HEARING AND NOISE IN
AVIATION
HEARING
he t
fun
He
physiolog
erm hearing describes the process,
ction, or power of perceiving sound.
aring is second only to vision as a
ical sensory mechanism to obtain critical
information during the operation of an aircraft. The
sense of hearing makes it possible to perceive,
process, and identify among the myriad of sounds
from the surrounding environment.
Anatomy and Physiology of the
Auditory System
The auditory
system
consists of
the external
ear, ear canal,
eardrum,
auditory
ossicles,
cochlea (which
resembles a
snail shell and is fi lled with fl uid), and the auditory
nerve.
Ambient sound waves are collected by the external
ear, conducted through the ear canal, and cause
the eardrum to vibrate. Eardrum vibration is
mechanically transmitted to the ossicles, which, in
turn, produce vibration of a fl exible window in the
cochlea. This vibration causes a pressure wave
in the fl uid located inside the cochlea, moving
thousands of hair-like sensory receptors lining the
inner walls of the cochlea. The movement of these
receptors resembles the gentle movement of a
crop fi eld caused by the wind. The stimulation of
these sensors produces an electrical signal that is
transmitted to the brain by the auditory nerve. This
signal is then processed by the brain and identifi ed
as a particular type of sound.
is used to describe the mechanical
SOUND
The term sound sound sound is used to describe the mechanical
radiant energy that is transmitted by longitudinal
pressure waves in a medium (solid, liquid, or gas).
Sound waves are variations in air pressures above
and below the ambient pressure. From a more
practical point of view, this term describes the
sensation perceived by the sense of hearing. All
sounds have three distinctive variables: frequency,
intensity, and duration.
Frequency. This is the physical property of sound
that gives it a pitch. Since sound energy propagates
in a wave-form, it can be measured in terms of wave
oscillations or wave cycles per second, known as
hertz (Hz). Sounds that are audible to the human
ear fall in the frequency range of about 20-20,000
Hz, and the highest sensitivity is between 500 and
4,000 Hz. Sounds below 20 Hz and above 20,000
Hz cannot be perceived by the human ear. Normal
conversation takes place in the frequency range
from 500 to 3,000 Hz.
Intensity. The correlation between sound intensity
and loudness. The decibel (dB) is the unit used
to measure sound intensity. The range of normal
hearing sensitivity of the human ear is between
-10 to +25 dB. Sounds below -10dB are generally
imperceptible. A pilot who cannot hear a sound
unless its intensity is higher than 25 dB (at any
frequency) is already experiencing hearing loss.
Duration. Determines the quality of the perception
and discrimination of a sound, as well as the
potential risk of hearing impairment when exposed
to high intensity sounds. The adverse consequences
of a short-duration exposure to a loud sound can be
as bad as a long-duration exposure to a less intense
sound. Therefore, the potential for causing hearing
damage is determined not only by the duration of a
sound but also by its intensity.
NOISE
The term noise refers to a sound, especially one
which lacks agreeable musical quality, is noticeably
unpleasant, or is too loud. In other words, noise is
is used to describe the mechanical
any unwanted or annoying sound. Categorizing a
sound as noise can be very subjective. For example,
loud rock music can be described as an enjoyable
sound by some (usually teenagers), and at the same
time described as noise by others (usually adults).
Sources of Noise in Aviation. The aviation
environment is characterized by multiple sources of
noise, both on the ground and in the air. Exposure
of pilots to noise became an issue following the
introduction of the fi rst powered aircraft by the
Wright Brothers, and has been a prevalent problem
ever since. Noise is produced by aircraft equipment
powerplants, transmission systems, jet effl ux,
propellers, rotors, hydraulic and electrical actuators,
cabin conditioning and pressurization systems,
cockpit advisory and alert systems, communications
equipment, etc. Noise can also be caused by
the aerodynamic interaction between ambient air
(boundary layer) and the surface of the aircraft
fuselage, wings, control surfaces, and landing
gear. These auditory inputs allow pilots to assess
and monitor the operational status of their aircraft.
