmental Ergonomics XI, Proeedings. of the 11th  International Conference; May 2005.
[Link] “An outdoor thermal comfort index (OUT-SET*) - Part I - The model and its assumptions.”  Richard de Dear and J. Pickup.  Proceedings of the 15th International Congress of Biometeorology and International Conference on Urban Climatology; January 1999.
[Link] "An Outdoor Thermal Comfort Index (OUT_SET*) - Part II – Applications."  Richard de Dear and J. Pickup.  Proceedings of the 15th International Congress of Biometeorology and International Conference on Urban Climatology; January 1999.
[Link] “Thermal Indices and Thermophysiological Modeling for Heat Stress.”  George Havenith and Dusan Fiala.  Comprehensive Physiology; January 2016.
[Link] “Threshold Limit Values for Chemical Substances and Physical Agents.”  American Conference of Governmental Industrial Hygienists (ACGIH); latest edition.
[Link] “UTCI - Universal Thermal Climate Index.”
[Link] “UTCI - why another thermal index?”  Gerd Jendritzky, Richard de Dear, and George Havenith.  International Journal of Biometeorology; December 21, 2011.
[Link] “The Universal Thermal Climate Index UTCI in operational use.”  Peter Bröde, Gerd Jendritzky, Dusan Fiala, and George Havenith.  Proceedings of Conference: Adapting to Change: New Thinking on Comfort Cumberland Lodge; April 2010.
[Link] “Deriving the operational procedure for the Universal Thermal Climate Index (UTCI).”  Peter Bröde, et al.   International Journal of Biometeorology; May 2012.
[Link] “The UTCI-clothing model.”  George Havenith, et al.  International Journal of Biometeorology; May 2012.
[Link] “The Universal Thermal Climate Index UTCI Compared to Ergonomics Standards for Assessing the Thermal Environment.”  Peter Bröde, et al.  Industrial Health; February 2013.
[Link] “Mean radiant temperature.”  Wikipedia.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Thermal Work Environments – Part 9:  Managing Conditions in Cold Environments]]>Wed, 29 Nov 2023 07:00:00 GMThttp://jaywinksolutions.com/thethirddegree/thermal-work-environments-part-9-managing-conditions-in-cold-environments     Throughout the range of possible workplace temperatures, safeguarding the health and well-being of employees is paramount.  Despite equal importance, the development of a coordinated program to prevent cold injury receives much less attention than its heat-related counterpart.
     An effective cold injury prevention program consists of the same components as a heat illness prevention program.  These include the measures used in environmental assessment, exposure limits, policies and procedures, training plans, program assessment processes, and other information relevant to work in a cold environment.  Like its heat-related counterpart, this is nominally a prevention program; however, information about the proper response to the occurrence of cold injury, such as first aid practices, is also included.
     Given the similar natures of the heat- and cold-related programs, it should come as no surprise that this installment of the “Thermal Work Environments” series parallels that of “Part 5:  Managing Conditions in Hot Environments.”  In the outline for a cold injury prevention program that emerges, cold stress hygiene and various control mechanisms are introduced.  This outline can be customized to the specific needs of an organization or workplace.
     The content of a cold injury prevention program is presented in five (5) sections:
  • Training
  • Hazard Assessment
  • Controls
  • Monitoring
  • Response Plans
To reiterate, the information presented here is only an overview.  An exhaustive treatment is not feasible in this format, given the range of potential workplace scenarios that exists.  Instead, it is intended to identify avenues of inquiry to be explored in the context relevant to developers of a specific program.

     Every person that works in or has responsibility for a cold workplace should be trained on the ramifications of overexposure to cold conditions.  An effective training program includes information discussed in the following four sections.  Topics important to all team members include:
  • basics of human biometeorology and heat balance,
  • environmental, personal, and behavioral risk factors,
  • methods used to monitor conditions,
  • controls in place to prevent cold injury,
  • signs and symptoms of cold injury, and
  • first aid and emergency response procedures.
Training of supervisors and team leaders should emphasize proper use of controls, signs and symptoms, and appropriate responses to cold injury.
     A complete training plan includes the content of the training and a schedule for delivery.  It may be best to distribute a large amount of information among multiple modules rather than share it in a single, long presentation.  Refresher courses of reduced duration and intensity should also be planned to combat complacency and to update information as needed.  Refreshers are particularly helpful when dangerous conditions exist intermittently or are seasonal.
Hazard Assessment
     An initial hazard assessment consists of identifying the elements of job design (see Part 1) that are cold-related.  These include site-specific environmental factors, such as:
  • atmospheric conditions (e.g. temperature, humidity, sun exposure),
  • air movement (natural or forced), and
  • precipitation (i.e. rain or snow that can wet clothing or exposed skin).
Job-specific attributes are also documented.  These may include:
  • intensity of work (i.e. strenuousness and rate),
  • dexterity requirements of work,
  • tools and materials to be handled,
  • personal protective equipment (PPE) and other gear required, and
  • access to food, water, and a warm recovery area.
As many relevant factors as possible should be identified.  Special attention must be paid to compounding risks.  For example, a task requiring high dexterity in the presence of fluids compounds the risk of exposed skin (gloves limit dexterity) with the risk of wetted skin and clothing.
     The information collected in the hazard assessment is used to create a risk profile for each task or logically-grouped series of tasks.  Development of controls and modifications of job design are prioritized according to the risk profiles generated.
     Readers are reminded that there is a hierarchy of controls that can be implemented to address a hazard.  The hierarchy is represented by an inverted pyramid, as shown in Exhibit 1.  The most-effective types of controlselimination and substitution – are not realistic options in many thermal environments.  Repair of a bridge or other structure cannot be postponed until spring and a construction site cannot be relocated to a more hospitable environment.
     Therefore, the focus of this presentation is, necessarily, on the remainder of the hierarchy which represents feasible protection opportunities.  Engineering controls modify the tasks performed, equipment used, or the operating environment.  Administrative controls reduce cold injury by guiding workers’ behavior.  Finally, PPE is used to manage cold stress not eliminated by other measures.
     A comprehensive cold injury prevention program considers every term in the heat balance equation (see Part 6), developing appropriate mitigations for each.  Examples of engineering controls include:
  • Heaters of various types – area heaters warm the entire workspace; spot or infrared (IR) heaters warm the worker without significantly heating the surroundings; chemical heat packs and electrically-heated garments provide a heat source that moves with the person.
  • Wind barriers reduce the wind chill effect.
  • Tool covers minimize conductive heat loss (K) by covering metals with lower-conductivity material, such as plastic or rubber.
  • Splash/spray guards limit contact with fluids, minimizing evaporative heat loss (E) and limiting loss of insulation capability of clothing.
  • Equipment designed with large buttons, knobs, dials, etc. facilitate use while wearing gloves or mittens.
  • Use of alternative control devices, such as a stylus for a touchscreen, avoid removal of protective gear for routine tasks.
  • Use of lift assists or other physical aids maintain work rate with minimum risk of sweating (i.e. stabilize metabolic heat generation, M).
Administrative control examples include:
  • Implementing a “buddy system” allows two (or more) people to effectively manage their work rates (minimize sweating, etc.) by sharing work and provide each other constant monitoring for signs of cold injury.
  • Developing a balanced work cycle avoids periods of intense effort followed by periods of very low intensity.  Long periods of sitting or standing are also avoided.
  • Implementing an acclimatization program provides an adjustment period to new or returning workers.  While there is consensus that heat acclimatization is effective in preventing many heat illnesses, the same does not appear to be true for cold acclimatization.  It is recommended, nonetheless, because evidence suggests that cold acclimatization does, in fact, occur to some degree, improving safety.
  • Implementing a work/warm-up schedule limits the duration of exposure.  The schedule shown in Exhibit 2 has been cited in Canadian provincial regulations and as ACGIH TLVs.  Exhibit 3 provides a visual representation of a 4-hour work cycle in the no-wind condition.  The supplement shown in Exhibit 4 combines the two styles of representation in a single reference.  All work/warm-up schedules assume that workers’ clothing is dry.
  • Encouraging consumption of snacks and warm drinks throughout the workday helps workers maintain energy and hydration.
     Cold-related PPE includes all garments worn for insulation, wind protection, or waterproofing.  This includes insulated boots, goggles, gloves, hats, neck gaiters or scarves, balaclavas, etc.
     Clothing should be worn in loose-fitting layers.  Multiple layers provide improved insulation performance and facilitate adjustment as needs change.  The innermost layers should be capable of wicking moisture away from the skin.  Garments made of synthetic fibers are often used for this purpose; cotton is not recommended, as it wets easily, reducing its insulating capability.
     A waterproof outermost layer or windbreaker may be needed, depending on conditions.  Adjustable closures (waist, neck, arms) and vents (armpits, etc.) allow the wearer to accommodate a wider range of conditions before changing is necessary.  Intermediate layers must be selected in accordance with environmental conditions and work performed (e.g. work rate or intensity).
     It is important to remember that clothing that is wet, dirty, or compressed loses insulation capacity.  This is particularly important to remember in regards to socks.  Thick, insulated socks may be counterproductive if their bulk causes boots to be tight-fitting.  Restricted circulation and impaired insulation accelerates the onset of trench foot and/or frostbite.

     Controls used in conjunction must be evaluated to ensure suitability of the combination.  For example, a well-balanced work cycle may render heated garments excessive and, therefore, counterproductive.  Likewise, using a physical aid to move a light load may cause a problem; effort is reduced, lowering M, while heat loss (K) in the extremities may be increased via physical contact with the equipment.  Another example is the construction of an effective wind barrier that, in addition to its direct benefit, precludes the need for a splash guard that hinders task performance.  Sometimes less is more.
     Monitoring is a multifaceted activity and responsibility.  In addition to measuring environmental variables, the effectiveness of controls and the well-being of workers must be continually assessed.  A monitoring plan includes descriptions of the methods used to accomplish each.
     Measurement of environmental variables is the subject of Part 8 of this series.  As discussed in that installment, decisions regarding work cycle modifications or stoppages is often based on wind chill calculations.  Though imperfect, it is a useful guide that provides early warnings that additional precautions may be needed to protect workers during particularly dangerous periods.  In addition to providing wind chill charts, the National Weather Service (NWS) issues advisories when dangerous conditions are forecast.
     After controls are implemented, they must be monitored for proper use and continued effectiveness.  This should be done on an ongoing basis, though a formal report may be issued only at specified intervals (e.g. quarterly) or during specific events (e.g. modification of a control).  Verification test procedures should be included in the monitoring plan to maintain consistency of tests and efficacy of controls.
     Monitoring the well-being of workers is a responsibility shared by a worker’s team and medical professionals.  Prior to working in a cold environment, each worker should be evaluated on his/her overall health and underlying risk factors for cold injury.  An established baseline facilitates monitoring a worker’s condition over time, including the effectiveness of acclimatization procedures and behavioral changes.
     Suggestions for behavioral changes, or “lifestyle choices,” can be made to reduce a worker’s risk; these include diet, exercise, consumption of alcohol or other substances, and other activities.  Recommendations to an employer regarding one’s fitness for certain duties, for example, must be made in such a way that protects both safety and privacy.  Cold-related issues may be best addressed as one component of a holistic wellness program such as those established by partnerships between employers, insurers, and healthcare providers.
Response Plans
     There are three (3) response plans that should be included in a cold injury prevention program.  Like heat-related response plans (see Part 5), two of them are concerned with cold injury that was not prevented.
     The first response plan details the provisioning of first aid and subsequent medical care when needed.  Refer to Part 7 for an introduction to cold injuries and first aid.
     The second outlines the investigation required when a serious cold injury or cold-related accident occurs.  The questions it must answer include:
  • Were defined controls functioning and in proper use?
  • Had the individual(s) involved received medical screening and been cleared for work?
  • Had recommendations from prescreens been followed by individual(s) and the organization?
  • Had the individual(s) been properly acclimatized?
  • Were special circumstances involved (e.g. wind chill advisory, emergency situation, etc.)?
The investigation is intended to reveal necessary modifications to the program to prevent future cold injuries and related accidents.
     The final response plan needed defines the review process for the cold injury prevention program.  This includes the review frequency, events that trigger additional scrutiny and revision, and required approvals.

