Hearing Protection Devices
Michael R. Chial, Ph.D.
Professor Emeritus
Department of Communicative Disorders
University of Wisconsin-Madison
Hearing protection devices (HPD’s) are, as a practical matter, the first line of
defense against hearing loss caused by excessive noise. Other ways to reduce exposure
to loud sound (engineering control of noise sources, reduction of noise in the
transmission path between a source and an individual) can indeed be more effective, but
are often more costly or more difficult to manage.
HPDs can be classified by type (active vs. passive), by form (e.g., ear plugs and
ear muffs of various types) and by effect. In all cases, the goal is the same: to attenuate
the magnitude of sound reaching the cochlea, thus limiting acoustical insult to the endorgan
of hearing. Passive devices accomplish this through blockage of the air-borne transmission path to the inner ear. Active devices seek to mechanically or electronically respond to noise to reduce
signal amplitudes presented to the auditory system.
Ear plugs fall into five categories (Berger, 2000): (1) closed-cell foam devices
designed to be manually compressed, then inserted in the ear canal where they expand to
approximate their initial size (e.g., the Aearo Company E-A-R plug), (2) pre-formed
devices available in different diameters to accommodate different ear canals (e.g., the
PlastiMed, Inc. V-51R plug), (3) malleable devices intended to fit a range of ear cannals,
(4) semi-insert devices held in the ear canal by means of a plastic or metal band, and (5)
devices made from ear mold impressions taken from individual ear canals. Most ear
plugs are created from plastics (polyvinyl chloride, polyurethane, silicone or acrylic);
malleable earplugs often consist of wax-impregnated cotton or fiberglass enclosed in a
thin plastic container. Most earplugs are passive, that is, they are not intended to respond
differently to differing noise exposures. Some active earplugs employ metal slugs or
other material intended to move within the plug when stimulated by sudden acoustic
overpressures, thus increasing attenuation in response to impulsive noise. For various
reasons, it is difficult or impossible to objectively measure the attenuation of such
devices, or to estimate their real-world benefit. A recent addition to the array of earplugs
are those offered by Etymotic Research for users with specific needs (e.g., musicians)
who seek flat attenuation across specified frequency ranges.
Ear muffs are designed as integral components of safety helmets, or as separate
devices which surround the outer ear and are held in place by headbands that extend over
or behind the head, or beneath the chin. Some of these are combined with
communication systems intended to increase the signal-to-noise ratio of messages
electronically routed to earphones placed within headsets. At present, most ear muffs are
passive devices, but several have been developed as active systems. Indeed, most active
hearing protectors are based on earmuffs, due to the space required for sound sensing and
processing components. Active protectors employ one of two (or both) methods to
attenuate sound. One method senses sound with a microphone, then processes sound
delivered through an earphone by means of automatic gain control circuitry: when
incident sound exceeds a certain level, further increases are electronically clipped or
otherwise squelched. The other method samples incident sound, reverses the phase of the
signal, and electronically adds the reversed signal within the muff enclosure to partially
cancel the incident sound. Due to incident signal changes and processing speed
requirements, devices employing additive cancellation techniques are more effective at
relatively low frequencies (e.g., below 500 Hz; see Nixon and others 1992). Some active
noise reduction methods appear similar to (or may benefit from) methods employed in
hearing aids.
Beyond type and form, HPDs differ in weight, comfort, uniformity of fit to
individuals, compatibility with other protective or prosthetic devices, and compatibility
with individual user health status. Using eye glasses with ear muffs, for example, can
create acoustic leaks that reduce attenuation performance. Similarly, a subject with
excessive cerumen or a middle ear effusion should not use ear plugs. Use of hearing
protectors in hot, humid environments can be uncomfortable and can cause skin irritation.
If hearing protectors (perhaps combined with hearing loss) render speech communication
difficult, or if they limit audibility of other signals deemed important, users may reject
them. For obvious reasons, ear muffs should not be used in conjunction with hearing
aids. These and other issues are discussed in detail by Berger (2000).