All pilots know the sounds of a normal-functioning
aircraft. On the other hand, unexpected sounds
or the lack of them, may alert pilots to possible
malfunctions, failures, or hazards. Every pilot has
experienced a cockpit or cabin environment that was
so loud that it was necessary to shout to be heard.
These sounds not only make the work environment
more stressful but can, over time, cause permanent
hearing impairment. However, it is also important
to remember that individual exposure to noise is a
common occurrence away from the aviation working
environment—at home or work, on the road, and in
public areas. The effects of pre-fl ight exposure to
noise can adversely affect pilot in-fl ight performance.
Sources of Sound/Noise
SOURCES LEVEL (dB)
Whispered Voice 20-30
Urban Home, Average Offi ce 40-60
Average Male Conversation 60-65
Noisy Offi ce, Low Traffi c Street 60-80
Jet Transports (Cabin) 60-88
Small Single Plane (Cockpit) 70-90
Public Address (PA) Systems 90-100
Busy City Street 80-100
Single Rotor Helicopter (Cockpit) 80-102
Power Lawn Mower, Chain Saw 100-110
Snowmobile, Thunder 110-120
Rock Concert 115-120
Jet Engine (Proximity) 130-160
Types of Noise
Steady: Continuous noise of sudden or gradual
onset and long duration (more than 1 second).
Examples: aircraft powerplant noise, propeller noise,
and pressurization system noise.
According to the
Occupational Safety and Health Administration
(OSHA), the maximum permissible continuous
exposure level to steady noise in a working
environment is 90 dB for 8 hours.
Impulse/blast: Noise pulses of sudden onset and
brief duration (less than 1 second) that usually exceed an intensity of 140dB. Examples: fi ring a
handgun, detonating a fi recracker, backfi ring of
a piston engine, high-volume squelching of radio
equipment, and a sonic boom caused by breaking
the sound barrier. The eardrum may be ruptured by
intense levels (140dB) of impulse/blast noise.
EFFECTS OF NOISE EXPOSURE
Physiologic
• Ear discomfort: May occur during exposure to a
120 dB noise.
• Ear pain
: May occur during exposure to a 130 dB
noise.
• Eardrum rupture: May occur during exposure to a
140 dB) noise.
• Temporary hearing impairment. Unprotected
exposure to loud, steady noise over 90 dB for a
short time, even several hours, may cause hearing
impairment. This effect is usually temporary and
hearing returns to normal within several hours
following cessation of the noise exposure.
• Permanent hearing impairment: Unprotected
exposure to loud noise (higher than 90dB) for
eight or more hours per day for several years,
may cause a permanent hearing loss. Permanent
hearing impairment occurs initially in the vicinity
of 4,000 Hz (outside the conversational range)
and can go unnoticed by the individual for some
time. It is also important to remember that hearing
sensitivity normally decreases as a function of age
at frequencies from 1,000 to 6,000 Hz, beginning
around age 30.
Psychologic
• Subjective effects: Annoying high-intensity
noise can cause distraction, fatigue, irritability,
startle responses, sudden awakening and poor
sleep quality, loss of appetite, headache, vertigo,
nausea, and impair concentration and memory.
• Speech interference: Loud noise can interfere
with or mask normal speech, making it diffi cult to
understand.
• Performance: Noise is a distraction and can
increase the number of errors in any given task.
Tasks that require vigilance, concentration,
calculations, and making judgments about time
can be adversely affected by exposure to loud
noise higher than 90 dB.