     Currently, management of cold work environments is governed by the “General Duty Clause” of the Occupational Safety and Health Act of 1970.  The General Duty Clause provides umbrella protections for hazards that are not explicitly detailed elsewhere in the regulations.  It is a generic statement of intent that provides no specific guidance for assessment of hazards or management of risks.
     Though OSHA has issued an “advance notice of proposed rulemaking” (ANPRM) to formalize heat-related safety regulations and launched a National Emphasis Program for heat-related hazards, no counterpart for cold conditions has yet been publicized nor is cold stress addressed in the OSHA Technical Manual; in fact, the major players in US industrial hygiene (OSHA, NIOSH, ACGIH) do not prescribe a cold injury prevention program.
     That a standard promulgated by OSHA or other prominent organization will reduce illness and injury in thermal work environments is a reasonable expectation.  However, it must be recognized that it, too, is imperfect.  No standard or guideline can account for every person’s unique experience of his/her environment; therefore, an individual’s perceptions and expressions of his/her condition (i.e. comfort and well-being) should not be ignored.  A culture of autonomy, or “self-determination,” where workers are self-paced, or retain other responsibility for thermal stress hygiene, is one of the most powerful tools available for safety and health management.
     For additional guidance or assistance with complying with OSHA regulations, developing a cold injury prevention program, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Thermal Work Environments” entries on “The Third Degree,” see Part 1:  An Introduction to Biometeorology and Job Design (17May2023).
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “Threshold Limit Values for Chemical Substances and Physical Agents.”  American Conference of Governmental Industrial Hygienists (ACGIH); latest edition.
[Link] “Thermal Environment.”  Student Manual, Occupational Hygiene Training Association; February 2016.
[Link] “Fire and Ice: Protecting Workers in Extreme Temperatures.”  Donald J. Garvey.  Professional Safety; September 2017.
[Link] “Hierarchy of Controls.”  NIOSH; January 17, 2023.
[Link] “Preventing Cold-related Illness, Injury & Death among Workers.”  NIOSH Publication No. 2019-113; September 2019.
[Link] “Cold Environments - Working in the Cold.”  Canadian Centre for Occupational Health and Safety (CCOHS); June 13, 2023.
[Link] “Working in Cold Conditions Fact sheet.”  Canadian Centre for Occupational Health and Safety (CCOHS); October 28, 2022.
[Link] “Staying safe in the cold - & tips for safety pros and workers.”  Barry Bottino.  Safety + Health; January 29, 2023.
[Link] “Working in the cold – Stay safe when temperatures drop.”  Alan Ferguson.  Safety + Health; November 22, 2020.
[Link] “Cold Stress and its Safety Measures.”  OSHA Outreach Courses: July 30, 2021.
[Link] “Recommendations to Improve Employee Thermal Comfort When Working in 40°F Refrigerated Cold Rooms.”  Diana Ceballos, Kenneth Mead, and Jessica Ramsey.  Journal of Occupational and Environmental Hygiene; August 17, 2015.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Thermal Work Environments – Part 8:  A Measure of Comfort in Cold Environments]]>Wed, 15 Nov 2023 07:00:00 GMThttp://jaywinksolutions.com/thethirddegree/thermal-work-environments-part-8-a-measure-of-comfort-in-cold-environments     Development of effective cold stress indices has garnered significantly less attention than that of heat stress indices (see Part 4).  Perhaps this is explained, at least in part, by the lesser threat to life posed by cold stress, as explained in Part 7.  Whatever the reason, this difference does not indicate lesser importanceCold stress and cold injuries are serious conditions that effect workers in many ways and have both short- and long-term consequences.  Monitoring environmental conditions and worker well-being is as critical a responsibility in cold environments as it is in hot ones.
      This installment of the “Thermal Work Environments” series parallels the discussion in Part 4, beginning with a widely-reported, if not widely-understood, index used in weather forecasting, followed by a discussion of application in industrial settings.  Readers are encouraged to review the discussions of heat and cold indices in conjunction.
Popular Meteorology
     Several independent weather-forecasting organizations have developed versions of “feels like” temperature indices to convey the level of discomfort one can expect to experience in cold, windy conditions.  Others simply defer to the National Weather Service (NWS) in the US or the Canadian Meteorological Service (MSC) in Canada, using the “New Improved Wind Chill Index” developed by an international consortium.  This is a logical choice, as these national agencies are generally recognized as the experts in weather-related matters in North America.  Outdated references may be encountered in resources that remain readily-available, such as websites and journal articles, however.
     The likelihood of encountering obsolete material warrants a brief review of the history of the development of wind chill as a comfort index.  Understanding input variables and calculation methods makes the variety of wind chill indices one may encounter more meaningful.
     The concept of wind chill, as we know it today, originated in the Antarctic in 1945.  Two explorers, Paul Siple and Charles Passel, measured the time required for a container of water to freeze at various temperatures and wind speeds.  From the data gathered, Siple and Passel derived a formula for the rate of heat loss in cold, windy conditions:
where H is “wind chill” or rate of heat loss (kcal/m^2/hr), v is wind speed (m/s) and T is air temperature (°C).
     This heat loss rate has little meaning outside the research community; therefore, conversion to a recognizable form is needed for practical use.  This seminal work’s greatest contribution has been to inspire development of better indices.
     To this end, further experiments were conducted, ultimately resulting in a revised formula for wind chill:
where WCI is the Wind Chill Index (W/m^2), v is wind speed (m/s), and T is ambient temperature (°C).  WCI is converted to an apparent temperature with the following relation:
where Tch is the equivalent chilling temperature (°C) and WCI is the Wind Chill Index (W/m^2) calculated above.
     In 1992, NWS published a wind chill index that came into wide use and public familiarity.  This formulation calculates an apparent temperature, in a single step, according to the following:
where Twc is the apparent or “wind chill temperature” (°F), T is ambient temperature (°F), and v is wind speed (mph).  Published wind chill tables are convenient; their widespread use makes them the most-likely references to this obsolete formulation to be encountered.
     In 1998, Robert Quayle and Robert Steadman advocated for the Steadman Wind Chill to replace the existing index.  Deficiencies of the NWS wind chill index cited by Quayle and Steadman include:
  • Data from water-freezing experiments are not representative of human physiology or behavior.
  • Wind speeds below 5 mph are assumed to have no cooling effect and may even have a warming effect.
  • Wind speeds above 40 mph are assumed to contribute no additional cooling effect.
  • The resultant wind chill values are lower than apparent temperatures actually experienced by humans in given conditions.
     To compensate for these deficiencies, the Steadman Wind Chill equation was developed:
where TSF is the Steadman wind chill equivalent temperature (°F), v is wind speed (mph), and T is ambient temperature (°F).  An alternate formulation, for use with metric units, was also developed:
where TSC is the Steadman wind chill equivalent temperature (°C), v is wind speed (m/s), and T is ambient temperature (°C).
     The broader meteorological community was clearly aware of deficiencies in the existing wind chill index when, in 2000, an international consortium convened to update it.  This effort resulted in “The New Improved Wind Chill Index.”  The updated index was adopted by NWS and MSC in 2001 and continues to be cited in meteorological reports and forecasts across North America.  In the US:
where Twc is the wind chill equivalent temperature (°F), T is ambient temperature (°F), and v is wind speed (mph).  In Canada:
where Twc is the wind chill equivalent temperature (°C), T is ambient temperature (°C), and v is wind speed (km/hr or kph).
     Though NWS and MSC did not simply adopt the Steadman equations for their improved indices, results are in much-greater alignment with Steadman’s than those of previous iterations.  A comparison of wind chill equivalent temperature calculations is shown in Exhibit 1 for two hypothetical conditions.  As conditions become extreme (i.e. very low temperature and high wind speed), discrepancies among the wind chill equivalent temperature calculations become more pronounced.  The modern indices better reflect human physiology – the basis of Steadman’s arguments in the 1990s.  Readers are encouraged to compare these values to those obtained by interpolating from wind chill index charts.
     A wind chill index table remains the most-convenient resource, as precision is often unnecessary; the variability of human experience typically exceeds the error inherent in interpolation of tabulated values.  NWS, MSC, and other national and independent organizations publish such tables.  Going a step further, the supplement to this post, shown in Exhibit 2, provides a single reference for use with either US or metric units.  Converted values are also included in the supplement tables to facilitate approximation of wind chill values when available measurements are in mixed units.

Industrial Application

     Wind chill equivalent temperatures are very useful for outdoor settings; however, significant shortcomings render them much less helpful in most indoor settings.  Wind chill and heat index (see Part 4) are subject to a similar criticism:  only two factors are included in calculations.
     Wind chill calculations neglect the influence of radiation, though exposure to direct sunlight can increase the apparent temperature by 8 – 15° F (5 – 9° C).  This can be accounted for with a wet bulb globe temperature (WBGT) measurement in hot conditions (see Part 4).  However, most sources also conclude that humidity is not relevant to cold stress.  This leaves only the dry bulb (ambient) temperature with no adjustment.
     The ambient temperature alone may be sufficient for many indoor workplaces, where the air is calm (i.e. no appreciable air movement).  In others, such as large freezer facilities, where significant air flows are necessary, the wind chill index provides a better assessment of conditions.
     The required clothing insulation (IREQ) index incorporates aspects of human physiology to determine proper clothing for the existing work conditions.  Two versions of the index have been defined:
  • IREQmin establishes the clothing insulation required to limit body temperature reduction to 97° F (36° C).  This is the maximum heat loss deemed safe for workers in cold conditions.
  • IREQneutral defines the clothing insulation required to maintain normal body temperature.  This is associated with comfort, as only minimal cooling occurs.
     Clothing outside the range from IREQmin to IREQneutral requires additional attention.  Below IREQmin, work must be time-limited to prevent excessive heat loss.  Above IREQneutral, a worker becomes subject to the risks associated with clothing dampened by sweat (see Part 7).
     Determining IREQ values requires complex calculations involving the heat balance equation (see Part 6).  Therefore, these indices provide more value as conceptualizations of conditions than in direct application.  The chart in Exhibit 3 visually reinforces the idea behind IREQ, though the index values remain abstract.  Instituting the IREQ indices fully requires data collection and computation that are impractical in most occupational settings and, therefore, beyond the scope of this treatise.

The Conclusion

     While several practical options exist to assess potential heat stress (see Part 4), cold stress options are much more limited.  In fact, simplicity being key to practical application in occupational settings renders all but wind chill infeasible.  Some choice does remain, however.  Wind chill should be cited in the scale (°F or °C) most familiar to the group effected.  This may define the equation used or table referenced to determine the index.  Though a “conservative” index may invite criticism for overprotection, an extra margin of safety reduces the risk inherent in the variability of human experience.  Ultimately, it is individual experience, as subjective and unreliable as it may be, that must be the determining factor in many workplace decisions.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Thermal Work Environments” entries on “The Third Degree,” see Part 1:  An Introduction to Biometeorology and Job Design (17May2023).
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “Thermal Environment.”  Student Manual, Occupational Hygiene Training Association; February 2016.
[Link] “Understanding Wind Chill.”  University of Kentucky Weather Center.
[Link] “The Ridiculous History of Wind Chill.”  Rachel Z. Arndt.  Popular Mechanics; December 12, 2016.
[Link] “The Steadman Wind Chill:  An Improvement over Present Scales.”  Robert G. Quayle and Robert G. Steadman.  Weather and Forecasting; December 1, 1998.
[Link] “The New Improved Wind Chill Index.”  National Weather Service; November 1, 2001.
[Link] “Wind chill – text version.”  Government of Canada; June 14, 2022.
[Link] “Wind Chill Calculator (Celsius).”  CalcuNation; 2022.
[Link] “Calculate the wind chill.”  Lenntech.
[Link] “Wind chill.”  Wikipedia; July 13, 2023.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Thermal Work Environments – Part 7:  Cold Injury and Other Cold-Related Effects]]>Wed, 01 Nov 2023 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/thermal-work-environments-part-7-cold-injury-and-other-cold-related-effects     Loss of heat balance in a cold environment leads to cold injury, an umbrella term for several afflictions, of varying severity, resulting from overexposure to low temperatures.  Recognizing symptoms of cold injuries is critical to timely treatment and successful recovery.
     This installment is a companion to Part 3 (“Heat Illness”) of the “Thermal Work Environments” series, in which a range of cold-related effects and injuries are presented.  The objective of this discussion is to raise awareness of the risks of working in cold environments and the severity of potential outcomes.  These are serious conditions, all but the mildest of which require medical attention from trained healthcare professionals.
Cold Injury
     The following descriptions of cold injuries include information to aid in their identification and understanding of proper treatment.  Minor issues can often be resolved by the affected individual or nearby coworkers.  Though information is shared regarding identification and treatment of cold injuries and related effects, it should not be construed as “medical advice.”  As the severity of injury increases, the more critical professional medical care becomes to survival and recovery.
     The ensuing presentation begins with the least-severe issue and progresses to the most-severe at its conclusion.  The intervening sequence, however, is not a reliable reflection of the relative severity of all occurrences of injury; individuals’ circumstances, sensitivities, and, therefore, experience of injuries differ.  Also, the absence of recognizable symptoms of “low-level” injury does not preclude the onset of a serious condition.  All signs of cold stress must be acknowledged and treated accordingly.
Cold Discomfort
     Discomfort is not an injury, but is, nonetheless, worth noting at the outset.  It is often the first warning or reminder that cold stress is an important element of the workplace that requires attention and proper management.  Discomfort is a common precursor to more-serious injury.
     A chilblain is a swelling of a foot or hand; ears and cheeks may exhibit a similar condition.  It is characterized by redness, itching, and pain.  Bare skin can develop chilblains with repeated exposure to temperatures below 60° F (16° C).  Permanent damage increases susceptibility to recurrence of redness and itching upon subsequent exposure.  Keeping affected areas warm and dry is the default treatment.
Frozen Cornea
     The combination of low temperature, strong wind, and the absence of eye protection (i.e. goggles) can result in frozen cornea.  Treatment usually consists of warming the closed eye with one’s hand or a warm compress, followed by 24 – 48 hours of complete coverage with an eye patch.
Trench Foot
     Trench foot is caused by exposure to cold, wet conditions.  Wet feet suffer from accelerated heat loss, with increased risk of trench foot in temperatures as high as 59° F (15° C).  The hypothalamus responds to the increased heat loss by restricting circulation to the feet, causing them to become cold and numb.  As the condition progresses, hot, shooting pain may be experienced, with swelling, redness, and blisters appearing.
     Tissue damage caused by reduced circulation becomes permanent after approximately 6 hours of exposure with vasoconstriction.  Tissue is damaged further by walking, as it is soft and weak.  After 24 hours of exposure with vasoconstriction, amputation may be necessary.
     Treatment of trench foot is limited to gentle warming and drying and slight elevation of the feet.  Use of over-the-counter pain medication and bed rest (i.e. no walking) are common during the recovery period.
     To prevent trench foot, waterproof insulated boots that are not constricting (tight fitting) should be worn.  Socks should be changed when they become damp and the inside of the boots should be dried regularly (e.g. overnight).
     This condition is also called “immersion foot.”  A similar condition can develop in the hands; the same cause, treatment, and prevention principles are applicable.  Generalizing, this type of injury can be called cold-immersion injury.
     Freezing of the top layers of skin tissue is called frostnip; it is most common in the cheeks, earlobes, fingers, and toes.  It is characterized by numbness, white, waxy appearance, and a hard, rubbery feeling of the skin while the tissue underneath remains soft.  Frostnip is usually reversible with gentle warming; rubbing effected areas should be avoided as this can damage the frozen tissue.
     Freezing that extends through all layers of skin is called superficial frostbite; deep frostbite includes freezing underlying tissue, such as muscle, and can extend into bone.  The extremities – fingers, toes, nose, ears, etc. – are most susceptible to frostbite.  Skin in frostbitten areas is white, with a “wooden” feel, and may develop a bluish hue.  Numbness and stinging are also common symptoms.
     Superficial frostbite is treated similarly to frostnip.  Ice crystals that form in the skin make the tissue susceptible to damage; rubbing or other stress on effected tissue must be avoided.  Treatment of deep frostbite introduces additional risks and is best left to medical professionals whenever possible.  Areas of deep frostbite should not be warmed until the victim is safe from potential refreezing.  Refreezing of frostbitten areas can result in damage and loss of tissue in excess of that caused by the initial frostbite.
     Warming is accomplished in a water bath maintained at 105 – 110 ° F (41 – 43° C).  Dry heat can cause burns and should not be used.  When thawing is complete, the water bath is discontinued and effected areas are wrapped in gauze, separated (i.e. fingers, toes), and immobilized.  Attempting to use rewarmed body parts can cause further damage.
     A core temperature below one’s normal (diurnal) range is termed hypothermiaHypothermia advances from mild to moderate to severe as core temperature drops.  The temperature ranges that characterize each level of severity differ among sources; there is greater agreement on the progression of symptoms.  A summary of this progression and one possible temperature range breakdown are shown in Exhibit 1.
     One possible element of a “field diagnosis” of hypothermia is “the –umbles.”  If observations of a person’s behavior include “stumbles, mumbles, fumbles, and grumbles,” a closer look for other signs of hypothermia is warranted.  This assumes, of course, that these observations represent a deviation from the person’s normal behavior.  Severity of the –umbles typically correlates with that of the hypothermia of which it is symptomatic.
     Significant physiological changes can occur during the earliest stages of hypothermiaMild hypothermia can cause vasoconstriction, limiting circulation to the extremities, loss of fine motor skills (“fumbles”) and shivering.  Onset can occur in ambient temperatures as high as 50° F (10° C).
     Fine motor skills continue to degrade in moderate hypothermia; tasks such as zipping a coat can become very difficult or impossible.  Shivering intensifies and can become uncontrollable.  Slurred speech (“mumbles”) and irrational behavior (“grumbles”) also manifest at this stage.
     When hypothermia becomes severe, shivering becomes intermittent, then ceases.  The person loses his/her ability to walk (“stumbles”) and becomes stiff.  Pulse and respiration rates decline and the person loses consciousness; cardiac arrest may be induced.  Severe hypothermia brings a person to the brink of death; emergency medical care is critical to survival.

