**2.3 Noise Exposure:**

Basic anatomy and physiology of the auditory system

Definition of sound

Physical properties of sound

Definition of noise

Sources of noise in aviation operations

Types of noise

Symptoms and signs of noise exposure

Hearing testing techniques

Temporary and permanent threshold shifts

Communications and performance effects of noise exposure

Prevention of noise exposure, hearing conservation

Excessive noise is an undesirable byproduct of aviation. Permanent
hearing loss and its associated problems are the most critical of the
various consequences. The extent of damage to hearing is related to the
amount of acoustic energy reaching the auditory system. Damage cannot be
estimated accurately for an individual because of the wide variance of
susceptibility of human ears to noise. Engineering controls successfully
treat many sources yet it is still not practical, economical or feasible
to rely on such controls to reduce aircraft noises to acceptable (safe)
levels. Personal hearing protection is the most effective and widely
utilized alternative. Good hearing conservation programs preserve
hearing. Overall, the primary consequences of excessive noise exposures
are damage to the auditory system and degradation of audio
communications.

![](media/image1.wmf)BASIC ANATOMY AND PHYSIOLOGY OF THE AUDITORY SYSTEM

Ear Canal Malleus Incus Stapes Oval Window Inner Ear Auditory Nerve

Pinna Outer Ear Middle Ear Round Window Eustachian Tube

Eardrum

Anatomy. The auditory system is comprised of the outer, middle, and
inner ears, and a neural network leading to the brain. The outer ear is
the pinna (auricle) and ear canal (meatus). The pinna collects and
amplifies frontal sounds and lessens those from the rear. The ear canal
connects the pinna to the eardrum (tympanic membrane) and conveys sound
waves to the middle ear. The canal contains a waxy substance and hairs
that repel insects and keep small particles from the eardrum.
Accumulations of this wax can reduce audition at the ear. The eardrum
separates the outer ear from the middle ear and is translucent as viewed
through an otoscope.

The middle ear is an air-filled space that includes the eardrum,
ossicular chain, two small muscles, and the oval and round windows. It
connects to the back of the mouth via the collapsed Eustachian tube that
opens as needed to equalize middle ear air-pressure. The ossicular chain
of three tiny bones (malleus, incus, and stapes) attaches the eardrum to
the oval

window at the inner ear. The two small muscles influence the incoming
acoustic stimuli. The tensor tympani muscle attached to the malleus
enhances the ossicular coupling increasing the sensitivity of the
eardrum. The stapedius muscle attached to the stapes limits the
ossicular motion, diminishing the impact on the inner ear of acoustic
signals above 75 dB to 85 dB. The stapes is physically attached to the
oval window. This figure is a crosscut of the cochlear duct.

![](media/image2.wmf) Scala vestibuli Reissner\'s membrane Scala media
Tectorial membrane

Auditory nerve Scala tympani Basilar membrane Organ of Corti

The inner ear contains both the auditory and the equilibrium organs (not
shown). The hearing organ (cochlea) is shaped like a coiled-snail shell
and filled with incompressible fluids. Three scala or ducts run the
length of the coiled cochlea and are separated by the basilar and
Reissner membranes. The scala vestibuli and scala tympani ducts are
continuous and joined at the coil\'s apex by a small open duct, the
helicotrema. The oval window is in the scala vestibuli and the round
window is in the scala tympani at the other end of the duct. The third
canal, the cochlear duct (ductus cochlearis), is between the other two
ducts. It contains the organ of corti positioned on the basilar
membrane. The organ of corti contains 20,000 to 30,000 hair cells in
four rows that reach and attach to a gelatinous layer called the
tectorial membrane. The hair cells are connected in a complex manner to
nerve fibers in the core of the cochlea where they unite to form the
auditory nerve, the acoustic branch of the VIII th cranial nerve. The
auditory nerve follows the auditory pathway through various nuclei to
the auditory cortex in the brain.