Real environments in which hearing protectors might provide benefit differ
tremendously in noise amplitude, spectrum and duration. Noise exposure is normally
indexed by time-weighted average (TWA) sound pressure levels sampled using
integrating meters or personal noise dosimeters. In the United States, such measurements
are specified by Federal regulation (OSHA, 1983; CFR Part 1910.95) and the subject of
technical standards (ANSI S12.19, 1996). Among other details, exposure is to be
indexed using a slow meter ballistic characteristic and an A-weighting network (a highpass
filter useful in predicting the effects of broad-band noise on hearing). Timeweighted
average levels are single-number values used to describe noise exposure and
determine actions to protect workers from noise-induced hearing loss in the work-place (OSHA, 1983).
In 1979, the Environmental Protection Agency (EPA) issued a regulation intended
to promote laboratory measurement of hearing protector attenuation for the purpose of
combining such information with exposure data to estimate protective effect. The EPA
regulation (CFR40 Part 211) built on previous technical standards (ANSI, 1974), and
invoked a single-number index, called the Noise Reduction Rating (NRR) to be included
in hearing protector product labels. Computation of NRRs from averaged behavioral
real-ear attenuation-at-threshold (REAT) data assume temporally continuous bandlimited
noise stimuli with equal energy per octave (pink noise), and address inter-subject
variability by doubling the standard deviation of threshold shifts, then subtracting that
value from mean threshold shift for each noise band. Adjusted attenuation values are
summed logarithmically across stimulus noise bands to yield a NRR in decibels. Because
the NRR method also assumes measurement of unprotected levels indexed with a Cweighting
network (which has a flatter frequency response that the A-weighting network
used to measure exposure), an additional 7 dB must be subtracted from the NRR to
estimate A-weighted noise levels when a hearing protector is in place (see OSHA, 1983,
Appendix B). Finally, because it was recognized that how hearing protectors are placed
in a subject’s ears (plugs) or on a subject’s head (muffs) could affect outcomes, the EPA
method specified experimenter-fit of HPDs during laboratory testing.
Because real-ear attenuation methods performed following the procedures
stipulated by EPA designate experimenter fitting of HPDs, it is to be expected that
resulting NRRs will be larger than what would be found with subject fitting of HPDs.
Because all extant methods for measuring REATs use temporally continuous noise,
results of such measurements cannot be generalized to impulse noise (e.g., gunfire) or
impact noise (e.g., forging).
Shortly after the inception of the current OSHA Hearing Conservation Rule
(OSHA, 1983), the National Institute of Occupational Safety and Health (NIOSH)
recommended that labeled NRRs be derated to estimate effectiveness in the field. Six
schemes are noted in Appendix B of the Hearing Conservation Rule. These differ based
upon available measurement devices and data, but generally reduce the estimated benefit
of hearing protectors. For example, if only A-weighted noise exposure data are available,
7 dB is subtracted from the NRR. For both A-weighted and C-weighted exposure data,
the resulting corrected NRR is further reduced by 50%.
Subsequent research over two decades suggests that NRRs derated in this manner
still over-estimate the attenuation of hearing protectors in real-world situations. Various
factors contribute to this inaccuracy, including over-estimation associated with (1)
experimenter fit, (2) highly trained test subjects (whose small standard deviations of
REATs produce higher NRRs), and (3) differences in patterns of use of hearing
protectors in laboratory and field settings (NIOSH, 1998).
Other pertinent generalizations include (1) overall, earmuffs provide the most
protection, foam and formable earplugs provide the next greatest protection, and all other
insert types provide less, and (2) ideally, individuals should be fitted individually for
hearing protectors (NIOSH, 1998). Generally, both ear plugs and earmuffs provide
greater attenuation at frequencies above 500 Hz than at lower frequencies (Berger, 2000).
Chapter 4 of the revised NIOSH criteria document (NIOSH, 1998) offers details
about estimated real-world NRRs for 84% of wearers of hearing protectors based upon
several independent studies. Labeled NRRs for single protectors range from 11 to 29
dB, while weighted mean NRR84 values range from 0.1 to 14.3 dB.