     Recovery from mild hypothermia is fairly straightforward.  Increasing physical activity increases the metabolic rate of heat generation, offsetting heat loss.  Moving to a warm shelter, removing wet clothing and replacing with additional dry layers, if necessary, may be sufficient to rebound from a mild case of hypothermia.
     The techniques for treating mild hypothermia are also applicable to moderate cases.  However, as a case of hypothermia worsens, the response must be scaled accordingly.  The human body requires fuel to generate heat; carbohydrate-rich foods provide the fastest conversion to energy.  Proteins are converted more slowly, over a longer period.  Fats are also converted slowly over a long period, but more water and energy are consumed in the conversion.  In short, carbohydrates facilitate recovery, while proteins and fats (to a lesser degree, with sufficient hydration) are better for long-term sustenance.
     Warm to hot (but not too hot) drinks are very beneficial.  They can provide immediate heat, an energy source (calories), and hydration simultaneously.  Alcohol and caffeine should be avoided because their consumption causes counterproductive physiological responses.
     If increased physical activity and warm shelter are insufficient, or unavailable, an external heat source may be needed.  A nearby fire or heater can warm the person and dry his/her clothing before redressing.  Hot water bottles, chemical heat packs, or similar heat source applied to the neck, armpits, and groin effectively warm the core.  Another person can also serve as an external heat source, provided that person is not also experiencing a heat deficit (i.e. s/he is normothermic).
     If the victim is able to drink, s/he should be given warm sugar water.  In severe conditions, the digestive system is incapable of processing solid food; a sugar mixture provides fuel the body needs to generate heat in a form it can process.  A gelatin dessert mix can also be used; the combination of sugar and protein provides fast- and slow-release energy.  Any drink must be dilute for the body to convert it to energy.
     A severe case may require a hypothermia wrap and transport to a medical facility.  Multiple blankets and sleeping bags can be used to create the wrap.  It is imperative that the victim and the wrap remain dry; this may require a wicking layer next to the skin and a waterproof outer layer.
     The heart is the organ most vulnerable to functional disruption in cold conditions.  The combined stress of hypothermia and physical shocks, such as those caused by being moved or carried, can induce cardiac fibrillation.  Performing CPR can also hasten the death it is intended to prevent because of the heart’s hypersensitivity in these conditions.
     “Rescue breathing” is the practice of a normothermic person gently blowing warm air into the victim’s mouth.  The pre-warmed air reduces respirational heat loss; it may also add oxygen needed to metabolize sugar and generate heat.
     To reiterate, severe hypothermia is a life-threatening condition; any missteps during treatment can hasten death.  Seek emergency medical care.
     Afterdrop is a dangerous drop in core temperature that occurs while rewarming a victim of hypothermia.  It is caused by vasodilation in the extremities allowing very cold blood to return to the core.  Blood stagnated in the arms and legs also becomes acetic; upon recirculation, it may cause the sensitive heart to become arrhythmic.
     Prevention of afterdrop requires a carefully-controlled warming process; only the core should be warmed.  Exposure to extreme heat, such as moving into a hot room, can cause superficial warming of the extremities that initiates vasodilation and recirculation before the core is warm enough to tolerate it.
     Recovery from a core temperature below 77° F (25° C) would be miraculous; death is a near-certainty.  A victim may lose consciousness and exhibit pulse and respiratory rates so low that they are difficult to detect, causing a premature declaration of death.  Entering such a state is the body’s final attempt at survival, reducing energy expenditure to its absolute minimum.

Other Cold-Related Effects
     Commonly-experienced effects of exposure to cold, such as numbness, redness, and stinging upon rewarming occur with such regularity that little attention is paid to them.  Many do not consider these to be cold injuries; it is only upon increased severity that they take note.  This is unfortunate, as proper attention to all occurrences of potential injury is key to the prevention of severe injury.
     Manual dexterity and flexibility are reduced with exposure to temperatures as high as 59° F (15° C).  A one-hour exposure at 45° F (7° C) can cause as much as a 20% loss of dexterity.  Continued exposure can reduce blood flow to the fingers to as little as 2% of normal.  Mild shivering exacerbates the loss of fine motor skill.  When shivering becomes severe, or violent, even coarse motor control becomes extremely difficult. The impact on task performance is intuitive; however, the potential for increased accident rates may be less so.
     Cognitive ability and psychomotor function, such as the ability to skillfully operate tools, also decline in cold environments.  The connection to safety may be more obvious here, though the extent of the decline may be surprising.  When core body temperature drops by as little as 7° F (4° C), a person loses the ability to make life-saving (“fight or flight”) decisions.  Core temperature may drop another 10° F (6° C) or more before the person loses consciousness; in the interim, a person can put him/herself in much greater peril with poor decision-making.
     Maximum vasoconstriction (i.e. minimum blood flow) in the extremities occurs at approximately 59° F (15° C).  If further cooled, to approximately 50° F (10° C), alternating periods of vasodilation and vasoconstriction begin.  This cold-induced vasodilation (CIVD) occurs in 5 – 10-minute cycles, providing some protection against cold injury via periodic rewarming of the extremities.  This phenomenon has been observed, but not fully explained; it remains unclear when this vascular behavior ceases and precisely why it occurs.
     Cold exposure can also modify the body’s response to heat upon rewarming.  During cold exposure, the sweating mechanism is disabled; upon rewarming, an increased threshold temperature and latent period delay the onset of sweating during subsequent heat exposure.  This shifting response reinforces the need for an acclimatization period, particularly for those exposed to both high- and low-temperature environments.

     Preparation for exposure and recovery between exposures is equally important in hot and cold environments.  The following cold-recovery guidelines are similar to those presented in Part 3 for heat exposure:
  • Spend the “downtime” in a warm, dry environment.
  • Tailor diet to the energy needs of the cold environment – proteins and fats for extended energy release and carbohydrates for quicker conversion to energy.
  • Replenish fluids.  Converting food to life-saving heat requires water.  Fluid loss is easily overlooked in a cold environment, but hydration must be given proper attention.  Alcoholic and caffeinated beverages should be avoided.
  • Use the time to verify that plenty of warm, dry layers are available for the next work shift, including hats, boots, and gloves.  Other gear, such as goggles, should also be checked to ensure proper protection will be provided.
  • Get plenty of rest.  Working in cold conditions is energy-intensive; the body needs time to recover and “rebuild.”
Risk Factors
     Several factors affect the risk one assumes when exposing him/herself to cold conditions.  A person’s overall condition, or general health, provides the baseline assessment.  In general, the better one’s physical fitness, the greater his/her resistance to cold injury.
     Specific health issues of concern include heart conditions and previous cardiac events (i.e. heart attacks).  As the heart is most susceptible to disruption by cold, any “imperfection” can become a significant liability as conditions degrade.
     Previous exposures, particularly overexposures, can reduce a person’s tolerance for cold conditions.  Previously-damaged tissue is susceptible to re-injury; each occurrence tends to be worse than the previous.
     Perhaps the greatest risk in cold conditions is overconfidence.  Overconfidence increases exposure unnecessarily when one convinces him/herself that additional precautions are excessive.  There are several workplace factors in which one might be overconfident, including:
  • One’s personal fortitude (i.e. ability to maintain efficacy in cold conditions; ego).
  • Performance capability of selected clothing and gear.
  • Stability of conditions during the planned work period (e.g. temperature, wind).
  • Stability of schedule (i.e. ability to complete tasks in allotted time).
     As a person’s condition deteriorates – hypothermia deepens – s/he becomes less aware of his/her condition and endangerment.  This makes coworkers that are aware of signs of cold injury in others critical to a team’s safety.  Individuals’ susceptibility to conditions varies widely; these are not always known in advance.  The ability to recognize changes in a person’s behavior or physical condition and respond accordingly is paramount.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Thermal Work Environments” entries on “The Third Degree,” see Part 1:  An Introduction to Biometeorology and Job Design (17May2023).

[Link] Human Factors in Technology.  Edward Bennett, James Degan, Joseph Spiegel (eds).  McGraw-Hill Book Company, Inc., 1963.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “Hypothermia and Cold Weather Injuries.”  Rick Curtis.  Princeton University Outdoor Action Program, 1995.
[Link] “Fire and Ice: Protecting Workers in Extreme Temperatures.”  Donald J. Garvey.  Professional Safety; September 2017.
[Link] “Cold Weather Exposure.”  Agricultural Safety and Health Program, Ohio State University Extension; May 17, 2019.
[Link] “Cold Stress – Cold Related Illnesses.”  National Institute for Occupational Safety and Health; June 6, 2018.
[Link] “Effect of body temperature on cold induced vasodilation.”  Andreas D. Flouris, David A. Westwood, Igor B. Mekjavic, and Stephen S. Cheung.  European Journal of Applied Physiology; June 21, 2008.
[Link] “Influence of thermal balance on cold-induced vasodilation.”  Andreas D. Flouris and Stephen S. Cheung.  Journal of Applied Physiology; April 2009.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Thermal Work Environments – Part 6:  Thermoregulation in Cold Environments]]>Wed, 18 Oct 2023 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/thermal-work-environments-part-6-thermoregulation-in-cold-environments     Many of the human body’s responses to cold mirror those initiated by exposure to heat.  Others are unique physiological mechanisms engaged to pursue diametrically-opposed objectives.  The risks associated with cold stress are very different from those of heat stress, requiring unique forms of strain for proper and effective management.
     This installment parallels Part 2 of the “Thermal Work Environments” series, providing an overview of thermoregulatory functions activated by cold stress.  The heat balance equation is also revisited, discussing each term in the context of cold environments.  These two installments are “companion pieces;” each can stand alone, but are most helpful when reviewed in conjunction.
Thermoregulatory Function
     The hypothalamus (see Exhibit 1) is responsible for regulation of core body temperature.  Heat-retention functions, such as vasoconstriction and shivering, are managed by the posterior hypothalamus.
     Vasoconstriction reduces blood flow to the outer regions of the body; heat is retained in the body’s core.  As core temperature drops, the heart rate decreases to further limit heat loss.
     Shivering is an involuntary response, engaged to offset heat loss.  A person can postpone the onset of shivering through force of will, but doing so could exacerbate an already-dangerous situation.  The onset of shivering should be treated as a warning; a warm refuge should be sought.
Heat Balance
     The objective of homeothermy requires viewing the body’s heat balance from different perspectives.  Heat stress requires a focus on the management (i.e. minimization) of heat gainCold stress, in contrast, requires a focus on heat loss which adds a layer of risk not present during heat stress.
     When experiencing heat stress, the temperature of a person’s extremities cannot be driven higher than his/her core temperature by thermoregulatory means.  Internal, involuntary functions are activated to transfer heat from the core to the extremities and the surroundings; heat transfer ceases when temperatures equalize.
     In contrast, cold stress can result in large temperature differentials between the core and the extremities.  As the hypothalamus activates thermoregulatory functions to maintain core temperature, the extremities are actively neglected.  The human body will sacrifice its appendages for the survival of the being.
     Therefore, thermoregulation and heat balance, in the context of cold environments, must be considered in two stages:  (1) retention of full physical and cognitive function, and (2) survival of the being, i.e. maintenance of core temperature at all costs.  These two objectives have very different thermal requirements.
     The second stage represents emergency situations, where circumstances are beyond control.  This presentation focuses on the first stage, as it is more-frequently applicable to occupational scenarios.  Successful management of heat balance in the first stage precludes the emergency responses of the second.
     The form of heat balance equation used in this series is
     S = M + W + C + R + K + E + Resp,
where S is heat storage rate, M is metabolic rate, W is work rate, C is convective heat transfer (convection), R is radiative heat transfer (radiation), K is conductive heat transfer (conduction), E is evaporative cooling (evaporation), and Resp is heat transfer due to respiration.  Each value is positive (+) when the body gains thermal energy (“heat gain”) and negative (-) when thermal energy is dissipated to the surrounding environment (“heat loss”).  Each term can be expressed in any unit of energy or, if time is accounted for, power, but consistency must be maintained.  The following discussion provides some detail on each component of the heat balance equation in the context of cold environments, contrasting with that of hot environments where it improves clarity.
     As discussed in Part 2, the hypothetical “perfect” equilibrium is attained at S = 0.  In a cold environment, two equilibria are possible, corresponding to the stages referenced above.  Here, S = 0 is the target average to maintain full function.  A person can suffer a loss of function when S < 0 for an extended period, trends downward, or becomes exceptionally low.
     The average “normal” core temperature is 98.6° F (37° C).  Ideally, the drop in core temperature will be limited to 97° F (36° C), the temperature below which most people suffer some loss of physiological function.  Increasing core temperature significantly above normal can also be hazardous if sweating is induced (discussed further below).  Oral temperatures in the 97 –99° F (36 – 37° C) range are safe for most people in most situations.
     The rate at which the body generates heat, M, varies with activity.  Precise values are difficult to obtain; representative estimates, such as those shown in Exhibit 2, are usually used instead.  The value of M when a person is at rest under normal conditions is called the basal metabolic rate (BMR).
     A person’s size and weight, growth stage, diet, fitness, and drug use can affect his/her metabolic rate.  A young, growing, physically-fit person has a higher BMR than a comparably-sized older adult.  A lower BMR must be compensated for by other means to maintain heat balance in a cold environment.
     When the combination of BMR and activity are insufficient, shivering may be induced, adding as much as 400W to a person’s metabolic rate.  Shivering is an involuntary, uncoordinated activation of skeletal muscles with the sole purpose of heat production.  It reduces further heat loss, but cannot generate sufficient heat to replace that already lost (i.e. raise body temperature).

     The work rate (W) represents the portion of energy consumed in the performance of work that is not converted to heat.  Many formulations of the heat balance equation exclude this negative (heat-reducing) term, deeming it safe to ignore, as it is usually less than 10% of M.  However, in critical situations, the work rate may gain significance.

     By definition, the air temperature in a cold environment is below safe body temperature.  Therefore, the convection (C) component is negative.  In a cold environment with significant air movement, convection causes the greatest heat loss.  Convective heat loss is combatted with insulating layers of clothing.  The most effective combination of clothing layers depends on several personal, environmental, and behavioral variables.  Choices must be made among breathable, waterproof, and zippered garments, boots, hats, gloves, and other options to match their performance with the environmental conditions and the work to be performed.  Any exposed skin increases convective heat loss.

     The radiation (R) term will also be negative in most cases.  In open air, in direct sunlight, for example, significant radiation may be received.  However, the insulating layers protecting against other forms of heat loss will likely prevent an appreciable heat gain.  In the vast majority of situations, it is those insulating layers that prevent radiative heat loss.  In the absence of insulation (i.e. clothing) and appreciable air movement, radiation causes the greatest heat loss.

     Heat loss via conduction (K) is typically associated with handling of tools and materials and walking on cold surfaces.  Gloves and boots appropriate for the type of work being performed should be worn to minimize this.  If dexterity requirements prohibit the use of gloves, appropriate work/warming cycles must be implemented to prevent loss of dexterity and cold injury.  Even when conductive heat loss is small relative to total heat loss, cold hands and feet can have a disproportionate effect on comfort and performance.

     Evaporation (E) remains a significant source of heat loss.  In cold stress situations, the goal is to minimize evaporative cooling; it cannot be eliminated and must be managed.  “Insensible water loss” continues in cold conditions; this moisture must be dissipated to avoid wetting clothing and degrading its insulating properties.  The challenge becomes greater if strenuous activity induces sweating.  Rapid heat loss due to the combined effect of reduced insulation and excessive evaporation could lower one’s body temperature to an uncomfortable, if not dangerous, level.  While the humidity in the surrounding air has a profound impact on evaporative cooling under conditions of heat stress, it plays no appreciable role in cold environments.