Tectorial membrane Hair cells

![](media/image3.wmf)

Auditory nerve Basilar membrane

*\
*

Physiology. Sound waves propelled through air are collected by the pinna
and conducted through the meatus to the eardrum. Ear canal resonance
amplifies sounds in the 3000 Hz region. The sound waves vibrate the
eardrum converting the sound energy to mechanical energy. The malleus
joined to the eardrum conveys this energy through the incus to the
stapes. The flexible round window extends outward when the oval window
is pushed inward by the stapes, and vice versa, creating fluid wave
motion in the vestibuli and tympani ducts. The middle ear increases
pressure to levels required to propagate waves through the
incompressible cochlear fluids. The fluid motion moves the tectorial
membrane and cochlear hair cells stimulating only those that respond to
the frequencies and intensities of the acoustic stimulus. Individual
hair cells are selective and respond only to specific frequencies and
intensities. High frequencies activate hair cells at the lower end and
low frequencies mostly activate hair cells at the higher or pointed end
of the cochlea. The stimulated hair cells send neural information to the
cortex by way of the auditory nerve network.

SOUND

Sound is a disturbance in an elastic media (such as displacement of air
molecules) that is audible.

*Auditory Sensitivity*. The range of human hearing is 1 Hz to 20,000 Hz
(following chart). The young, normal ear is sensitive to acoustic energy
of about 20 Hz to 20,000 Hz, called the audio frequency range. High
frequency sensitivity declines to maximums about 10,000 Hz to 12,000 Hz
in the third and fourth decades. Infrasound, acoustic energy about 20 Hz
and below, can be perceived at high intensity levels but not as tonal
sounds. Sounds above 20,000 Hz have been perceived through bone
conduction. Logarithmic scales were adopted to enable the expansive
range of human hearing to be represented on manageable charts and
displays. Hearing sensitivity of some non-human species reach as high as
40,000 Hz and 100,000 Hz.

Sound

Pressure

Level (dB)

PHYSICAL PROPERTIES OF SOUND

Sound Pressure Level (SPL). Sound pressure level is the ratio of the
pressure of any sound to the reference or ambient sound pressure at that
same point (currently 20 micropascals).

Frequency. Frequency of sound is defined in cycles per second.
Currently, the term Hertz (Hz) is used instead of frequency, i.e., 500
Hz.

*Propagation.* Sound propagates in all directions from its source
creating an expanding sphere when in the atmosphere (free of
reflections). The original sound pressure decreases to half of its value
when the distance from the source is doubled, which is a 6 dB reduction
in SPL. The speed of sound varies slightly with temperature and humidity
and is accepted as moving about 344 m/sec (1032 ft/sec) at a temperature
of 21 degrees centigrade (F).

Spectrum. The spectrum of a sound is displayed by sound pressure level
distributed across frequency. It is commonly described in terms of
levels in successive frequency bands of octave, half-octave, and
third-octave bandwidths but can be in a successive bandwidth of any
size. Noise exposures are usually described in nine octave bands
centered from 31.5 Hz to 8000 Hz. Noises dealing with hearing protector
sound attenuation are typically selected third-octave bands centered at
125 Hz to 8000 Hz.

Localization. The source of a sound is localized by the differences in
time and intensity of the same sound arriving at the separated ears.
Auditory localization is enhanced when the head is turned side to side
while listening. A sound source totally in the midline of the head is
difficult to localize without head movement.

Doppler Effect*.* The frequency of a sound changes while the source is
moving relative to the observer. The frequency of an approaching sound
increases or decreases relative to the source. A common example is the
escalating frequency of an approaching locomotive and the decreasing
frequency while it departs, as observed outside a train station.
Observers on the train hear the constant frequency of the source.

Time Histories (Duration). Time histories describe variations in the
sound pressure of a sound as a function of time. Sounds that are
interrupted in a systematic pattern are periodic. Those interrupted at
random or non-periodic intervals are aperiodic. Steady-state sounds have
durations greater than one second. Impulse sounds are individual
pressure pulses of sudden onset and brief duration and are described by
their rise time, peak level, duration, and number of events or
repetitions. Continuous sound is uninterrupted and of constant or
variable levels.

Pure Tone. A pure tone is a sound characterized by its singleness of
pitch (a sign wave; simple sinusoidal function of the time).

Bone Conduction Sound. Acoustic energy changes to vibratory energy when
it enters the body. Bone conduction sound is audition of vibratory
energy transmitted through tissue and bone stimulating the auditory
system.

Infrasound. Acoustic energy in the frequency range of 20 Hz and below.