To address these problems, existing standards for measuring HPD attenuation
were revised (Royster and others, 1996; Berger and others, 1998) to include subject-fit
methods with audiometrically proficient listeners naïve about HPDs. The resulting
standard is ANSI S12.6 (1997). (A companion standard, ANSI S12.42 (1995) specifies a
test fixture method and a microphone-in-real-ear method for measuring insertion loss
useful for quality control and product development work with ear muffs.)
The National Hearing Conservation Association (NHCA, 1995) proposed
alternative labeling requirements in which only subject-fit real-ear attenuation data
(ANSI S12.6-1997 Method B) are reported. The revised NRR(SF) information generally
suggests less protection than NRRs based upon experimenter-fit. Alternatively, the
NHCA (1995) suggests labeling to include high, medium, and low NRRs based upon
statistical distributions of measured subject-fit REATs. This proposal has been endorsed
by several other organizations. As this writing, however, the EPA NRR labeling
requirement remains based upon the experimenter-fit method specified in ANSI S3.19
(1974).
If only experimenter-fit data are available, NIOSH (1998) currently recommends
derating of NRRs based upon type of hearing protector: 25% for earmuffs, 50% for
formable earplugs, and 70% for all other earplugs. In the case of double protection
(plugs and muffs), the OSHA Technical Manual (OSHA, 1999) recommends using the
EPA NRR for the better protector, minus 7 dB, dividing the result by 2 (a 50% derating),
then adding 5 dB to the field-adjusted NRR to account for the second protector.
Rather clearly, much work remains to be done to improve the prediction of realworld
benefit of hearing protectors (Berger and Lindgren, 1992; Berger, 1999). One
promising approach involves methods similar to in vivo real-ear gain measurements of
hearing aids (now a common practice), together with modification of commonly used
personal noise dosimeters. This approach requires the ability to simultaneously measure
exposure level and the sound level generated within the ear canal of the wearer of a
hearing protector. If both are measured with the same filtering schemes (preferably, the
C-weighting network; ideally with both A and C networks), the signed difference
between the two would index attenuation due to the hearing protector. If such
measurements can be adapted to field use (e.g., with a two-channel noise dosimeter), it
may be possible to add useful information to what otherwise can be determined about the
performance of at least some HPDs. ANSI S12.42 (1995) addresses some of these issues
for ear muffs and communication headsets, but only for laboratory measurements.
Because the use of probe microphones with earplugs is likely to produce reactive
measurement effects, this approach may not be suitable for insert devices.
It is generally recognized that effective use of hearing protectors in the workplace
or elsewhere is influenced by factors that go beyond the physical performance of these
devices. As summarized by NIOSH (1998), these factors include convenience and
availability, comfort and ease of fit, compatibility with other safety equipment, and
worker belief that the device can be worn effectively, will indeed prevent hearing loss,
and will still permit hearing of important sounds.
Originally published in Kent, R. D. (2004). The MIT Encyclopedia of Communication Disorders. Pages 497-500.Cambridge, MA: The MIT Press.
References
American National Standards Institute, 1996, S12.19-1996 American National Standard
Measurement of Occupational Noise Exposure. New York, NY: author.
American National Standards Institute, 1974, S3.19-1974 American National Standard
Measurement of Real-Ear Protection of Hearing Protectors and Physical
Attenuation of Ear Muffs. New York, NY: author.
American National Standards Institute, 1997, S12.6-1997 American National Standard
Methods for Measuring of the Real-Ear Attenuation of Hearing Protectors. New
York, NY: author.
American National Standards Institute, 1995, S12.42-1995 American National Standard
Microphone-in-Real-Ear and Acoustic Test Fixture Methods for the Measurement
of Insertion Loss of Circumaural Hearing Protection Devices. New York, NY:
author.
Berger, E. H., 2000, Hearing protection devices. Chapter 10 in The Noise Manual, Fifth
Edition (Berger, E.H., Royster, L. H., Royster, J. D., Driscoll, D. P., and Layne,
M., Editors). Fairfax, VA: American Industrial Hygiene Association.
Berger, E. H., 1999, Hearing protector testing - let’s get real [using the new ANSI
Method-B data and the NRR(SF)]. Earlog 21. Indianapolis, IN: Aearo Company.