     Heat loss due to respiration (Resp) also requires management.  For example, a face covering that allows exhaled water vapor to dissipate while retaining some of the heat transferred to the air while in the lungs could be a helpful complement to a worker’s wardrobe.  In addition to any respiratory heat that may be recaptured, the heat loss due to convection and radiation are also lowered by the reduced skin exposure.  Perceived comfort may also improve significantly by warming the nose, cheeks, and ears.

     The analogous representation of the human body’s heat balance as a mechanical balance scale, as shown in Exhibit 3, continues to be a helpful visualization.  It is quite intuitive and identification of the diurnal range on the scale provides an important reminder that some variation in core temperature is normal and unrelated to our thermal work environments.
     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Thermal Work Environments” entries on “The Third Degree,” see Part 1:  An Introduction to Biometeorology and Job Design (17May2023).

[Link] Human Factors in Technology.  Edward Bennett, James Degan, Joseph Spiegel (eds).  McGraw-Hill Book Company, Inc., 1963.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “Hypothalamus” in Encyclopedia of Neuroscience.  Qian Gao and Tamas Horvath.   Springer, Berlin, Heidelberg; 2009.
[Link] “Thermal Indices and Thermophysiological Modeling for Heat Stress.”  George Havenith and Dusan Fiala.  Comprehensive Physiology; January 2016.
[Link] “Hypothermia and Cold Weather Injuries.”  Rick Curtis.  Princeton University Outdoor Action Program, 1995.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 6:  Measurement of Sound Exposure]]>Wed, 04 Oct 2023 06:09:39 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-6-measurement-of-sound-exposure     The measurement of sound pressure levels throughout a workplace is a fundamental component of noise-control and hearing-conservation initiatives.  It is the basis for exposure assessment and regulatory guidance.  Sound measurement and audiometry are opposite sides of the same coin.
     This installment of the “Occupational Soundscapes” series introduces basic concepts of sound level measurement and exposure assessment.  Equipment used, frequencies analyzed, calculation of a “dose,” and more are presented.  Like the presentation of audiometry (Part 5), its aim is to provide a level of understanding, within the constraints of this format, that engenders trust in an organization’s noise-related practices.
Types of Sound
     There are three types of sound to which a person may be exposed:  continuous, intermittent, and transient.  These are not unique to occupational settings, but understanding each is critical to defining an occupational soundscape.  Accurate definition of the soundscape is required to identify and implement appropriate noise-control and hearing-protection measures.
     Continuous or “steady-state” sound is nearly constant over long periods of time.  Generally, sound that varies less than ± 3 dB is considered continuous.  OSHA also treats sounds with level maxima at intervals up to one second as continuous.  Electric motor-driven equipment, fans, and turbines are common examples of continuous sound generators.  Continuous sounds are the most predictable and easily measured.  Thus, it is relatively straightforward to identify and implement appropriate countermeasures.
     Sounds that vary by more than ± 3 dB are termed intermittent.  This can include the “extreme” case of a sound being absent (“turned off”) at intervals during the measurement.  Less-extreme sound level variations are often called time-varying or fluctuating.  For example, automated equipment that performs a series of operations in a repeating cycle – workpiece transfers, pressurization and exhaust, enclosure closing and opening, clamping and unclamping, etc. – can generate highly-variable sound pressure levels (SPLs).
     Fluctuating sounds may or may not be predictable.  A single automated workcell may generate predictable sound levels as it executes highly-repeatable process cycles.  In contrast, the interaction of a variety of equipment, processes, and schedules can make predicting sound levels highly impractical.  Quantifying such a variable soundscape may require the use of averages or other sound level definitions for a “typical” workday.  For example, L10, L50, and L90 levels may be cited to describe the variation in the environment.  These correspond to the sound level exceeded during 10%, 50%, and 90%, respectively, of the measurement period.  For a “normal” workday, the notation L90,8hr, for example, may be used to specify the measurement period.  Varying sound levels may also necessitate the use of dose calculations (see “Exposure Indices,” below).
     Transient sounds exist only for a very brief period of time (often less than one second); these may also be called instantaneous sounds.  There are two types of transient sounds – impact and impulse (or impulsive) sounds.  Impact sounds are generated by the collision of two solid objects, such as parts falling into a bin.  Impulse sounds are generated by rapid releases of energy, such as explosions, gunfire, or pressurized fluid release (i.e. burst or relief).
     Transient sounds pose the greatest risk to hearing for three interrelated reasons.  They often occur at sufficiently-high levels (e.g > 140 dB) to cause immediate permanent hearing loss.  They are often unpredictable; therefore, proper protections may not be in place (e.g. use of HPDs) when they occur.  They are also more difficult to measure, defying the accurate definition needed to implement appropriate countermeasures.
     Results of some studies suggest that transient sounds exhibit a synergistic effect with continuous sound.  That is, adding transient sounds to a “background” of continuous sound has a greater effect on hearing than SPL addition might indicate.  This combined-sound scenario is common in industrial settings, reinforcing the need to pay special attention to transient sounds in the occupational soundscape.
Measured Frequencies
     Sound levels are measured in octave bands identified by their nominal center frequencies.  An octave band is a range of frequencies where the upper end is twice the lower end (fupper = 2 x flower Hz).  All sound transmitted at frequencies within an octave band are aggregated and associated with the band’s center, or nominal, frequency.  A “full” octave is called a 1/1 octave band; the “1/1” designation is often dropped, as it is assumed when absent.
     For a more-refined analysis of a soundscape, sound can be measured in 1/3 octave bands.  As the name implies, each octave is subdivided into three bands, each with its own center frequency to which measured sound levels correspond.  The 1/1 and 1/3 octave bands in common use for workplace noise assessments are tabulated in Exhibit 1.  Measurements can be conducted in narrower frequency ranges, with the right instrument, but the 1/1 and 1/3 octave bands should suffice until noise control practitioners gain experience and sophistication.
Frequency Weighting
     Several frequency-weighted sound level scales have been developed for various purposes.  Those relevant to hearing conservation programs are A-weighting and C-weightingA-weighting (dBA) is used to model human perception of loudness of sound.  C-weighting (dBC) is typically used in the assessment of hearing protection device (HPD) effectiveness.  Z-weighting (dBZ) is often used when measuring exposures to transient sounds.  It is an unweighted measurement scale (making it something of a misnomer); therefore, dBZ ≡ dB and the Z-weighting designation is often dropped.
     The A- and C-weighting factors are tabulated in Exhibit 2 and shown graphically in Exhibit 3.  Although 1/3 octave bands are shown, it is common practice to use 1/1 octave band (highlighted rows) measurements in workplace assessments.
     An example data set converted to A- and C-weighted sound levels is shown in Exhibit 4.  The sound level measurement in each frequency band is adjusted by the corresponding weighting factor.  To determine the overall weighted sound level, the sound levels at all frequencies are added (see “SPL Addition” in Part 4).  Comparing sound levels attained for the different scales demonstrates how significant weighting can be to assessment results.
     B-weighting (dBB) is also included in the example for comparison, though it is no longer in common use.  It was originally conceived of as a scale for “medium-level” sounds, but has failed to offer sufficient utility to justify its retention in addition to A- and C-weightingD-weighting (dBD) was developed for the unique characteristics of aircraft noise.  Its use is currently limited to non-bypass jet engines on military aircraft.  It is included in the graph of Exhibit 3, but precise weighting values are not readily available.
     If only A- and C-weighted sound levels (LA and LC, respectively) are known, some insight into the frequency composition of the sound can still be gained.  When higher frequencies are prevalent, LA > LC and when lower frequencies are prevalent, LA < LC.  When LA and LC are nearly equal, the sound is not dominated by frequencies at either end of the range.  Review the weighting factors in Exhibit 2 or the graph in Exhibit 3 to see why this is true.
Exposure Indices
     The magnitude of a person’s exposure to sound energy can be expressed in various ways.  The choice of exposure index may be influenced by several factors, such as:
  • measurement equipment in use,
  • purpose of the measurement; e.g. the standard to which compliance is sought,
  • type(s) of sounds to which a person is exposed,
  • perceived risk of exposure,
  • amount of measurement data attainable, and
  • characteristics of a person’s workday; e.g. assignment rotations, etc.
Some considerations relevant to the context of workplace noise assessments are explored below as the most-relevant indices are presented.
     Exposure to continuous sound is well-defined by a single measurement, utilizing A-weighting and expressed in dBA.  OSHA defines the maximum duration of exposure allowable at A-weighted SPLs from 80 – 130 dBA; this information is tabulated in Exhibit 5.  This “reference duration” can also be calculated:
The valid range for this calculation is also 80 – 130 dBA.  Below 80 dBA, indefinite exposure is acceptable; there is no appreciable risk to hearing.  Continuous exposures above 130 dBA are impermissible for any duration (TOSHA, >130dBA = 0).
     Other organizations determine the reference duration with different parameters.  The generic form of the reference duration calculation is:
  • Tn = reference (i.e. permissible) duration of exposure at LA, hours;
  • Tc = duration of exposure, hours;
  • LA = A-weighted sound level of exposure, dBA;
  • Lc = criterion level, dB;
  • ER = exchange rate, dB.
     The criterion level is the permissible sound level exposure for a “standard” 8-hour workday.  OSHA’s criterion level is 90 dBA, while NIOSH sets it at 85 dBA.
     The exchange rate is also called the “doubling rate.”  It is the increase in sound level exposure permissible when exposure duration is halved.  Alternatively, it is the decrease in sound level exposure required to double the permissible exposure duration.  OSHA’s exchange rate is 5 dB, while NIOSH prescribes a 3-dB ER.
     NIOSH, in its “Criteria for a Recommended Standard,” provides some history of ER development and the current disparity between its specified exchange rate and that of OSHA.  NIOSH demonstrates how a 3-dB ER accurately reflects the amount of sound energy a person is exposed to when conditions change.  That is, the exchanged time and sound level are shown to be equivalent in energy terms.  This equivalency is known as the equal-energy rule.
     Intermittent sounds are not fully defined by a single measurement.  The L10, L50, and L90 levels introduced previously can provide significant information about a soundscape, but its definition remains incomplete.  To fully account for the entirety of exposure, a person’s “noise dose” can be determined.  Doing so can require a large number of measurements if a person’s exposure changes frequently during a workday.
     A person’s noise dose, D, is calculated as follows:
where Ci is the duration of exposure at a specific sound level and Ti is the reference duration at that sound level.  The OSHA reference duration calculation is shown above; for NIOSH, use the generalized formula with Lc = 85 dB and ER = 3 dB.
     A 100% noise dose indicates that a person’s average exposure to sound levels equals the permissible exposure limit (PEL).  It is important to remember that a 100% dose may be reached long before the workday has ended!
     If the variations in a person’s exposure are cyclical and, therefore, predictable, the anticipated daily noise dose can be determined with fewer measurements.  For example, a worker performs a series of tasks that require two hours to complete and repeats the pattern four times per workday.  The person’s exposure data for one 2-hour cycle can be extrapolated to an anticipated 8-hour dose using the dose rate, DR:
For our example, the worker receives a 25% dose in 120 minutes; DR = 0.21%/min.  Multiplying by the full duration of the workday (480 min) yields a 100% dose (rounding error may be introduced).  Use of dose rates can significantly reduce the monitoring workload in predictable soundscapes.
     A time-weighted average (TWA) may be a more intuitive index.  To convert a daily dose to an 8-hour TWA, the following calculation is performed:
The exchange rate (ER) and criterion level (Lc) must be consistent with the standard used to determine the dose (e.g. NIOSH, OSHA, etc.).  At 100% dose, TWA = Lc.  OSHA provides a table of converted values (see Exhibit 6) that may expedite TWA determinations.
     An alternative representation of a person’s sound exposure is provided by the equivalent sound exposure level, LAeq,T, known in recent standards as the equivalent continuous sound level, LAT.  Conceptually, LAT is the logarithm of the ratio of sound exposure to exposure duration.  Exposure can be integrated over any time period; when the measurement period is eight hours, LAT ≡ TWA.
     The pressure changes associated with transient sounds can occur so rapidly that the sound exposure can be integrated over a one-second time period.  This “equivalent” measure is called the sound exposure level (SEL).  Integrating the same exposure over an 8-hour time period yields an Leq that is 44.6 dB lower than the SEL (SEL = Leq,8hr + 44.6 dB), an indication of the ferocity of transient sounds.
Sound-Measurement Equipment
     Development of sound-measurement equipment over several decades has provided sophisticated measurement and analysis tools to industrial hygienists and researchers alike.  A thorough treatment of these instruments and their capabilities is beyond the scope of this series.  However, an introduction to the types of equipment commonly used to support hearing conservation in occupational settings could prompt participants to conduct necessary research and enlist the assistance of knowledgeable and experienced specialists.
     Due to their use in musical and theatrical performances, public address systems, and other applications, microphones seem familiar to most people.  However, few understand the principles of operation or why a particular microphone is used in each application.
     Three types of microphones may be used in sound measurement, the choice depending on the nature of the sound field.  Proper orientation of a microphone in a sound field is defined by the type in use; a pictorial summary is provided in Exhibit 7.
     A direct-incidence microphone is placed parallel to the sound wave’s direction of travel, or “pointing at” the sound source.  This type is also known as a free-field or normal-incidence microphone.  This type of microphone is well-suited for use with a single sound source and in the absence of reflections.
     A pressure microphone is positioned perpendicular to the sound wave’s travel.  For this reason, this type is sometimes called a grazing microphone.  Pressure microphones are often used in calibration of audiometric equipment or flush-mounting in a wall, baffle, or similar surface.
     A random-incidence microphone should be oriented at approximately 70° from the primary sound source’s wave propagation direction.  This type is also called an omni-directional microphone to describe its ability to process multiple sound sources and reflections simultaneously.  It is this capability that makes them well-suited for many industrial environments.
     In addition to its type, a microphone’s physical characteristics (e.g. diameter) can determine its suitability to a particular application.  Exhibit 8 demonstrates this by comparing the frequency responses of several microphones.  To get the best results, properties of the sound field and measurement equipment must be compatible.
Sound Level Meters
     Though presented separately, above, microphones are typically integral to sound level meters (SLMs) used in occupational noise assessments.  These are usually handheld devices that also carry a display and setup capabilities onboard.  This discussion of SLMs is somewhat superficial by necessity.  The technology behind their operation is far beyond the scope of this series; here, the focus is on basic functionality and operator interface.
     Two classes of SLM are defined in current standards.  Class 2 devices are “general purpose” units and may suffice for preliminary assessments and other applications with less-stringent requirements.  Class 1 devices are precision instruments with higher performance characteristics.  A comparison of specifications for Class 1 and Class 2 SLMs is provided in Exhibit 9.
     Preparing an SLM for an assessment consists of selecting several parameters according to the sound field to be studied and the purpose of the study.  Modern SLMs typically offer A-, C-, and Z-weighting options (see “Frequency Weighting,” above) to accommodate a variety of needs.  The user can also choose to make 1/1 or 1/3 octave band measurements.  Some models offer additional octave band options to refine measurements further.
     The time constant (τ) to be used for measurements must also be selected.  An SLM’s time constant is a measure of its responsiveness to changes in sound levels; the shorter the time constant, the more rapidly the instrument responds to changes in the input.  The time constant is defined as the time required for a response curve to reach 63% of the maximum value of a step change in the input.
     In the occupational noise context, a slow (S) time constant (τ = 1 s) is typically used to smooth the response curve when the input is highly variable.  A fast (F) time constant (τ = 0.125 s) is used when the range of sound levels (i.e. min, max) is of greatest interest.  If fast-response (F) readings fluctuate more than ±4 dB, the relative stability of slow-response (S) readings may be preferable.
     Two other time constants are considered “legacy” settings and may not be available on all instruments.  The impulse (I) time constant (τ = 0.005 s) is well-suited for measurements of impact noises.  The peak hold (τ = 0.00005 s) setting is used to capture the maximum level of extremely-short-duration transient sounds, such as gunshots.
     Compliance with a standard will dictate some or all of the settings and equipment types needed.  Understanding the context of the noise assessment is critical to obtaining useful results.
     A dosimeter is, in essence, a sound level meter with additional computing capability and a modified physical form.  Either miniaturization of the instrument or separation of the microphone from the body of the unit makes it feasible for a person to wear a dosimeter for an entire workday without interfering with normal activities.  Positioning a microphone in a worker’s “hearing zone” – within 12 in (30.5 cm) of the ear canal – is untenable with a standard SLM.
     The requirements of various standards (OSHA, NIOSH, etc.) may be preprogrammed in the dosimeter; assessment per such standards are as simple as a setting selection.  Some units allow custom profiles, called virtual dosimeters, to be programmed; various exchange rates, criterion levels, and threshold levels can be entered manually.  The threshold level is the sound level below which the instrument does not integrate the exposure.  Stated another way, sounds below the threshold level are not included in the noise dose calculation.
     The large amount data that is often required for dose calculations is held in onboard memory until it can be transferred to a computer for further analysis or long-term retention.  Lengthy measurement periods, in addition to onboard data processing, make battery life another important factor for consideration in the selection of devices.
     Frequency analyzers, spectrum analyzers, real-time analyzers (RTAs), and similarly-named instruments are also variations on the SLM theme.  Narrower octave bands, simultaneous measurements, graphical displays, and other features support advanced analysis.  Further exploration of these devices here is unwarranted; readers are encouraged to investigate the capabilities of these instruments after mastering the foundational elements of sound measurement and hearing conservation covered in this series.
     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] “Criteria for a Recommended Standard - Occupational Noise Exposure, Revised Criteria 1998.”  Publication No. 98-126, NIOSH, June 1998.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “OSHA Technical Manual (OTM) - Section III: Chapter 5 - Noise.”  Occupational Safety and Health Administration; July 6, 2022.
[Link] ”29 CFR 1910.95 - Occupational noise exposure.’  OSHA.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications, 1996.