Ultrasound. Acoustic energy in the frequency range above 20,000 Hz,
although frequencies as low as 8,000 Hz are sometimes called ultrasound.

NOISE

Noise is any unwanted sound. The perception of the listener determines
if the sound is or is not noise. Pleasant sounds under some conditions
are considered noise and under different settings the reverse is true.

SOURCES OF NOISES IN AVIATION ENVIRONMENTS

Major aviation noise sources are propulsion systems, rotors, propellers,
exhaust, and airflow over the aircraft surfaces as well as radiation
from various maintenance facilities. The most powerful propulsion
systems radiate noises well exceeding 100 dB in areas close to airports
and 110 dB within 600 feet. Aircraft create noise levels at 30 dB to 40
dB above normal conversation levels. Sources of noise inside many
aviation vehicles include air conditioners, blowers, and pneumatic pumps
as well as intrusions of propulsion systems and aerodynamic noises. A
background noise of about 65 dB inside a commercial aircraft should
allow conversation within about three seats and mask those beyond that
distance.

Ground operations produce noise from propulsion systems operated for
maintenance and overhaul on the flight line and test stands. The use of
noise suppressors during engine run-up effectively reduces noise levels.
The controls of ground operation noises are implemented to also protect
maintenance and support personnel from overexposure.

Jet Propulsion Systems. Turbojet engines produce three types of noise:
inlet noise radiated from the air intake, noise radiated from vibrations
of the shell of the engine, and exhaust noise. The exhaust noise
includes some combustion and turbine noise and occurs outside the jet
nozzle where the jet stream mixes with the surrounding air. Turbofan
engines use thrust-producing fans and they create lower jet-exhaust
velocities than the turbo jet enabling a quieter operation for a given
thrust. The broadband aerodynamic-jet noise dominates the noise spectrum
of the turbofan engines at full power.

Propeller Aircraft. The noise of propeller aircraft propulsion systems
comes from the propeller and the engine exhaust. Propeller noise is
usually the significant component and is essentially a function of the
blade tip speed. The range of noises from different propeller aircraft
is very large.

Helicopters. Major helicopter noise sources are the main rotor system
and engine that produce broadband and periodic noise with a blade slap.
Brief peak levels of almost 100 dB can be experienced on the ground
directly under a vehicle at 500 ft elevation.

TYPES OF NOISE

Continuous Noise. Noises that are constant or varying but are not
interrupted.

White Noise*.* Noises that contain all audible frequencies at
approximately the same sound level.

Band-limited Noise. Selected bands comprised of limited frequencies.
Common band limits are octave-band, third-octave band, wide band, and
narrow band.

Impulse/Impact Noise. Exposures generated by heavy artillery (160-180
dB), handgun at the ear (140-170 dB), impact devices in industry
(120-140 dB), and construction site pile-driver (110-130 dB) are
examples of impulse or impact noise.

Blast Noise*.* Blast waves are effectively shock waves generated by
explosive actions including the detonation of TNT, pyrotechnics, and
ammunition.

Sonic Boom. Sonic boom is a shock wave generated by aircraft moving at
supersonic speed. This wave is generated in all directions and when it
reaches the ground is perceived as a boom. The loudness of the sonic
boom on the ground is related to the altitude of the aircraft

EFFECTS OF NOISE ON COMMUNICAITONS

The most expansive impact of noise is interference with audio
communication. Population surveys indicated that the primary complaint
of those near airports was the inability to communicate, use the
telephone, use the radio or hear the television in their own homes
during noisy aircraft operations. Exposed inhabitants have very strong
feelings against the noise, particularly those living under flight paths
and very near airports. These feelings often stimulate legal actions
attempting to resolve noise exposure issues. In spite of current
aviation regulations and initiatives, such as regulation of noise
emission characteristics and control of flight patterns and landing and
takeoff routes, the aviation noise exposure issue remains a very complex
problem.

Voice communication is influenced by personal, environmental, message
and equipment factors in aerospace operations. Personal features of
speech habits, dialects, word usage, experience, emotional state, and
hearing loss can adversely affect audio communication effectiveness. The
efficiency and quality of many communicators improve with practice and
others only with training. Those with such features as poor enunciation
or strong dialects may remain difficult to comprehend particularly when
communication channels are noisy. Familiar phrases and commands
associated with routine actions and events are perceived in noises that
would mask other speech.

