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 soundscape.  Likely 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 substitution.  Substitution 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.

The SPR/ETI/PAP Model
     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).

References
[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
[email protected]