[Link] “The Impact of Threshold, Criterion Level and Exchange Rate on Noise Exposure Data Results.”  TSI Incorporated; 2020.
[Link] “Noise and Vibration.”  Evan Davies in Plant Engineer’s Reference Book, 2ed.  Dennis A. Snow, ed.  Reed Educational and Professional Publishing Ltd.; 2002.
[Link] “Sound level meter.”  Wikipedia.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 5:  Audiometry]]>Wed, 20 Sep 2023 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-5-audiometry     Audiometry is the measurement of individuals’ hearing sensitivity using finely-regulated sound inputs.  It is a crucial component of a hearing loss prevention program (HLPP) with an emphasis on the range of frequencies prevalent in speech communication.  To be valid, audiometric testing must be conducted under controlled conditions and the results interpreted by a knowledgeable technician or audiologist.
     This installment of the “Occupational Soundscapes” series provides an introduction to audiometry, requirements for equipment, facilities, and personnel involved in audiometric testing, and the presentation and interpretation of test results.  It targets, primarily, those enrolled in – as opposed to responsible for – an HLPP.  Its purpose is to develop a basic understanding of a critical component of hearing conservation efforts to, in turn, engender confidence in the administration of procedures that may be foreign to many who undergo them.
The Audiologist
     Audiometric testing is conducted by an audiologist, audiometric technician, audiometrist, or physician.  Distinctions among these roles are not important to the present discussion; therefore, the term audiologist will be applied to any competent administrator of audiometric tests.
     Demonstration of competency in audiometric testing is typically achieved by attaining certification from the Council for Accreditation in Occupational Hearing Conservation (CAOHC) or equivalent body.  Physicians, such as otolaryngologists, are certified by their respective medical boards.
     The Occupational Safety and Health Administration (OSHA) requires that audiometric testing be administered by a licensed or certified audiologist, physician, or technician capable of obtaining valid audiograms (see “The Results,” below) and maintaining test equipment in proper working order.  OSHA does not require technicians operating microprocessor-controlled (i.e. automated) audiometers (see “The Equipment,” below) to be certified, but the National Institute for Occupational Safety and Health (NIOSH) rejects this exemption.
The Facility
     Audiometric testing is typically conducted in one of three types of test facility – onsite, mobile, or clinical.  Each has unique characteristics that must be considered to determine which is best-suited to an organization and its HLPP.
     An onsite test facility utilizes dedicated space within an organization where an audiometric test booth is permanently installed.  An onsite facility is typically feasible only for large organizations with more than 500 noise-exposed employees enrolled in an HLPP at a single location.  Dedicated facilities often require full-time professional staff, further limiting the range of organizations for which onsite facilities are appropriate.
     Mobile test facilities may be provided by a third-party contractor to support an organization’s HLPP.  This may be an appropriate solution for an organization with multiple operations throughout a region when the number of employees enrolled in the HLPP at each location is relatively small.
     A clinical test facility is an independent medical or occupational health practice.  Employees schedule audiometric tests as they would an eye exam, annual physical checkup, or other outpatient procedure.  For smaller entities or programs, this is often the most practical choice.  Administration by an independent brick-and-mortar medical practice may also increase employees’ confidence in the HLPP, providing a psychological benefit that is difficult to quantify.
     The facility, regardless of the type chosen, must be sufficiently isolated to prevent interference with audiometric testing.  Vibrations, ambient noise, and distracting sounds must be minimized to ensure a valid audiogramMaximum Permissible Ambient Noise Levels (MPANLs) are defined in standards and regulations (e.g. ANSI S3.1, CFR29 Part 1910.95) for various types of test equipment.  It is important to note that sounds below the required MPANL, such as phone alerts, conversation, or traffic, can still be distracting and should be avoided.
The Equipment
     The two pieces of equipment most relevant to this discussion are the audiometer and the earphone.  There are three types of audiometer that may be encountered in an HLPP – manual, self-recording, and computer-controlled.  In the context of occupational hearing conservation, pure-tone, air-conduction audiometers are used; other types (e.g bone-conduction) may be utilized for advanced analysis and diagnosis.
     Using a manual audiometer, the audiologist retains control of the frequency, level, and presentation of tones and manually records results.  This is the least sophisticated, thus least expensive, type of audiometer.  It is also the most reliant upon an audiologist’s skill and concentration.
     A self-recording, or Békésy audiometer (named for its inventor) controls the frequency and level of tones, varying each according to test-subject’s responses; test results can be difficult to interpret.  This type of audiometer is no longer in common use in occupational HLPPs; its use is more common in research settings where its finer increments of tone frequency and level control are advantageous.
     Computer-controlled audiometers are prevalent in modern practice.  Continually-advancing technology has improved reliability and added automated functions, such as data collection, report generation, and test interruption for excessive ambient noise.  Stand-alone units may be called microprocessor audiometers; they also perform automated tests, but have fewer capabilities and cannot be upgraded as easily as software residing on a PC.
     There are also three types of earphone available for audiometric testingsupra-aural, circumaural, and insert.  A more-precise (“technical”) term for an earphone is “transducer;” “headset” or “earpiece” is more colloquial.
     Supra-aural earphones consist of two transducers, connected by a headband, that rest on the test subject’s outer ears; no seal is created.  Therefore, little attenuation is provided, requiring increased diligence in control of ambient sounds.
     Circumaural earphones consist of two transducers, housed in padded “earmuffs” that surround the ears, connected by a headband.  The seal provided by the earmuffs, though imperfect, provides significantly greater attenuation of ambient sound than supra-aural earphones.
     Insert earphones consist of flexible tubes attached to foam tips that are inserted in the ear canal.  The foam tip seals the ear canal, providing the greatest attenuation of ambient sound.  Test tones are delivered directly to each ear via the flexible tubes; the lack of physical connection between the transducers reduces the opportunity for transmission of tones from the tested ear to the “silent” ear.
     Some test subjects may experience discomfort, particularly when using insert earphones, which could lead to distraction that influences test results.  Recognizing signs of discomfort, distraction, or other interference is among the required skillset of an effective audiologist.
     Evidence suggests that the choice of earphone does not significantly affect test reliability.  However, earphones and audiometers are not interchangeable; an audiometer must be calibrated in conjunction with a paired earphone to provide valid test results.
The Test
     A typical audiometric test does not evaluate the entire frequency range of human hearing capability (~20 ~ 20,000 Hz).  Instead, the focus of testing is on the range of critical speech frequencies introduced in Part 2 of the series.  Specific test frequencies used are 500, 1000, 2000, 3000, 4000, and 6000 Hz.  Testing at 8000 Hz is also recommended for its potential diagnostic value; testing at 250 Hz may also be included.
     Each ear is tested independently by delivering pure tones at each frequency and varying levels, usually in 5 dB increments.  The minimum level at which a subject can hear a tone a specified proportion of the times it is presented (e.g. 2 of 3 or 3 of 5) is the person’s hearing threshold at that frequency.  Consecutive tests indicating thresholds within ±5 dB are typically treated as “consistent,” as this level of variability is inherent to the test.
     A single audiometric test may identify a concern, but multiple tests are needed to identify causes and determine appropriate actions.  The first test conducted establishes the subject’s baseline hearing sensitivity.  The subject should limit exposure to less than 80 dB SPL for a minimum of 14 hours prior to a baseline test, without the use of hearing protection devices (HPDs).  Some test protocols reduce the quiet period to 12 hours minimum or allow use of HPDs, but an extended period of “unprotected rest” is preferred.
     A baseline test is required within 6 months of an employee’s first exposure to the loud environment, though sooner is better.  Ideally, a baseline is established prior to the first exposure, thus eliminating any potential influence on the test results.
     Monitoring tests are conducted annually, at minimum.  They are often called, simply, annual tests, though more frequent testing is warranted, or even required, in some circumstances.  Monitoring tests are conducted without a preceding “rest” period, at the end of a work shift, for example.  Doing so provides information related to the effectiveness of HPDs, training deficiencies, etc.
     A retest is conducted immediately following a monitoring test indicating a 15 dB or greater hearing loss in either ear at any of the required test frequencies.  This is done to correct erroneous results caused by poor earphone fitment, abnormal noise intrusions, or other anomaly in the test procedure.
     A confirmation test is conducted within 30 days of a monitoring test indicating a significant threshold shift (discussed further in “The Results,” below).  Confirmation test protocols mimic those of a baseline test to allow direct comparison.
     Exit tests are conducted when an employee is no longer exposed to the loud environment.  This may also be called a transfer test when the cessation of exposure is due to a change of jobs within the organization, rather than termination of employment.  Exit test protocols also mimic those of a baseline test, facilitating assessment of the impact of working conditions over the course of the subject’s entire tenure.
The Results
     The results of an audiometric test are recorded on an audiogram; a blank form is shown in Exhibit 1.  Tone frequencies (Hz) are listed on the horizontal axis, increasing from left to right.  On the vertical axis, increasing from top to bottom, is the sound intensity level scale (dB).  This choice of format aligns with the concept of hearing sensitivity; points lower on the chart represent higher intensity levels required for a subject to hear a sound and, thus, lower sensitivity to the tested frequency.
     The audiogram shown in Exhibit 2 places examples of familiar sounds in relative positions of frequency and intensity.  Of particular interest is the “speech banana” – the area shaded in yellow that represents typical speech communications.  Presented this way, it is easy to see why differentiating between the letters “b” and “d” can be difficult.  These letters hold adjacent positions at the lower end of the speech frequency range, where several other speech sounds are also clustered.  This diagram also reinforces the idea that the ability to hear chirping birds and whispering voices are among the first to be lost; they are high-frequency, low-intensity sounds.
     Visual differentiation of data for each ear is achieved by using symbols and colors.  Each data point for a subject’s left ear is represented by an “X,” while each data point for the right ear is represented by an “O.”  Colors are not required; when they are used, the convention is to show left-ear data in blue and right-ear data in red.  The increased visual discrimination facilitates rapid interpretation of test results, particularly when all data for a subject are shown in a single diagram.  When baseline data are shown on a monitoring audiogram, the baseline data is typically shown in grey to differentiate between historical and current test data.
     The vertical scale represents a person’s hearing threshold – the minimum sound intensity level required for the test tone to be heard.  An example audiogram, representing “normal” hearing using the formatting conventions described above, is shown in Exhibit 3.  Sound stimuli above the line on the audiogram are inaudible; only those on or below the line can be heard by the subject.  Widely-accepted definitions of the extent of hearing loss are as follows:
  • normal hearing:  < 20 dB hearing level;
  • mild hearing loss:  20 – 40 dB hearing level;
  • moderate hearing loss:  40 – 70 dB hearing level;
  • severe hearing loss:  70 – 90 dB hearing level;
  • profound hearing loss:  > 90 dB hearing level.
     The example audiogram in Exhibit 4 also demonstrates the use of symbols and colors to differentiate data, though the dual-chart format makes it less critical.  The data is also tabulated to provide precise threshold levels for each frequency.
     A significant drop in sensitivity, in both ears, at 4000 Hz is depicted in Exhibit 4.  This is the infamous “4K notch,” indicative of noise-induced hearing loss (NIHL).  The appearance of this notch or other deviation from normal hearing should elicit an appropriate response.
     The presence of a notch in a baseline audiogram suggests that permanent hearing loss has already occurred.  Appropriate measures must be taken to ensure that no further damage occurs.  Furthermore, additional assessments may be necessary to ensure that the subject’s abilities are compatible with the work environment.  If diminished communication abilities creates a hazard for the subject or others, for example, an appropriate reassignment should be sought.
     The appearance of a notch or other decline in hearing sensitivity in a monitoring audiogram should trigger follow-up testing.  A retest is conducted to ensure the validity of the data by verifying that the facility and equipment are operating within specifications and the test was conducted properly by both the subject and audiologist.  NIOSH recommends retesting when a monitoring audiogram indicates a 15 dB or greater increase in hearing level, relative to the baseline audiogram, at any frequency from 500 to 6000 Hz.
     If the monitoring and retest audiograms are consistent, two parallel paths are followed.  On one path, the subject undergoes a confirmation test to determine if the indicated hearing loss is permanent.  Appropriate follow-up actions are determined according to the results of this test.
     On the other path, HPD use and effectiveness is reviewed to determine necessary changes to the individual’s work process or to the HLPP more broadly.  Other changes to the work environment may also be necessary; noise-control strategies will be discussed further in future installments of this series.
     The decline in hearing sensitivity represented by a lower line on an audiogram is called a threshold shift.  When the arithmetic average of the differences between the baseline and monitoring audiograms at 2000, 3000, and 4000 Hz exceeds 10 dB in either ear, a standard threshold shift (STS) has occurred.  An STS is depicted in the comparative audiogram of Exhibit 5; calculation of the shift’s magnitude is shown in the table.
     If the change in hearing sensitivity is shown by confirmation testing to be irreversible, a permanent threshold shift (PTS) has occurred.  Some level of hearing loss is recoverable with “rest” in a quiet setting.  This change is called a temporary threshold shift (TTS).  Appropriate action must be taken to prevent a TTS from becoming a PTS.
     A baseline audiogram represents a person’s “best-case” hearing or maximum sensitivity.  Therefore, if subsequent testing results in a “better” audiogram than the baseline, the baseline is replaced by the new audiogram.  This can occur if influences on the baseline test were not noticed or properly addressed.  Examples include an insufficient rest period preceding the test, intrusive noise or vibration in the test chamber, and suboptimal earphone fitment.
     Results other than a pronounced 4K notch can also prompt additional testing.  The series’ focus remains on NIHL; therefore, only a brief overview will be provided here.  Interested readers are encouraged to consult other sources for additional information.
     Bone-conduction testing is performed with transducers placed behind the ears.  This type of test may be warranted to diagnose an occlusion of the ear canal, which can include impacted cerumen (“earwax”), or other condition of the outer or middle ear that limits air-conducted hearing.  Conductive hearing loss is suggested by differences between air- and bone-conduction thresholds of greater than 10 dB.  An example audiogram depicting this condition in one ear is shown in Exhibit 6.
     A positively-sloped audiogram, such as that shown in Exhibit 7, depicts higher sensitivity to higher frequencies, often indicative of a disorder of the middle or inner ear.  In the case of Meniere’s disease, for example, audiometric testing may be used to validate a medical diagnosis, whereas the reverse is often true for other conditions.
     A negatively-sloped audiogram, such as that shown in Exhibit 8, depicts lower sensitivity to higher frequencies, often indicative of the advancement of presbycusis (age-related hearing loss).  Guidance on the appropriate use of an audiogram of this nature in the context of an HLPP varies.  A non-mandatory age-adjustment procedure remains in the OSHA standard (CFR 29 Part 1910.95 Appendix F), though NIOSH has rescinded support for the practice of “age correction”.  Organizations utilizing age-adjusted audiograms should consider that OSHA regulations tend to follow NIOSH recommendations; the lag on this specific matter has been quite long already.
The Bottom Line
     Noise-induced hearing loss (NIHL) is the accumulation of irreparable damage to the inner ear, particularly the fine hairs of the cochlea (see Part 2).  Hearing loss usually occurs in higher frequencies first.  The focus of audiometric testing on speech communication leads us to define “high frequency” as the 3000 – 6000 Hz range, where the 4K notch is of particular concern.  Hearing loss in frequencies above 8000 Hz often go undiagnosed as the highest frequencies in the audible range are rarely tested.
     NIHL is one of several possible causes of hearing impairment.  Other causes include hereditary conditions, exposure to ototoxic substances, and illness (i.e. infection).  The various audiometric tests are valuable tools beyond the scope of NIHL; they can also aid diagnosis of several other conditions.  For example, a baseline audiogram may confirm the presence of a congenital disorder, or a confirmation test may reveal that an STS was caused by an illness from which, in the interim, the subject had recovered.
     A thorough, well-crafted health and wellness program will include audiometric testing.  In addition to the direct benefits of an HLPP, information about other conditions may also be obtained, further improving the work environment.  Psychological well-being of employees can be improved via increased effectiveness of verbal and nonverbal communication, in addition to the physical health benefits that participation in such a program can provide.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] “Criteria for a Recommended Standard - Occupational Noise Exposure, Revised Criteria 1998.”  Publication No. 98-126, NIOSH, June 1998.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “OSHA Technical Manual (OTM) - Section III: Chapter 5 - Noise.”  Occupational Safety and Health Administration; July 6, 2022.
[Link] ”29 CFR 1910.95 - Occupational noise exposure.’  OSHA.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications, 1996.
[Link] “Pediatric Audiology:  A Review.”  Ryan B. Gregg, Lori S. Wiorek, and Joan C. Arvedson.  Pediatrics in Review, July 2004.
[Link] “Familiar Sounds Audiogram:  Understanding Your Child’s Hearing.”  Hearing First, 2021.
[Link] “Hearing and Speech.”  University of California – San Francisco, Department of Otolaryngology – Head and Neck Surgery.
[Link] “Audiograms.”  ENT Education Swansea.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 4:  Sound Math]]>Wed, 06 Sep 2023 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-4-sound-math     Occupational soundscapes, as outlined in Part 1, are comprised of many sounds.  Each has a unique source and set of defining characteristics.  For some purposes, treating all sounds in combination may be appropriate.  For others, the ability to isolate sounds is integral to the purpose of measuring sound levels.
     Of particular importance to a hearing loss prevention program (HLPP) is the ability to add, subtract, and average contributions to the sound pressure level (LP, SPL) in a workplace.  The ratios and logarithms used to calculate SPLs, presented in Part 3, complicate the arithmetic, but only moderately.  This installment of the “Occupational Soundscapes” series introduces the mathematics of sound, enabling readers to evaluate multiple sound sources present in workers’ environs.
     As mentioned in Part 3, sound pressure is influenced by the environment.  The number of sources, the sound power generated by each, and one’s location relative to each source contribute to the sound pressure level to which a person is subjected.