Environmental noises interfere with or mask speech signals. Intense
noises may cause aural overload, distortion, and temporary hearing loss
causing additional obstruction. Masking effectiveness varies with the
frequency content of the noise and the ratio of the speech signal level
to the noise. The most effective noise masker contains the same acoustic
energy that is present in the 500 Hz through 6000 Hz speech band.
Average speech spectra vary slightly from talker-to-talker but
significantly with higher levels of vocal effort. Although there is some
upward spread of masking of noises, intelligibility is generally best
when the noise is outside the 500 Hz to 6000 Hz band.

The speech level must be greater than the noise level at the ear for
good intelligibility. Understanding varies with different types of
speech. Comprehension of typical speech is about zero in a 12 dB noise
and greater than 95% in noise equal to and less than the speech.
Comprehension of nonsense syllables is also zero in a 12 dB noise but
must exceed the noise by at least 15 dB to reach 95% correct. The speech
to noise ratio can be improved by increasing the level of the speech
when it is independent of the noise level. Both spectra and levels of
aviation noises must be controlled to minimize masking of the speech
signal and ensure successful communication.

Direct Communication

Face-to-Face*.* The vocal effort and quality of face-to-face
communications in background noises and at talker-listener separation
distances are reflected in terms of A-weighting sound level in the
table. Satisfactory communication, about 90 to 95% correct perception of
sentences, is expected with a normal voice level at a separation
distance of about 4 feet in a noise of 60 dB(A) Talking is difficult to
be understood at 8 feet separation in 80 dB(A) noise. Communication is
impossible at distances of about 6 feet and greater in 100 dB(A) noise.
To maintain good communication, voice level must increase from 3 dB (in
lower noise levels) to 6 dB (in higher noise levels) for every 10 dB
increase in noise level. The average male voice is about 4 dB higher in
noise level than the female voice.

Normal voice conversations are not possible in most high‑noises at
distances greater than a few feet. Above‑normal voice efforts place
additional stress on talkers and listeners. The amount of stress on the
vocal cords depends on both the level of effort and frequency of
communication. Infrequent or occasional raised voices and shouts may be
tolerated while sustained above‑normal vocal effort should be avoided.
Electronically aided communications should be used for these situations
to protect the health and wellbeing of personnel as well as minimize
errors due to inadequate communications.

Communication Equipment. State‑of‑the‑art communications systems and
accompanying terminal equipment are satisfactory for most communication
tasks in noise. Equipment is designed to optimize the speech signal.
Noise-canceling microphones significantly reduce low frequency noises
without affecting the speech signal. The use of insert earplugs under
headsets and helmets provides equal attenuation of the noise and speech.
Increasing the level of the speech does not affect the noise and
improves the speech-to-noise ratio and intelligibility. Noise-excluding
personal equipment is not always adequate for satisfactory
communications when used in the highest levels of noise. In more
moderate noise environs contemporary equipment is satisfactory.
Improvements are required in intense aviation noise environments such as
helicopters, flight decks, and jet engine test cells and some
maintenance environs.

SYMPTOMS AND SIGNS OF NOISE EXPOSURE

Characteristics of noise exposures include noise-induced hearing loss,
interference with routine living activities, reduced performance,
disturbed sleep, induced annoyance, and degraded voice communications.

The most critical effect is noise induced hearing loss prevalent among
people in high-level noise environs. Most noise-induced hearing losses
are in the high frequency region of the ear (500 Hz to 6000 Hz) where
speech is heard. Consequently, many symptoms are associated with
communication.

The higher frequency consonant sounds are difficult to distinguish
causing confusion while listening. Normal levels of speech are no longer
easily recognized and may be misunderstood or not heard. The presence of
background noises mask the speech signals causing the hard of hearing to
talk louder. Responses unrelated to comments and questions indicate
failure to comprehend them.

Elevated levels of music, radio and TV are common signs. An increase in
the use of the question, \"What did you say?\" Some people attempt to
hide their hearing loss by speaking softer than their normal speech.
Some people develop coughs, hoarseness and pain in their throats from
the strain of talking in a noisy environment. Tinnitus noises
(continuous sounds in the ear) can disturb and compete with sounds
received by the hard of hearing. Residual hearing is vulnerable to
masking.

