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 6.  Canal 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 Labeling.  Subpart 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

Conclusion
     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).

References
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Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
[email protected]