SPL Addition
     A representative example of a typical application will be used to place SPL addition in context.  This should make it easier to understand the process and its value to hearing conservation efforts.  Consider a manufacturing operation with several machines running in one department.  The company’s industrial hygienist has tasked the department manager with reducing the noise to which operators are exposed.  With a capital budget insufficient to replace machines or make significant modifications to the facility, the manager concludes that the only feasible option is to schedule work within the department such that, at all times, some machines remain idle (i.e. quiet).  To determine a machine schedule that will yield acceptable noise exposures while meeting production demands, SPLs generated by each machine are added in various combinations.
     A baseline for comparison must be established to evaluate sound-level reduction results.  The baseline SPL includes sounds from all sources and can be established by either the formula method or the table method.
     Using the formula method, the total (i.e. combined) SPL generated by n sources is calculated with the following equation:
where LPt is the total SPL and LPi is the SPL generated by the ith source.  The calculation of total SPL for our example, which includes five machines, is tabulated in Exhibit 1, where it is found to be 96.8 dB.
     To use the table method, first sort the source SPLs in descending order.  Compare the two highest SPLs and determine the difference.  Find this value in the left column of the table in Exhibit 2 and the corresponding value in the right column.  Interpolation may be necessary, as only integer differences are tabulated.  Add the value from the right column to the higher SPL to obtain the combined SPL to be used in subsequent iterations.
     Compare the combined SPL to the next source in the sorted list, repeating the process described until all source SPLs have been added or they no longer contribute to the total SPL.  When adding a large number sources, sorting SPLs first may allow the process to be abbreviated; once the difference between the combined SPL and the next source exceeds 10 dB, the remainder of the list need not be considered.
     A pictorial representation of the cascading calculations performed in the table method of SPL addition is provided in Exhibit 3, where the total SPL for our example is found to be 96.5 dB.  This result differs slightly from that attained by the more-precise formula method, but this need not be a concern.  The reduced complexity of computation often justifies the sacrifice of accuracy.  A 0.3 dB difference, like that found in our example, is imperceptible to humans and is, therefore, inconsequential.  While some circumstances warrant use of the formula method, the table method of SPL addition provides a convenient estimate without the need for a calculator.
     The department manager proposes running the machines in two groups – machines 1, 3, and 5 will run simultaneously, alternating with machines 2 and 4.  Total SPL calculations for each machine grouping, using the formula method and the table method, are shown in Exhibit 4 and Exhibit 5, respectively.
     Total SPL results are the same for both methods – 93.3 dB for machine group (1, 3, 5) and 94.3 dB for machine group (2, 4).  This represents 3.5 dB and 2.5 dB reductions, respectively, from the baseline SPL (all machines running).  These are consequential reductions in noise exposure; the proposal is accepted and the new machine operation schedule is implemented.  The total SPL remains high, however, and further improvements should be sought.

SPL Subtraction
     To determine the SPL contribution of a single source, the “background” SPL is subtracted from the total.  Like SPL addition, there are two methods.
     Consider the 5-machine department presented in the SPL addition example; for this example, the SPL attributed to machine 3 is unknown.  With machine 3 turned off, the total SPL in the department is 95.5 dB; this is considered the background level with respect to machine 3.  Recall that the total SPL with all machines running is 96.8 dB.
     To subtract the background SPL by the formula method, use the following equation:
where LPi is the SPL of the source of interest, LPt is the total SPL (all machines running), and LPb is the background SPL (source of concern turned off).  In our example, LP3 = 10 log (10^9.68 – 10^9.55) = 90.9 dB.  The result can be verified using SPL addition:  LPt = 10 log (10^9.55 + 10^9.09) = 96.8 dB.
     To determine the SPL attributed to machine 3 by the table method, find the difference between the total and background SPLs (96.8 dB – 95.5 dB = 1.3 dB) in the left column of the table in Exhibit 6.  Subtracting the corresponding value in the right column of the table (~ 6.0 dB) from the total SPL gives the machine 3 SPL (96.8 dB – 6.0 dB = 90.8 dB).  Again, interpolation causes a small variance in the results that remains inconsequential.

SPL Averaging

     Measurements may be repeated across time or varying conditions.  In our 5-machine department example, this may be to document different machine combinations or sounds generated during specific operations.  In the latter scenario, an average SPL may be a useful, though simplified, characterization of the environment.
     SPLs are averaged using the following formula:
where n is the number of measurements to be averaged and LPi is an individual measurement.
     As an example, the SPLs of the 5-machine department example will be reinterpreted as multiple measurements in a single location.  Averaging the five SPL values (88.0, 92.5, 91.0, 89.5, and 83.5 dB) using the equation above gives LPavg = 88.9 dB.  When the range of SPLs to be averaged is small (e.g. < 5 dB), the arithmetic average can be used to approximate the decibel average.  The arithmetic and decibel average calculations for this example are shown in Exhibit 7.  Arithmetic averaging provides a convenient estimation method, but the decibel average should be calculated for any “official” purpose, as the two rapidly diverge.

     In the coming installments of the “Occupational Soundscapes” series, the connections between previous topics and hearing conservation begin to strengthen.  The discussion of audiometry brings together the physiological functioning of the ear (Part 2), speech intelligibility (introduced in Part 2), and the decibel scale (Part 3) to lay the foundation of a hearing loss prevention program.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] An Introduction to Acoustics.  Robert H. Randall.  Addison-Wesley; 1951.
[Link] “OSHA Technical Manual (OTM) - Section III: Chapter 5 - Noise.”  Occupational Safety and Health Administration; July 6, 2022.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications, 1996.
[Link] “Noise Navigator Sound Level Database, v1.8.” Elliot H. Berger, Rick Neitzel, and Cynthia A. Kladden.  3M Personal Safety Division; June 26, 2015.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 3:  The Decibel Scale]]>Wed, 23 Aug 2023 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-3-the-decibel-scale     In all likelihood, readers of this series have encountered the decibel scale many times.  It may have been used in the specifications of new machinery or personal electronic devices.  Some may be able to intuit the practical application of these values, but it is likely that many lack knowledge of the true meaning and implications of the decibel scale.
     This installment of the “Occupational Soundscapes” series introduces the decibel (dB) and its relevance to occupational noise assessment and hearing conservation.  Those with no exposure to the scale and those that have a functional understanding, but lack foundational knowledge, benefit from understanding its mathematical basis.  The characteristics of sound to which it is most-often applied is also presented to continue developing the knowledge required to effectively support a hearing loss prevention program (HLPP).
The Decibel
     Two key characteristics of the decibel scale define its use and contribute greatly to its lack of common understanding.  First, it is a logarithmic scale.  Linear scales are more common, which may lead those unfamiliar with the decibel scale to assume it, too, is linear.
     Second, the decibel scale is a comparative measure, incorporating the ratio of the measured quantity to a reference value.  Absolute scales are more common, potentially leading to another erroneous assumption.
     Making either assumption leads to gross misinterpretation of the information provided by cited values.  Mathematically, the general expression of the decibel scale is:
(all logarithms cited are base 10, log10, unless otherwise specified).  The multiplication factor of 10 converts Bels to decibels.  One Bel is defined as the increase corresponding to a tenfold increase in the ratio of values.  A decibel (dB) is, therefore, one tenth of a Bel.  The nature of the scale yields a dimensionless value that is valid for any system of units.

Sound Parameters
     To use the decibel scale effectively, in the context of occupational soundscapes, the interrelationships of power, intensity, and pressure must be understood.  Differentiating these measures is critical to understanding the true nature of the sound environment under scrutiny.
     Sound power (W), measured in watts (W), is the amount of acoustical energy produced by a sound source per unit time.  It is a characteristic of the source and is, therefore, independent of its location or surroundings.  In this discussion, it is assumed that sounds are generated by point sources, with sound dispersing spherically; variations will be introduced later.
     Sound intensity (I), measured in watts per square meter (W/m^2), is the sound power per unit area.  It is dependent on location, as it accounts for the dispersion of sound energy at a specified radial distance from the source:
The equation reveals that intensity decreases with the square of the distance from the source.  This inverse square law is depicted in Exhibit 1.
     Sound intensity, I, is a vector quantity.  In free-field conditions, however, the lack of obstructions and reflecting surfaces renders the specification of direction moot.  The intensity at a given distance from the source is equal in all directions.
     Sound pressure (P), measured in newtons per square meter (N/m^2) or, equivalently, Pascal (Pa), is the variable air pressure (force per unit area) superimposed on atmospheric pressure.  Propagation of pressure fluctuations as sound waves was introduced in Part 2; root mean square pressure (PRMS) is typically used.  Sound pressure is an effect of sound power generated by a source; it is influenced by the surrounding environment and distance from the source.
     Of the three parameters described, only pressure can be measured directly.  With adequate pressure data, however, it is possible to work backwards to obtain intensity and power values.  To do this, first calculate the RMS pressure of the sound wave.
     Sound intensity is calculated using the following formula:
where P is the RMS pressure (Pa), ρ is the density of air, and c is the speed of sound in air.
     At standard conditions, ρ = 1.2 kg/m^3; though the density of air varies, this approximation provides sufficient accuracy for most purposes.  Likewise, the approximation of c = 343 m/s will typically suffice.
      With the intensity at a known distance from the source, calculating sound power is simple:
where A is the spherical area at distance r (A = 4 Π r^2).
Sound Levels
     In the previous section, sound power, intensity, and pressure were discussed in absolute terms.  More often, however, these measures are referenced by their levels, using the decibel scale.  Doing so makes the very wide range of values encountered more manageable.
     The sound power level (LW or PWL) is calculated using the general expression of the decibel scale, rewritten as:
where Wref is the reference power value; Wref = 10^-12 W.
     Likewise, the sound intensity level (LI or SIL) is calculated with the general expression rewritten as:
where Iref is the reference intensity value; Iref = 10^-12 W/m^2.
     Using the expression for LI and the inverse square law, it can be shown that 6 dB of attenuation is attained by doubling the distance from the source.  Choosing an arbitrary value, (I/Iref) = 40, at distance r, we get LI(r) = 10 log (40) = 16 dB.  Doubling the distance increases the area of the hypothetical sphere by a factor of 4 (see Exhibit 1).  With power constant, this increased area reduces intensity by a factor of 4, which, in turn, reduces (I/Iref) by the same factor.  Therefore, for our example, (I/Iref) = 10 at distance 2r and we get LI(2r) = 10 log (10) = 10 dB, a reduction of 6 dB.
     Equating the two expressions for I, above, and rearranging, we get
In this form, it is easy to see that the square of pressure varies with r^2, while power and intensity (i.e. first power) vary with r^2.  Thus the general expression is rewritten for the sound pressure level (LP or SPL) as:
Pref is the reference pressure value; Pref = 2 x 10-5 N/m^2 = 20 μPa, corresponding to the threshold of human hearing at 1000 Hz.  Exhibit 2 provides examples of decibel scale levels and corresponding absolute values of sound power, intensity, and pressure.  The following should be noted in the table:
  • Each of the reference values correspond to 0 dB – when the ratio = 1, log (1) = 0.
  • Power and intensity are numerically equal at equal dB levels – numerically equal reference values are used.
  • All decimal places are shown, but small values of power and intensity are typically expressed in scientific notation (e.g. 10^-12) and small values of pressure are typically expressed in μPa.
  • Values in the table range from the threshold of human hearing to far beyond the threshold of pain (the range of human hearing will be discussed further in a future installment).
     Sound power and intensity levels are useful for acoustics projects – designing sound systems, venues, etc. – but sound pressure levels are most useful in quantifying occupational environments and supporting hearing conservation programs.  Examples of typical sound pressure levels encountered in commercial, recreational, and other settings are shown in Exhibit 3.  The “Noise Navigator,” an extensive database compiled and published by 3M Corporation, is available online.  In it, measurements of numerous sound levels are recorded, providing more useful data for research and planning purposes.