Behavioral cues are not uncommon. Withdrawal from communications can be
easier than struggling to understand meaningful sounds.

Fatigue and irritation at home can be symptoms of exposure during work.
Agitation and annoyance are often blamed on those accused of speaking
too softly. Performance errors can occur with incomplete understanding
of guidance and instructions. Typical behavior of cupping the hand at
the ear to hear better is obvious. Many of these representative
symptoms, and others, are present in the workplace as well as at home
and other places. Fortunately, most hearing losses benefit from the use
of contemporary hearing aids.

HEARING TESTING TECHNIQUES

There are three classes of audiometers. Monitoring audiometers are used
to determine if a hearing loss is present, diagnostic audiometers to
determine the cause of the hearing loss, and research audiometers to
conduct research on speech and hearing problems. Diagnostic and research
audiometers are not directly related to hearing conservation programs.
Bone-conduction audiometry is very complex and should be evaluated only
by an otologist or certified administrator.

Monitoring audiometers use pure tone signals presented over headphones
to detect the presence of hearing loss, measure the amount of loss, and
identify the type of loss. The zero dB value on audiometers is the
average hearing of otologically normal hearing young adults. Audiograms
display the numbers of decibels above the reference zero of the test
frequencies heard by the listener. Normal hearing for adults is within
the audiometric range of -10 dB to 25 dB. Hearing threshold levels
higher than 25 dB indicate non-normal hearing. The effect of hearing
loss on speech perception is evaluated with standardized speech sounds,
words and sentences instead of pure tones. Test materials for
communication ability are based on the comprehension of speech materials
by normal hearing listeners.

BASIC AUDIOMETRIC PROCEDURE

Each test signal is presented to the subject below or near the threshold
of hearing and gradually increased until the subject hears and responds.
The signal is then gradually decreased until no longer heard. This
alternating presentation of signals is continued (usually 30 seconds)
until a threshold is determined for the test frequency. The procedure is
repeated for each standard test signal (500, 1000, 2000, 3000, 4000 and
6000 Hz) and both ears. This bracketing of threshold of each test
frequency is the most common procedure.

STANDARD AUDIOMETERS

*Manual audiometers* are hand-operated by experienced administrators.
The basic procedure is applied until both ears are measured at all test
frequencies. The subject\'s responses are recorded manually or by the
audiometer. Self-Recording Audiometers are operated by the subject who
controls the test signals. The basic procedure is followed until all
measurements are completed. Subject responses are automatically recorded
on audiograms. Microprocessor (computer) Audiometers automatically
control the measurement procedure. The test signals are presented at
levels close to the subject\'s thresholds. The computer controls the
basic procedure until the measurements are completed. Measured threshold
data are printed for each audiometric test.

TEMPORARY AND PERMANENT HEARING LOSSES

Noise-induced hearing loss is either temporary or permanent and is
related to fatigue and damage of hair cells. The locations of abnormal
hair cells appear related to the frequencies of the acoustic stimuli
causing the hearing loss. Temporary hearing loss appears to be
associated with hair cells that are fatigued but not damaged. Very high
magnification displays hairs that appear abnormal during temporary
hearing loss and return to the normal positions when hearing is
recovered. Permanent hearing losses are related to areas where hair
cells are missing.

Temporary threshold shift (TTS) is a loss of sensitivity that returns to
pre-exposure hearing levels within a reasonable time after cessation of
the noise. Permanent threshold shift (PTS) persists with no recovery
regardless of the time away from noise. Relations have been established
among TTS, PTS and noise experienced daily over many years.

Noise induced TTS is considered to be a precursor to noise induced PTS.
It is assumed that (1) noise exposures that do not produce TTS will not
produce PTS; (2) PTS develops similarly to TTS but on a slower time
scale; and, (3) different noise exposures that produce equal amounts of
TTS are considered equally noxious to PTS. Hearing risk criteria that
relate noise exposure with hearing loss are based on TTS data from
laboratory studies and both TTS and PTS data from population field
studies.