HOW TO PROTECT YOUR HEARING
Limiting duration of exposure to noise. OSHA-
established permissible noise exposure limits for the
workplace (including the cockpit of an aircraft):
Noise Exposure Level Limits
Noise Intensity
(dB)
Exposure Limit
(hrs. per day)
92 6
95 4
97 3
100 2
102 1.5
105 1
110 .5
115 .25
Use Hearing Protection Equipment. If the ambient
noise level exceeds OSHA’s permissible noise
exposure limits, you should use hearing protection
devices—earplugs, earmuffs, communication
headsets, or active noise reduction headsets. Even
if an individual already has some level of permanent
hearing loss, using hearing protection equipment
should prevent further hearing damage. These
protection devices attenuate noise waves before
they reach the eardrum, and most of them are
effective at reducing high-frequency noise levels
above 1,000 Hz. It is very important to emphasize
that the use of these devices does not interfere with
speech communications during fl ight because they
reduce high-frequency background noise, making
speech signals clearer and more comprehensible.
• Earplugs. Insertable-type earplugs of fer a very
popular, inexpensive, effective, and comfortable
approach to provide hearing protection. To be
effective, earplugs must be inserted properly to
create an air-tight seal in the ear canal. The wax-
impregnated moldable polyurethane earplugs
provide an effective universal fi t for all users and
provide 30 to 35 dB of noise protection across all
frequency bands.
• Communication headsets. In
general, headsets provide the
same level of noise attenuation
as earmuffs, and are also more
easily donned and removed that
earplugs, but the microphone
can interfere with the donning of
an oxygen mask.
• Active noise reduction headsets.
This type of
headset uses active noise reduction technology
that allows the manipulation of sound and signal
waves to reduce noise, improve signal-to-noise
ratios, and enhance sound quality. Active noise
reduction provides effective protection against low-
frequency noise.
The electronic coupling of a low-
frequency noise wave with its exact mirror image
cancels this noise.
. The • Combinations of protection devices
combination of earplugs with earmuf
fs or
communication headsets is recommended when
ambient noise levels are above 115dB. Earplugs,
combined with active noise reduction headsets,
provide the maximum level of individual hearing
protection that can be achieved with current
technology.
SUMMARY
• Hearing is second only to vision as a sensory
mechanism to obtain critical information during the
operation of an aircraft.
•
All sounds have three distinctive variables:
frequency, intensity, and duration.
• Normal conversation takes place in the frequency
range from 500 to 3,000 Hz.
• Daily exposure to noise levels higher than 90dB
can cause hearing impairment. This can go
unnoticed initially because it occurs in the vicinity
of 4,000 Hz (outside the conversational range)
• If the ambient noise level reaches 90dBA, you
must use hearing protection equipment to prevent
hearing impairment.
• Exposure to loud noise before fl ying (at home,
while driving, at a party, etc.) can be as harmful as
exposure to aircraft noise.
MEDICAL FACTS FOR PILOTS
Publication: AM-400-98/3
Written by: Melchor J. Antuñano, M.D.,
and James P. Spanyers
Prepared by: FAA Civil Aerospace Medical Institute
Aeromedical Education Division
AAM-400, P.O. Box 25082
Oklahoma City, OK 73125
Other Pilot Safety Brochures Available
Number Title
AM-400-94/2 Alcohol and Flying: A Deadly
Combination
OK05-0270 Carbon Monoxide: A Deadly Threat
AM-400-03/2 Deep Vein Thrombosis and Travel
AM-400-91/1 Hypoxia: The Higher You Fly, the Less
Air...
AM-400-97/1 Introduction to Human Factors in Aviation
AM-400-92/1 Over the Counter Medications and Flying
AM-400-98/2 Pilot Vision
AM-400-95/1 Smoke!
AM-400-00/1 Spatial Disorientation: Visual Illusions
AM-400-03/1 Spatial Disorientation: Why You Shouldn’t
Fly By the Seat of Your Pants
AM-400-01/1 Physiological Training Courses for Civil
Aviation Pilots
AM-400-05/1 Sunglasses for Pilots: Beyond the Image
To view these pilot and passenger safety brochures,
visit the Federal Aviation Administration’s Web Site: www.faa.gov/pilots/safety
Physiological Training Classes for
Pilots
www
If you are interested in taking a one-day aviation
physiological training course with altitude chamber
and vertigo demonstrations or a one-day survival
course, learn about these courses by visiting this
FAA
Web site:
www.faa.gov/pilots/training/airman_education