     Thus far, sounds have been treated as if generated by a singular point source in free-field conditions (no interference in spherical transmission).  Realistic soundscapes, however, are comprised of multiple complex sounds from various sources in environments where obstructions and reflective surfaces are ubiquitous.  In the next installment, the “Occupational Soundscapes” series begins to tackle the challenges of real-world conditions, presenting methods for assessing the effects of multiple simultaneous sounds on sound pressure levels.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] An Introduction to Acoustics.  Robert H. Randall.  Addison-Wesley; 1951.
[Link] “OSHA Technical Manual (OTM) - Section III: Chapter 5 - Noise.”  Occupational Safety and Health Administration; July 6, 2022.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications, 1996.
[Link] “Noise Navigator Sound Level Database, v1.8.” Elliot H. Berger, Rick Neitzel, and Cynthia A. Kladden.  3M Personal Safety Division; June 26, 2015.
[Link] “Sound Intensity.”  Brüel & Kjaer; Septermber 1993.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 2:  Mechanics of Sound and the Human Ear]]>Wed, 09 Aug 2023 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-2-mechanics-of-sound-and-the-human-ear     A rudimentary understanding of the physics of sound and the basic functions of the human ear is necessary to appreciate the significance of test results, exposure limits, and other elements of a hearing loss prevention program (HLPP).  Without this background, data gathered in support of hearing conservation have little meaning and effective protections cannot be developed and implemented.
     This installment of the “Occupational Soundscapes” series provides readers an introduction to the generation and propagation of sound and the structure and function of the human ear; it is not an exhaustive treatise on either subject.  Rather, it aims to provide a foundation of knowledge – a refresher, for many – on which future installments of the series build, without burdening readers with extraneous or potentially confusing detail.
Sound Generation and Propagation
     Sound can travel through solid, liquid, and gaseous media.  As our primary interest is in human hearing, this presentation focuses on sound propagation through air.  It should be noted, however, that vibrations in other media can be transferred to surrounding air and, therefore, ultimately perceptible by the human ear.  In fact, structure-borne noise is a prominent component of many occupational soundscapes.
     In air, sound is propagated via longitudinal pressure waves.  The movement of particles in a longitudinal wave is parallel to the wave’s direction of travel.  In contrast, particles move perpendicular to the travel direction of a transverse wave; a common example is a ripple in water.  A pressure wave consists of alternating regions of high and low pressure.  These are known as compressions and rarefactions, respectively, as the air pressure oscillates above and below ambient atmospheric pressure as portrayed in Exhibit 1.
     Sound waves are most-often referenced by their amplitude (A) and frequency (f).  A sound wave’s amplitude is the maximum pressure deviation from the ambient (μPa).  It is related to the perception of “loudness” of the sound.
     Instantaneous sound pressure is often a less useful measure than one that is time-based, such as an average.  However, the average of a sine wave’s amplitude is zero and, thus, unhelpful.  For a metric comparable across time and events, the root mean square (RMS) sound pressure (PRMS) is used.  To calculate PRMS:
(1) Consider the sound pressure waveform over a specified time period; the period of time considered is important for comparison of sound environments or events.
(2) Square (multiply by itself) the waveform (i.e. pressures) [P^2].
(3) Average the pressure-squared waveform (mean pressure squared) [P^2avg].
(4) Take the square root of the mean pressure squared (RMS pressure) [PRMS].
The steps of this process are represented graphically in Exhibit 2 and mathematically by the equation:
Squaring the pressures ensures that RMS values are always positive, simplifying use and comparison.
     A wave’s frequency is the number of complete wave cycles to pass a fixed point each second (Hz or cycles per second).  Frequency is related to the perception of a sound’s pitch.  A wave’s period (T), the time required for one complete wave cycle to pass a fixed point, is simply the inverse of its frequency:  T = 1/f (s).
     The wavelength (λ), the length of one complete wave cycle, is related to frequency and the speed of sound (c):  λ = c/f (m).  Though the speed of sound in air is influenced by temperature and density (i.e. elevation), an approximation of c = 343 m/s (1125 ft/s) is often used in lieu of calculating a more precise value.
     Speech is generated by forcing air through the larynx.  Movement of the vocal cords creates pressure fluctuations that manifest as complex sound waves.  These complex waves include carriers and modulation superimposed upon them.  Timing of modulation differentiates similar sounds, such as the letters “b” and “p;” therefore, resolution of these timing differences in the auditory system is integral to speech intelligibility.

     Maintaining speech communication abilities is paramount to a hearing loss prevention program (HLPP).  As such, understanding typical characteristics of voiced sounds is critical to its success.  Exhibit 3 shows the average (dark line) and range (shaded region) of sound pressures created by a small, homogeneous sample of subjects (seven adult males) reciting the same nonsensical sentence.  Though unrepresentative of the diversity of the broader population, the results are indicative of the variability that can be expected in a wider study.  A more-generalized data set is depicted in Exhibit 4, suggesting that the most-critical speech frequencies lie in the range of 170 – 4000 Hz (dB scales and the relation to speech communication will be explored in a future installment of the series).
     In addition to the voiced sounds of “normal” speech, humans generate unvoiced, or breath, sounds.  These occur when air is passed through the “vocal equipment” (i.e larynx, mouth) without activating the vocal cords.  Breath sounds, acting as low-energy carriers, make whispering possible.
     All sounds in an environment – wanted and unwanted – impinge on occupants indiscriminately.  It is up to the auditory system of each occupant to receive, process, resolve, differentiate, locate, and interpret these sounds collectively and/or individually as circumstances dictate.  Much of this work is performed by a sophisticated organ that often garners little attention:  the seemingly underappreciated ear.
Structure and Function of the Human Ear
     Exhibit 5 provides a pictorial representation of the ear’s complexity; a detailed discussion of each component is impractical in this introductory presentation.  Instead, a brief description of some critical components and their roles in the perception of sound is offered.  It won’t make “experts” of readers, but will provide the basic understanding needed to support hearing conservation efforts.
     Hearing – the perception of sound – takes place in three “stages” corresponding to the three regions of the ear:  outer, middle, and inner.  Common use of the word “ear” often refers only to the outer ear, the region highlighted in Exhibit 6.  Many times, it is intended to reference only the visible, cartilaginous portion called the pinna or auricle.  The pinna is most famous for adornment with jewelry and being the part of a misbehaving child pulled by a TV sitcom mom.
     Sound waves in the environment impinge upon the outer ear, where the pinna helps direct them into the external auditory canal, or simply ear canal.  The structure of the ear canal causes it to resonate, typically, in the range of 3 – 4 kHz, providing an amplification effect for sounds at or near its resonant frequency.
     The terminus of the outer ear is the tympanic membrane, commonly known as the eardrum.  The variable pressure of the impinging sound waves causes this diaphragm-like structure to flex inward and outward in response.
     In the middle ear, highlighted in Exhibit 7, the vibrational energy of the flexing eardrum is transmitted to another membrane in the inner ear via a linkage of three small bones, collectively called the ossicles or ossicular chain.  The malleus is attached to the eardrum and the stapes is attached to a membrane in the oval window of the cochlea.  Between these two lies the incus, the largest of the three bones.  The malleus, incus, and stapes are commonly known as the hammer, anvil, and stirrup, respectively.
     The configuration of the ossicular chain provides approximately a 3:1 mechanical advantage.  In conjunction with the relative sizes of the eardrum and oval window, the middle ear provides an overall mechanical advantage of approximately 15:1.  The ability to hear very soft sounds is attributed to the amplification effect produced in the middle ear.
     The Eustachian tube connects the middle ear to the nasal cavity, enabling pressure equalization with the surrounding atmosphere.  Blockage of the Eustachian tube, due to infection, for example, results in pressure deviations that can reduce hearing sensitivity, potentially to the extent of deafness.
     Two small muscles, the stapedius and the tensor tympanic, serve a protective function against very loud sounds.  These muscles act on the bones of the ossicular chain, changing its transmission characteristics to reduce the energy transmitted to the inner ear.  This protection mechanism is only available for sustained sounds, as the reflexive contraction of these muscles, known as the acoustic reflex, does not engage rapidly enough to attenuate sudden bursts of sound, such as explosions or gunshots.
     The inner ear, highlighted in Exhibit 8, is comprised primarily of the cochlea.  The cochlea is an extraordinary organ in its own right; its presentation here is, necessarily, an extreme simplification.  Many of its components and functional details will not be discussed, as a descriptive overview is of greater practical value with respect to hearing conservation.
     The motion of the stapes (stirrup) in the oval window induces pressure fluctuations in the fluid in the chambers of the cochlea.  The mechanical advantage provided by the middle ear serves to overcome the impedance mismatch between the air in the outer and middle ear and the liquid in the inner ear.  As mentioned previously, this maintains sensitivity to low-intensity sounds.
     The fluid movement, in turn, causes tiny hair cells in the cochlea to bend in relation to the sound energy transmitted.  These hairs are selectively sensitive to frequency; the extent of bending is proportional to the loudness of the sound.  It is this bending of hair cells that is translated into electrical signals that are sent to the brain for interpretation.  Damaging these sensitive hairs leads to reduced hearing sensitivity.  They are also nonregenerative; therefore, hearing loss caused by damaging these hairs is permanent and irrecoverable.  Though other mechanisms of damage exist, NIHL is a prominent and important one.
     The basilar membrane, separating the chambers of the cochlea, is also selectively sensitive to frequency, due to its varying mass and stiffness.  This results in the tonotopic organization of the cochlea, as depicted in Exhibit 9.  The highest frequencies (~ 20kHz) are detected near the basil end of the membrane (i.e. nearest the oval window); sensitivity shifts to progressively lower frequencies along the cochlear spiral.  Sensitivity to frequencies above ~ 2 kHz, including critical speech frequencies, is concentrated in the first 3/4 “coil” of the cochlea.  The range of human hearing and the critical speech frequencies will be discussed further in a future installment.
     The semicircular canals are highly recognizable, projecting from the cochlea’s distinct snail-like shape, but they play no significant role in hearing.  They are, however, integral to the critical function of maintaining balance which enables humans to walk upright.  Sharing a fluid supply with the cochlea, issues with hearing and balance can be correlated during a traumatic event.
     The introduction to Part 1 of the series called attention to several parallels between occupational soundscapes and thermal work environments.  The fluids contained in the cochlea may demonstrate a more-direct link.  There are two key fluids contained in chambers of the cochlea.  One, perilymph, is sodium-rich and the other, endolymph, is potassium-rich.  As discussed in the “Thermal Work Environments” series (Part 3), depletion of these electrolytes (salts) can be caused by profuse sweating and/or ingesting large quantities of water without balanced replenishment.  In addition to causing heat cramps and other afflictions, it seems heat stress could affect your hearing!