Retrieved April 12, 2002 from
http://www.cabotsafety.com/html/industrial/earlog21.htm
Berger, E. H., Franks, J. R., Behar, A., Casali, J. G., Dixon-Ernst C. , Kieper, R. W.,
Merry, C. J., Mozo, B. T., Nixon, C. W, Ohlin, D., Royster, J. D., and Royster, L.
H., 1998, Development of a new standard laboratory protocol for estimating the
field attenuation of hearing protection devices. Part III. The Validity of Using
Subject-Fit Data. Journal of the Acoustical Society of America, 103(2), 665-72.
Berger, E. H., and Lindgrin, F., 1992, Current issues in hearing protection. Chapter 33 in
Noise-Induced Hearing Loss (Dancer, A. L., Henderson, D., Salvi, R. J., and
Hamernik, R. P., Editors). St. Louis, MO: Mosby Year Book.
Environmental Protection Agency, 1979, Noise labeling requirements for hearing
protectors. Code of Federal Regulations 40CFR Part 211. Retrieved April 12,
2002 from http://www.nonoise.org/lawlib/cfr/40/40cfr211.htm
National Hearing Conservation Association, 1995, Recommendations of the NHCA Task
Force on Hearing Protector Effectiveness. Retrieved April 12, 2002 from
http://www.hearingconservation.org/pos6.htm
National Institute of Occupational Safety and Health, 1998, Criteria for a Recommended
Standard: Occupational Noise Exposure, Revised Criteria 1988. (DHHS
(NIOSH) Publication NO. 98-126). Cincinnati, OH: National Institute of
Occupational Safety and Health. Retrieved April 12, 2002 from
http://www.cdc.gov/niosh/98-126.html
Nixon, C. W., McKinley, R. L., and Steuver, J. W., 1992, Performance of active noise
reduction headsets. Chapter 34 in Noise-Induced Hearing Loss (Dancer, A. L.,
Henderson, D., Salvi, R. J., and Hamernik, R. P., Editors). St. Louis, MO:
Mosby Year Book.
Occupational Safety and Health Administration, 1999, Noise Measurement (Section III:
Chapter 5 of OSHA Technical Manual) Retrieved on April 12, 2002 from
http://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_5.html
Occupational Safety and Health Administration, 1983, Occupational noise exposurehearing
conservation amendment. Code of Federal Regulations 29 CFR Part
1910.95. Retrieved April 12, 2002 from
http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDAR
DS&p_id=9735
Royster, J.D., Berger, E. H., Merry, C.J., Nixon, C. W., Franks, J. R., Behar, A., Casali, J.
G., Dixon-Ernst, C., Kieper, R. W., Mozo, B. T., Ohlin,D., and Royster, L. H. ,
1996, Development of a new standard laboratory protocol for estimating the field
attenuation of hearing protection devices. Part I. Research of Working Group 11,
Accredited Standards Committee S12, Noise. Journal of the Acoustical Society of
America, 99 (3), 1506-152.
Further Reading
Alberti, P. W. (Editor.), 1982, Personal Hearing Protection in Industry. New York, NY:
Raven Press.
American Industrial Hygiene Association [Archive]. Retrieved April 12, 2002 from
http://www.aiha.org/
American National Standards Institute [Archive]. Retrieve April 12, 2002 from
http://www.ansi.org/
Berger, E. H. Earlog Series (1-21) [Archive]. Retrieved April 12, 2002 from
http://www.cabotsafety.com/html/industrial/earlog.htm
Council for Accreditation in Occupational Hearing Conservation [Archive]. Retrieved
April 12, 2002 from http://www.caohc.org/
Hearing Protection Devices Page 12
Franks, J.R. and Berger, E. H., 1998, Hearing protection-personal protection-overview.
Encyclopedia of Occupational Health and Safety, 31.11-31.15. Geneva,
Switzerland: International Labour Organization.
National Hearing Conservation Association, 2002, Contemporary References: Hearing
Protection Research. Retrieved April 12, 2002 from
http://www.hearingconservation.org/cr3.html
Noise Pollution Clearinghouse [Archive]. Retrieved April 12, 2002 from
http://www.nonoise.org/