Recovery of TTS appears related to the amount and duration of the
exposure and not the frequency of the stimulus. Temporary threshold
shifts exceeding 30 dB to 40 dB tend to have long-lasting recoveries.
Shifts less than about 15 dB recover rapidly. Hearing loss does not
continue to grow with ongoing exposures. Threshold levels reach an
asymptote after about 10 to 12 hours of continuous exposures and TTS
does not increase even though the noise exposure continues. The sound
levels of asymptotic thresholds vary with the nature and level of the
noise stimuli. Studies and experiments on asymptotic threshold shift
focused on effects of continuous noise exposures of 8, 24 and 48 hours
duration.

TTS and PTS can be accompanied by tinnitus, a persistent tonal or
noise-like sound in the ear. It often occurs among workers in noise
environments and those using tools such as air hammers or chain saws.
Most moderate and severe hearing losses occur without tinnitus. Some
people with normal hearing and no history of noise exposure are
afflicted with tinnitus.

PREVENTION OF NOISE EXPOSURE

The Noise Control Act of 1972 established the Environmental Protection
Agency (EPA). The EPA instituted a regulation requiring all
noise-protecting devices to be labeled. The regulation requires the
attenuation of any device worn on the head or in the ears and sold for
its ability to reduce sound entering the ear to be evaluated according
to American National Standards Institute (ANSI) standards.

The Noise Reduction Rating (NRR) describes how much the overall noise is
reduced by the hearing protector. It is mandated by EPA and is based on
the ANSI measurement procedure. It is calculated using the one-third
octave-band attenuation data obtained in the laboratory and provides a
single number value of the hearing protector attenuation. The NRR
calculation uses a standard noise level of 107.9 dB and is independent
of the noise spectrum to which it applies. The calculated single hearing
protection value number is subtracted from the standard noise exposure
level to determine the noise level at the ear under the protector.

Another preferred method is the A-weighted sound level that is a little
more accurate than the NRR. The A-weighted sound level is the
equal-loudness contour of normal hearing subjects at a level of 40 dB
reflecting decreasing sensitivity with decreasing frequencies from 1000
Hz and lower. The laboratory one-third octave-band values are subtracted
from the A-weighted octave band levels of the noise. The resulting
values are logarithmically summed providing an overall A-weighted sound
pressure level at the ear under the protector. The A-weighted
calculation includes the noise environs where the protector will be
worn.

ALLOWABLE DAILY EXPOSURE

A daily exposure is a noise dose experienced during an 8-hour period.
The Occupational Safety and Health Administration (OSHA) has adopted a
90 dB(A) level limit with a 5-dB level vs duration trading relationship
for an 8-hour exposure. A continuous exposure for 8 hours at a level of
90 dB(A) is 100% of the allowable daily noise dose. The levels and
durations of exposures can be traded so long as the exposure does not
exceed 100%. The 5 dB noise is traded for a halving or doubling of the
exposure time. A 5 dB increase to 95 dB(A) would reduce the allowable
exposure time to 4 hours while a 5 dB decrease to 85 dB(A) would permit
16 hours. These traded values are called Time Weighted Averages (TWA).
Employees experiencing a noise dose of 50% or a TWA of 85 dB must be
included in a hearing conservation program. Other trading relationships
and criteria are in use. The EPA uses a criterion of 85 dB(A) with a
trading value of 3 dB which is more protective than the OSHA criterion
and trading value.

NOISE MEASUREMENT

The basic instruments used in the definition of noise environments and
noise exposures are the sound level meter and the noise dosimeter.

Sound Level Meter. The sound level meter (SLM) is a basic instrument for
sound measurements (SPL). It provides a single number overall reading of
the sound pressure level in the audible frequency range. Most SLMs
contain the three standardized weightings of A, B, and C. They measure
the approximate loudness response of the human ear at the respective
sound levels of 40 dB (A-weighting), 70 dB (B-weighting) and 100 dB
(C-weighting). This meter is very important because it measures the
A-weighted sound level that is the basis of many noise exposure
standards and criteria.

Noise Dosimeters*.* Noise dosimeters are the preferred instruments for
rating noise exposure for potential effects on hearing. They are
integrating sound level meters that quantify daily noise exposures.
Portable dosimeters are small units that can be hand-held or left at a
specific location, unattended, for the duration of an exposure. Personal
dosimeters fit into a shirt pocket and have a small microphone on a thin
cable worn at the ear. The dosimeter output indicates the percentage of
the allowable daily noise dose actually experienced by the individual
wearing the unit.

NOISE REDUCTION

Engineering. The control of stationary noises is treated with
conventional engineering controls including isolation, shock mounting,
damping, shielding, and enclosing the source. Mufflers, hush houses, and
noise suppressors reduce noise levels also extending allowable
maintenance time. Inside aviation vehicles, internal noises originate
from air conditioners, blowers, and pneumatic pumps. External noise from
the propulsion systems and the aerodynamic noise flowing over the
fuselage also radiate to the inside. Noise reduction is usually designed
to assure safe exposure conditions and effective communication
capability. A background noise of about 65 dB inside a commercial
aircraft is intended to allow conversations within about three seats and
mask those beyond that area or distance.

Hearing Conservation Programs. These programs are established to protect
the hearing of employees from damage due to hazardous noise exposures in
the workplace. A basic program includes a sound survey, engineering and
administrative controls, education for staff and employees, hearing
protection, and audiometric monitoring. The sound survey identifies
areas where employees require hearing protection, where noises interfere
with communication or annoy workers. The sound survey also documents the
noise conditions for future (regulatory) purposes. Sound surveys should
be repeated periodically (annually) and when there are changes of
facilities and equipment.

Engineering controls treat current facilities and replace older
equipment with quieter machinery. However, the elimination of all
undesirable noises is not feasible. Administration controls can reduce
exposures by moving susceptible workers to quieter task areas and
limiting daily time in noise.

All personnel, staff and employees, must be well educated about noise
exposure, the various undesirable consequences and the values of
participation that provides the required protection. Education is very
important and should be repeated and organized so that employees clearly
feel a part of the program.

Audiometry. Audiometric monitoring is the periodic measurement of the
hearing thresholds of noise-exposed employees. A reference or baseline
audiogram should be obtained before the employee is exposed to the noisy
area and repeated on an annual basis. Audiograms identify existing
hearing loss that might defer assignment to a noisy area and also reveal
hearing changes not associated with noise exposure. They inform the
audiologist of hearing loss and indicate what action should be taken.
Typically, workers with normal or allowable hearing changes return to
work. Employees with significant hearing changes must return for a
second (validation) audiogram in the near future. Those with persistent
significant changes on the second audiogram are referred to an
otologist. The otologist\'s examination and evaluation determine the
disposition of the worker.

HEARING PROTECTION

Passive Hearing Protection Devices

Passive hearing protectors are categorized as earplugs, earmuffs, and
helmets. Insert earplugs are pre-molded, custom molded and formable.
Pre-molded earplugs are provided in one or more sizes to fit the ears of
most people. Custom molded earplugs are individually made to fit the ear
of the user. Formable earplugs are made of pliable materials and formed
by the user to fit into and seal the ear canal. Foam earplugs are made
of slow-recovery closed-cell foam that is compressed and inserted into
the ear canal where it expands to form a seal. Semi-insert earplugs are
designed to seal the ear canal at its entrance and are suspended on a
headband.

Earmuffs are noise excluding cups that completely enclose the ears and
are supported on a headband or hard hat. Earmuffs attached to a hard hat
are more difficult to fit properly than those with a headband.

Helmets enclose a substantial portion of the head and can provide good
attenuation when fit with ear cups or a snug fitting cushion around the
edge. When the auditory pathway is effectively occluded sound may reach
the inner ear through air leaks, vibration of the hearing protector,
transmission through materials and the bone and tissue conduction
pathways to the inner ear.

Premolded earplugs provide similar amounts of attenuation of about 25 dB
at frequencies up to 1000 Hz and as much as 40 dB at higher frequencies.
Attenuation among formable earplugs varies with as little as 10 dB at
the lowest frequencies and as much as 30 dB to 45 dB above 2000 Hz. Good
insert earplugs attenuate infrasonic frequencies (sound below 20 Hz)
similar to that at 125 Hz. They provide good protection at all
ultrasonic frequencies.

Custom-molded earplug attenuation varies widely because of differences
among materials, production and fitting. Performance of custom-molded
earplugs is less than that of some premolded devices. The performance of
semi-insert devices also varies with the best providing an average
attenuation of about 20 dB below and 35 dB above 2000 Hz.

The best overall protection is obtained with slow-recovery foam earplugs
with attenuations of 20 dB to 40 dB at frequencies below 2000 Hz and 30
dB to 45 dB above 1000 Hz. Attenuation from deep inserted foam earplugs
can exceed 35 dB at all test frequencies. The attenuation of earmuffs is
influenced by differences among the manufacturer\'s models. Attenuation
as low as 5 dB to 15 dB can be experienced at frequencies less than 1000
Hz with an average of 35 dB at frequencies above 2000 Hz.

Earmuffs provide very little attenuation against infrasound. They
provide very good protection against ultrasonic frequencies. Normal
hearing persons wearing earplugs or earmuffs can understand
communications in broadband noises of 85 to 105 dB. Eyeglass temples
worn under earmuffs cause a typical 3 dB to 7 dB loss of attenuation.

Earplugs worn with earmuffs or helmets provide more protection than
either device but not the sum of both. The amount of increase varies
with frequency and ranges from 0 dB to as much as 15 dB. Attenuation
changes very little when an earplug is used with different earmuffs but
changes greatly when an earmuff is used with different earplugs. Most
good earplug-earmuff combinations provide attenuation above 2000 Hz
(about 40 dB to 50 dB) that reaches the threshold of bone conduction
pathways to the ear.

Auditory warning signals are usually recognized whether ears are or are
not covered with hearing protectors. Non-normal hearing individuals
might experience difficulties with recognition depending on factors such
as type of hearing protector and degree of hearing loss. Auditory
localization relies on utilization of important high-frequency sounds by
the pinna. Earmuffs and helmets effectively deprive the pinna of the
sounds essential for localization. Insert earplugs are less disruptive
because the pinnae is uncovered.

Typical low and high attenuation expected of laboratory experimenter fit
(best fit) or experimenter-assisted fit hearing protectors is shown in
the previous table. Attenuation of all hearing protectors will not
necessarily fall within these values. The amount of protection actually
achieved varies widely among individuals due to fit, comfort, care, use,
condition and effectiveness of the protector. Protectors that are
misfit, damaged or modified in any way are usually ineffective.

Employees should be inspected unannounced in the workplace to ensure the
hearing protector in use is effective. Unfortunately, for numerous and
varied reasons many employees obtain significantly less protection than
is available with the provided devices and suffer the consequences.

Active Hearing Protective Devices.

Active Hearing Protectors are earmuffs containing electronic components
that enhance communications for persons wearing hearing protection in
low-level noises. Audio information is automatically conveyed into the
earmuff when outside noise levels are below a criterion such as 85
dB(A). When the outside noise exceeds the criterion level the
electronics do not operate and the earmuff performs as a passive device.

Active Noise Reduction (ANR) Headset.

The ANR headset or helmet electronically detects the noise inside the
earmuff, processes it, and returns it to inside the earmuff 180 degrees
out of phase. This effectively cancels a significant amount of the
low-frequency noise below 2000 Hz and increases the attenuation 15 to 25
dB. These actions substantially reduce low-frequency sounds at the ear,
improve intelligibility and comfort and decrease fatigue.

Communication Earplug (CEP).

The communication earplug is a widely used insert device containing a
miniature microphone inserted into the ear canal. The insert device
comes in different sizes and the depth of insertion protects the
microphone from external noise enhancing speech communication. The CEP
also provides good insert earplug protection against adverse effects of
noise on hearing. It is used alone and with both earmuffs and helmets.
The CEP must be connected to the communication system.

Noise exposure is pervasive. In spite of federal legislation,
engineering controls, hearing protection devices, and hearing
conservation programs adverse effects of noise persist. It is widely
believed that most noise induced hearing loss is preventable, that
contemporary hearing protection devices are effective, and the system
seems to breakdown at the noise site.

Smith, Suzanne D. and Nixon, Charles W. (2002) \"Chapter 7: Vibration,
Noise, and Communications\". In: Fundamentals of Aerospace Medicine 3rd
Ed, DeHart, Roy L. and Davis, Jeffrey R., eds, Lippincott Williams &
Wilkins, Philadelphia.

Nixon, Charles W. and Berger, Elliott, H. (1991) \"Chapter 21: Hearing
Protection Devices\". In: Handbook of Acoustical Measurements and Noise
Control 3rd Ed, Harris, Cyril M., ed, McGraw-Hill, Inc, New York.