     There is a great deal more detail available from various sources to explain the mechanics of hearing, particularly the inner workings of the cochlea.  It is a fascinatingly complex organ; intrigued readers are encouraged to consult the references at the end of this post, as well as medical texts, to learn more.  Despite the requisite simplification of this presentation, sufficient information has been included to enable readers to continue on this journey of sound exploration in pursuit of the ultimate goal: effective hearing conservation practices.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).
[Link] ”29 CFR 1910.95 - Occupational noise exposure.’  OSHA.
[Link] “Noise Control Design Guide.” Owens Corning; 2004.
[Link] Engineering Noise Control – Theory and Practice, 4ed.  David A. Bies and Colin H. Hansen.  Taylor & Francis; 2009.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications, 1996.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] An Introduction to Acoustics.  Robert H. Randall.  Addison-Wesley; 1951.
[Link] “How Hearing Works.”  Hearing Industries Association; 2023.
[Link] “OSHA Technical Manual (OTM) - Section III: Chapter 5 - Noise.”  Occupational Safety and Health Administration; July 6, 2022.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
<![CDATA[Occupational Soundscapes – Part 1:  An Introduction to Noise-Induced Hearing Loss]]>Wed, 26 Jul 2023 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/occupational-soundscapes-part-1-an-introduction-to-noise-induced-hearing-loss     Exposure to excessive noise in the workplace can have profound effects, both immediate and long-term.  Some consequences are obvious, while others may surprise those that have not studied the topic.
     Some industries, such as mining and construction, are subject to regulations published specifically for them.  This series presents information, including regulatory controls, that is broadly applicable to manufacturing and service industries.
     Several parallels exist between exposure to noise and heat stress (see the “Thermal Work Environments” series).  These include the relevance of durations of exposure and recovery, the manifestation of cognitive, as well as physical, effects on workers, and the importance of monitoring exposure and risk factors.
     To take advantage of these parallels, the “Occupational Soundscapes” series follows a path similar to that taken in the “Thermal Work Environments” series.  Terminology, physiological implications, measurement, and guidance for managing the risks are each discussed in turn.
Terms in Use
     The title “Occupational Soundscapes” was chosen to maintain the focus of the series on two important aspects.  First, “occupational” reminds readers that the subject matter context is the workplace.  Managing sound and preventing occupational noise-induced hearing loss (NIHL) – hearing loss caused by workplace noise – is the objective of the series.  This differentiates occupational hearing loss from other causes.  Other forms of hearing loss can occur in addition to NIHL; these include:
  • presbycusis – naturally-occuring due to aging.
  • sociocusis – caused by recreational or non-occupational activities, such as music, aviation, motorsports, or arena sports.
  • nosocusis – caused by environmental factors such as exposure to chemicals, behaviors such as drug use, or underlying health conditions such as hypertension.
These types of hearing loss are presented to provide clarity to occupational causes, but will not be discussed in detail.
     The second term of the title, “soundscapes,” serves to remind readers that workplaces are filled with a combination of sounds; some are desired, others are detrimental to working conditions.  Each contribution to the soundscape has a unique source and set of parameters.
     Much of this series focuses on the reduction, control, and protection from noise – the unwanted portion of the soundscape – but readers should not lose sight of the wanted sound.  One very important reason to control noise is to maintain accessibility of desired sounds.  Speech communication is of particular importance and is the primary focus of audiometric testing and industrial noise-control regulation.
     In its “Criteria for a Recommended Standard – Occupational Noise Exposure, Revised Criteria” (1998), NIOSH declares that its focus is on prevention of hearing loss rather than conservation of hearing.  This emphatic declaration is somewhat bizarre, as this is a distinction without a difference.  The terms are functionally equivalent, particularly in practical matters, to which “The Third Degree” is committed.  Readers will be spared a detailed explanation of why this is true; suffice to say that references to hearing loss prevention, hearing conservation, and hearing preservation are considered interchangeable.
     While paralleling the information presentation of the “Thermal Work Environments” series, the objectives pursued in this series will also mimic those of its predecessor series.  In brief, each installment is limited in scale and scope to be palatable to busy practitioners, easily referenced, edited, or expanded as future development requires it.  To further promote a holistic approach to job design, the two series should be read as companion pieces.  Side-by-side review of thermal and aural requirements of a workplace may reveal complementary or synergistic solutions, increasing the efficiency of industrial hygiene improvement efforts.
     Links to the entire series are provided at the end of this post for easy reference.
     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.
[Link] “Criteria for a Recommended Standard - Occupational Noise Exposure, Revised Criteria 1998.”  Publication No. 98-126, NIOSH, June 1998.
[Link] ”29 CFR 1910.95 - Occupational noise exposure.’  OSHA.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
Directory of “Occupational Soundscapes” entries on “The Third Degree.”
Part 1:  An Introduction to Noise-Induced Hearing Loss (26Jul2023)
Part 2:  Mechanics of Sound and the Human Ear (9Aug2023)
Part 3:  The Decibel Scale (23Aug2023)
Part 4:  Sound Math (6Sep2023)
Part 5:  Audiometry (20Sep2023)
Part 6:  Measurement of Sound Exposure (4Oct2023)
Part 7:  Perceptions (10Jan2024)
Part 8:  Effects of Exposure (24Jan2024)
Part 9:  Concepts in Communication (7Feb2024)
Part 10:  Communication Systems (6Mar2024)
Part 11:  Concepts in Noise Control (20Mar2024)
Part 12:  Source Noise Control (3Apr2024)
Part 13:  Path Noise Control (17Apr2024)
Part 14:  Receiver Noise Control (1May2024)
Part 15:  Community Noise (15May2024)
Part 16:  A Rainbow of Noise (29May2024)
Part 17:  Total Soundscape Management (12Jun2024)
<![CDATA[Thermal Work Environments – Part 5:  Managing Conditions in Hot Environments]]>Wed, 12 Jul 2023 06:00:00 GMThttp://jaywinksolutions.com/thethirddegree/thermal-work-environments-part-5-managing-conditions-in-hot-environments     Safeguarding the health and well-being of employees is among the critical functions of management.  In hot workplaces, monitoring environmental conditions and providing adequate protection comprise a significant share of these responsibilities.  The details of these efforts are often documented and formalized in a heat illness prevention program.
     An effective heat illness prevention program consists of several components, including the measure(s) used for environmental assessment, exposure limits or threshold values, policies defining the response to a limit or threshold being reached, content and schedule of required training for workers and managers, and the processes used to collect and review data and modify the program.  Other information may be added, particularly as the program matures.  Though it is nominally a prevention program, response procedures, such as the administration of first aid, should also be included; the program should not be assumed to be infallible.
     In this installment of the “Thermal Work Environments” series, the components of heat stress hygiene and various control mechanisms are introduced.  Combined with the types of information mentioned above, an outline of a heat illness prevention program emerges.  This outline can be referenced or customized to create a program meeting the needs of a specific organization or work site.
     The content of a heat illness prevention program is presented in five (5) sections:
  • Training
  • Hazard Assessment
  • Controls
  • Monitoring
  • Response Plans
A comprehensive review of each would be unwieldy in this format.  Instead, an overview of the information is provided as an introduction and guide to further inquiry when one begins to develop a program for his/her team.

     Every person that works in or has responsibility for a hot workplace should be trained on the ramifications of excess heat.  Information relevant to the following four sections is included in an effective training program.  Examples of important topics for all team members include:
  • basics of human biometeorology and heat balance,
  • environmental, personal, and behavioral risk factors,
  • methods used to monitor conditions,
  • controls in place to prevent heat illness,
  • signs and symptoms of heat illness, and
  • first aid and emergency response procedures.
Training of supervisors and team leaders should emphasize proper use of controls, signs and symptoms, and appropriate responses to heat illness and failure of control mechanisms.
     A complete training plan includes the content of the training and a schedule for delivery.  It may be best to distribute a large amount of information among multiple modules rather than share it in a single, long presentation.  Refresher courses of reduced duration and intensity should also be planned to combat complacency and to update information as needed.  Refreshers are particularly helpful when dangerous conditions exist intermittently or are seasonal.
Hazard Assessment
     An initial hazard assessment consists of identifying the elements of job design (see Part 1) that are heat-related.  These include environmental factors, such as:
  • atmospheric conditions (e.g. temperature, humidity, sun exposure),
  • air movement (natural or forced), and
  • proximity to heat-generating processes or equipment.
Also included are job-specific attributes, such as:
  • intensity of work (i.e. strenuousness and rate),
  • personal protective equipment (PPE) and other gear required, and
  • availability of potable water and a cool recovery area.
Other relevant factors may also be identified.  A compound effect could be discovered, for example, between concentration required for task performance and an increase in heat stress due to resultant anxiety.
     Using the information collected in the hazard assessment, a risk profile can be created for each job.  The risk profile is then used to prioritize the development of controls and modifications to the job design.
     Similar to that for quality [see “The War on Error – Vol. II:  Poka Yoke (What Is and Is Not)” (15Jul2020)], there is an implied hierarchy of controls used to manage heat-related effects on workers.  Engineering controls modify the tasks performed or the surrounding conditions, while administrative controls guide workers’ behavior to reduce heat stress.  Finally, personal protective equipment (PPE) is used to manage heat stress that could not otherwise be reduced.  PPE is often the first protection implemented and is used until more-effective controls are developed.
     A comprehensive heat stress control plan is developed by considering each term in the heat balance equation (see Part 2).  Examples of engineering controls include:
  • To reduce metabolic heat generation, M, provide lift assists, material transport carts, or other physical aids to limit workers’ exertion.
  • To reduce radiative heat load, R, install shields between heat sources (e.g. furnaces or other hot equipment) and workers, just as an umbrella is used to block direct sunlight.
  • Use fans to increase evaporative cooling, E, when air temperature is below 95° F (35° C).
  • Reduce air temperature with water-mist systems if relative humidity (RH) is below 50% and general air conditioning is not practical.
     Administrative control examples include:
  • Establish policies that limit work during periods of excessive heat; thresholds can be based on Heat Index (HI), Wet Bulb Globe Temperature (WBGT), or other index.  The American Conference of Governmental Industrial Hygienists (ACGIH) regularly publishes threshold limit values (TLVs) based on WBGT with adjustments for clothing and work/rest cycles.  ACGIH TLVs often serve as the basis for standards and guidelines developed by other organizations and government agencies.
  • Reduce M by increasing periods of rest in the work cycle.
  • Implement an acclimatization program for new and returning workers that allows them to develop “resistance” to heat stress.
  • Encourage proper hydration; ensure availability of cool potable water.
     Heat-related PPE examples include:
  • Reflective clothing to reduce radiative heat load, R.
  • A vest cooled with ice, forced air, or water increases conductive, K, and/or convective, C, heat loss.
  • Bandana, hat or similar item that can be wetted to enhance evaporative cooling, E, particularly from the head and neck.
  • Hydration backpack.
     Many controls are used in conjunction to achieve maximum effect.  Tradeoffs must be considered to ensure that the chosen combination of controls is the most-effective.  For example, cooling with a water-mist system increases humidity; if it begins to inhibit evaporation from skin, its use may be inadvisable.
     Monitoring is a multifaceted activity and responsibility.  In addition to measuring environmental variables, the effectiveness of controls and the well-being of workers must be continually assessed.  A monitoring plan includes descriptions of the methods used to accomplish each.
     Measurement of environmental variables is the subject of Part 4 of this series.  As discussed in that installment, multiple indices may be used to inform decisions regarding work cycle modifications or stoppages.  Those used in popular meteorology, such as Heat Index (HI), are often insufficient to properly characterize workplace conditions; however, they can be useful as early warnings that additional precautions may be needed to protect workers during particularly dangerous periods.  See “Heat Watch vs. Warning” for descriptions of alerts that the National Weather Service (NWS) issues when dangerous temperatures are forecast.
     After controls are implemented, they must be monitored for proper use and continued effectiveness.  This should be done on an ongoing basis, though a formal report may be issued only at specified intervals (e.g. quarterly) or during specific events (e.g. modification of a control).  Verification test procedures should be included in the monitoring plan to maintain consistency of tests and efficacy of controls.
     Monitoring the well-being of workers is a responsibility shared by a worker’s team and medical professionals.  Prior to working in a hot environment, each worker should be evaluated on his/her overall health and underlying risk factors for heat illness.  An established baseline facilitates monitoring a worker’s condition over time, including the effectiveness of acclimatization procedures and behavioral changes.
     Suggestions for behavioral changes, or “lifestyle choices,” can be made to reduce a worker’s risk; these include diet, exercise, consumption of alcohol or other substances, and other activities.  Recommendations to an employer regarding one’s fitness for certain duties, for example, must be made in such a way that protects both safety and privacy.  Heat-related issues may be best addressed as one component of a holistic wellness program such as those established by partnerships between employers, insurers, and healthcare providers.
Response Plans
     There are three (3) response plans that should be included in a heat illness prevention program.  Somewhat ironically, two of them are concerned with heat illness that was not prevented.
     The first response plan details the provisioning of first aid and subsequent medical care when needed.  Refer to Part 3 for an introduction to heat illnesses and first aid.
     The second outlines the investigation required when a serious heat illness or heat-related injury or accident occurs.  The questions it must answer include:
  • Were defined controls functioning and in proper use?
  • Had the individual(s) involved received medical screening and been cleared for work?
  • Had recommendations from prescreens been followed by individual(s) and the organization?
  • Had the individual(s) been properly acclimatized?
  • Were special circumstances involved (e.g. heat advisory, other emergency situation, etc.)?
The investigation is intended to reveal necessary modifications to the program to prevent future heat illness or heat-related injury.
     The final response plan needed defines the review process for the heat illness prevention program.  This includes the review frequency, events that trigger additional scrutiny and revision, and required approvals.
     Currently, management of hot work environments is governed by the “General Duty Clause” of the Occupational Safety and Health Act of 1970.  The General Duty Clause provides umbrella protections for hazards that are not explicitly detailed elsewhere in the regulations.  It is a generic statement of intent that provides no specific guidance for assessment of hazards or management of risks.
     In 2021, OSHA issued an “advance notice of proposed rulemaking” (ANPRM) to address this gap in workplace safety regulations.  A finalized standard, added to the Code of Federal Regulations (CFR), will add specific enforcement responsibilities to OSHA’s current role of education and “soft” guidance on heat-related issues.
     That an OSHA standard will reduce heat-related illness and injury is a reasonable expectation.  However, it must be recognized that it, too, is imperfect.  No standard or guideline can account for every person’s unique experience of his/her environment; therefore, an individual’s perceptions and expressions of his/her condition (i.e. comfort and well-being) should not be ignored.  A culture of autonomy, or “self-determination,” where workers are self-paced, or retain other responsibility for heat stress hygiene, is one of the most powerful tools for safety and health management imaginable.
     For additional guidance or assistance with complying with OSHA regulations, developing a heat illness prevention program, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.
     For a directory of “Thermal Work Environments” entries on “The Third Degree,” see Part 1:  An Introduction to Biometeorology and Job Design (17May2023).
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “NIOSH Criteria for a Recommended Standard Occupational Exposure to Heat and Hot Environments.”  Brenda Jacklitsch, et al.  National Institute for Occupational Safety and Health (Publication 2016-106); February 2016.
[Link] “Threshold Limit Values for Chemical Substances and Physical Agents.”  American Conference of Governmental Industrial Hygienists (ACGIH); latest edition.
[Link] “National Emphasis Program – Outdoor and Indoor Heat-Related Hazards.”  Occupational Safety and Health Administration (OSHA); April 8, 2022.
[Link] “Ability to Discriminate Between Sustainable and Unsustainable Heat Stress Exposures—Part 1:  WBGT Exposure Limits.”  Ximena P. Garzon-Villalba, et al.  Annals of Work Exposures and Health;  June 8, 2017.
[Link] “Ability to Discriminate Between Sustainable and Unsustainable Heat Stress Exposures—Part 2:  Physiological Indicators.”  Ximena P. Garzon-Villalba, et al.  Annals of Work Exposures and Health;  June 8, 2017.
[Link] “The Thermal Work Limit Is a Simple Reliable Heat Index for the Protection of Workers in Thermally Stressful Environments.”  Veronica S. Miller and Graham P. Bates.  The Annals of Occupational Hygiene; August 2007.
[Link] “Thermal Work Limit.”  Wikipedia.
[Link] “The Limitations of WBGT Index for Application in Industries: A Systematic Review.”  Farideh Golbabaei, et al.  International Journal of Occupational Hygiene; December 2021.
[Link] “Evaluation of Occupational Exposure Limits for Heat Stress in Outdoor Workers — United States, 2011–2016.”  Aaron W. Tustin, MD, et al.  Morbidity and Mortality Weekly Report (MMWR).  Centers for Disease Control and Prevention; July 6, 2018.
[Link] “Occupational Heat Exposure. Part 2: The measurement of heat exposure (stress and strain) in the occupational environment.”  Darren Joubert and Graham Bates.  Occupational Health Southern Africa Journal; September/October 2007.
[Link] “Heat Stress:  Understanding factors and measures helps SH&E professionals take a proactive management approach.”  Stephanie Helgerman McKinnon and Regina L. Utley.  Professional Safety; April 2005.
[Link] “The Heat Death Line: Proposed Heat Index Alert Threshold for Preventing Heat-Related Fatalities in the Civilian Workforce.”  Zaw Maung and Aaron W. Tustin.  NEW SOLUTIONS: A Journal of Environmental and Occupational Health Policy; June 2020.
[Link] “Loss of Heat Acclimation and Time to Re-establish Acclimation.”  Candi D. Ashley, John Ferron, and Thomas E. Bernard.  Journal of Occupational and Environmental Hygiene; April 2015.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC