Noise and Vibration in Army Aviation
Aircraft, both rotary and fixed wing, produce perhaps the
most severe noise and vibration environments experienced by
aircrew members. These biomechanical force environments,
singly and in combination, threaten the health, safety, and
well-being of people associated with or exposed to aircraft
operations. Mechanical vibration transmitted to human
operators can induce fatigue, degrade comfort, interfere with
performance effectiveness, and under severe conditions,
influence operational safety and occupational health.
Excessive exposure to airborne acoustic energy may interfere
with routine living activities, induce annoyance, degrade
voice communication, modify physiological functions, reduce
the effectiveness of performance, and cause noise-induced
hearing loss. Both noise and vibration effects may occur
simultaneously with the initial exposure or may be manifested
only after the passage of time and repeated exposure. The
impact of most exposures can be minimized by focusing on the
source, the propagation of the energy, and the exposed crew
member. Monitoring the influence of such exposures over time
with hearing tests and medical observations can also
determine the impact of these combined factors. This chapter
addresses the physiology of both noise and vibration and ways
to minimize their short-term and long-term exposures. Aircrew
members must use their knowledge and training to protect
themselves and to prevent injuries caused by noise and
NOISE CHARACTERISTICS AND EFFECTS
7-1. Noise is sound that is loud, unpleasant, or
unwanted. Vibration is the motion of objects
relative to a reference position, which is
usually the object at rest. In aviation, both may
cause annoyance, speech interference, fatigue,
and hearing loss.
7-2. Noise energy is undesirable when attention
is called to it unnecessarily or when it
interferes with routine activities in the home or
while flying an aircraft. Individuals become
annoyed when the amount of interference becomes
significant. High-frequency noises and vibration
are especially irritating and can cause a
subjective sense of fatigue.
7-3. When noise and vibrations reach a certain
loudness or amplitude, they mask normal speech
communication. Thus, words become difficult to
7-4. The most important and common undesirable
effect of noise is permanent hearing damage.
Excessive vibrations can manifest themselves in
terms of internal organ malfunctions and skeletal
disabilities. Damage may be rapid when noise is
either extremely intense or prolonged. More
often, it is insidious in onset and results from
continual exposure at lesser intensities. All
aviation personnel need to recognize that the
damage may become permanent.
SOUND AND VIBRATIONAL MEASUREMENT
7-5. Sound and vibration energy have measurable
characteristics. These characteristics are
frequency, intensity (or amplitude), and
7-6. Frequency is the physical characteristic
that gives a sound the quality of pitch.
Frequency of periodic motion is the number of
times per second that the air pressure
oscillates. The number of oscillations, or cycles
per second, is measured in hertz.
Human Hearing and Speech Range
7-7. The human ear is very sensitive and can
detect frequencies from 20 to 20,000 hertz.
Speech involves frequencies from 200 to 6,800
hertz, the range in which the ear is most
7-8. Speech Intelligibility
7-9. People must be able to hear in the range of
300 to 3,000 hertz to understand speech
communication. Speech outside these ranges may
result in incoherence or misinterpretation.
7-9. Vibration affects the body most in low
frequencies, usually confined to frequency ranges
below 100 hertz to displace body parts. These
effects vary greatly with the direction, body
support, and restraint.
7-10. Intensity is a measure that correlates
sound pressure to loudness. Amplitude (for
vibration) is the maximum displacement about a
position of rest.
7-11. Aviation personnel need
to understand the relationship of decibels to
sound pressure (vibration). For every 20-decibel
increase in loudness, sound pressure increases by
a factor of 10. At 80 decibels, sound pressure is
10-thousand times greater than at 0 decibel; at
100 decibels, sound pressure is one-million times
greater than at 0 decibel. The same sound
pressure moving through the air that stimulates
the ear to hear may also cause hearing loss under
certain conditions. Table 7-1
shows the effects of various sound intensities on
Table 7-1. Effects of Various Sound
Intensities on the Listener
7-12. Duration is the length of time that an
individual is exposed to noise or vibrations. It
is a variable factor that may be measured in
seconds, minutes, hours, or days or any other
selected unit of time.
NATURAL BODY RESONANCE
7-13. Natural body resonance is the mechanical
amplification of vibration by the body occurring
at specific frequencies. Table
7-2 shows resonant frequencies for various
parts of the human body.
Table 7-2. Resonant Frequencies for Various
Parts of the Human Body
7-14. Damping is the loss of mechanical energy in
a vibrating system. This loss causes the
vibration to slow down.
NOISE AND HEARING LEVELS
7-15. Army aviation personnel are exposed to two
types of sound levels that can impair their
hearing. The sound levels that affect the
duration of noise exposure are steady-state noise
and impulse noise.
7-16. Aviation personnel encounter this type of
continuous noise around an operating aircraft.
The noise is usually at a high intensity over a
wide range of frequencies. The Surgeon General
has established 85 decibels, at all frequencies,
as the maximum permissible sound level for
continuous exposure to steady-state noise
(damage-risk criteria). There is a direct link
between duration of exposure and intensity; the
louder the sound, the shorter the time required
to cause hearing loss. Table
7-3 shows the recommended allowable sound
intensities for the various durations of
exposure. Exposure to noise above recommended
duration levels could result in noise-induced
hearing lossthe primary risk to Army
Table 7-3. Recommended Allowable
7-17. Weapons fire produces this type of noise.
It is an explosive sound that builds rapidly to a
high intensity and then falls off rapidly.
Although the entire cycle usually lasts only
milliseconds, this sound is detrimental to
hearing when the intensity exceeds 140 decibels.
Looking at Army aircraft as both fixed and rotary
wing, certain generalizations can be made.
Overall noise levels generally are equal to 100
or more decibels. This level exceeds the average
85-decibel damage-risk criteria. Table 7-4 shows the estimated
noise levels for both rotary- and fixed-wing Army
Table 7-4. Rotary-Wing and Fixed-Wing
Aircraft Noise Levels
7-19. The frequency that generates the most
intense level is 300 hertz. Low-frequency noise
will produce a high-frequency hearing loss.
Providing adequate hearing protection for lower
frequencies is very difficult. Exposures to these
levels without hearing protection will cause
permanent, noise-induced hearing loss.
7-20. The human body reacts in various ways to
- Vibration can cause short-term acute
effects because of the biomechanical
properties of the body.
- The human body acts like a series of
objects connected by springs.
- The connective tissue that binds the
major organs together reacts to vibration
in the same way as springs do.
- When the body is subjected to certain
frequencies, the tissue and organs will
begin to resonate (increase in
- When objects reach their resonant
frequencies, they create a momentum,
which increases in intensity with each
- Without shock absorption, vibration will
damage the mass or organ.
7-21. Helicopters subject aircrew members to
vibrations over a frequency range that coincides
with the resonant frequencies of the body (Table 7-5). Prolonged contact
with vibration causes short-term effects, as well
as long-term effects, to the body. Minor
amplitudes of the vibration and the ability of
the body to provide some dampening are reasons
why humans do not receive injuries every time
they fly. Vibration can affect the respiratory
system as well as cause
- Motion sickness.
- Microcirculatory effects.
- Visual problems.
Table 7-5. Vibration Frequency Levels for
the UH-1 Helicopter
7-22. Such factors as age, health, and the noise
environment cause hearing loss. There are three
types of hearing loss: conductive, presbycusis,
7-23. This type of hearing loss occurs when some
defect or impediment blocks sound transmission
from the external ear to the inner ear. Wax
buildup, middle-ear fluid, and calcification of
the ossicles can all impede the mechanical
transmission of sound. A conductive hearing loss
affects mainly the low frequencies. In most
cases, this type of hearing loss can be treated
medically. A hearing aid is often beneficial
because the inner ear can still pick up sounds if
they are loud enough. The aviator may fly with a
hearing aid if he or she is given a waiver to
continue on flight status.
7-24. This type of hearing loss usually results
from old age. The hair cells of the cochlea
become less resilient as people age.
7-25. Sensorineural hearing loss occurs when the
hair cells of the cochlea are damaged in the
inner ear. The primary cause is noise exposure,
but disease or aging also can cause this type of
hearing loss. Sensorineural hearing loss caused
by noise exposure usually occurs first in the
higher frequencies. In some cases, a hearing aid
may benefit, but generally, no known medical cure
exists for this type of hearing loss.
7-26. A crew member may have an ear infection
that could cause conductive hearing loss and have
been diagnosed with a senorineural hearing loss.
The ear infection is treatable; sensorineural
hearing loss is not.
HEARING PROTECTION AND REDUCTION OF VIBRATIONAL THREAT
7-27. Pilots, aircrew members, ground-support
troops, and passengers should wear hearing
protection at all times. Hearing loss is one
hazard of the aviation environment that adequate
protective measures can minimize.
amount of sound protection that a protective
device provides is determined by its fit and
condition and, most importantly, by the
willingness and ability of the individual to use
it properly. Using individual devices in
combination provides the best hearing protection.
7-29. While individual devices are not
foolproof, virtually all noise-induced hearing
loss is preventable if these devices fit properly
and are worn on all flights. Even if hearing has
already been affected somewhat, these devices
will help prevent further damage. Hearing
protection is ultimately each individuals
7-27. Aircraft noise levels interfere with the
speech communication of Army aircrew members and
pose the risk of hearing loss. Protective
measures can reduce the undesirable effects of
noise. These measures include
- Use of personal protective measures.
- Isolation or distancing of crew members
from the noise source.
The HGU-56P (Figure 7-1)
and SPH-4B (Figure 7-2)
aviator helmets are excellent means of personal
protection from the standpoint of noise and crash
attenuation. The helmets, designed primarily for
noise protection, provide noise attenuation
exceptionally well in the range of 3,000 to 8,800
Figure 7-1. HGU-56P Helmet
Figure 7-2. SPH-4B Flight Helmet
7-32. When worn alone, the SPH-4B and the HGU-56P
helmets reduce the noise exposure to safe limits
for every aircraft in the Army inventory except
for the UH-60 (Black Hawk) and CH-47 (Chinook). Table 7-6 shows the estimated
attenuation levels for various types of helmets.
The UH-60 and CH-47 aircraft require both helmet
and earplug use to attenuate noise and prevent
Table 7-6. Estimated Attenuation Levels for
Helmets and Other Protective Devices
7-33. Ancillary devices worn with the
aviators helmet can significantly
compromise hearing protection. For example,
eyeglass frames break the ear seal, creating a
leak and producing a sound path from outside to
inside the earcup.
7-34. The communications
earplug, Figure 7-3,
improves hearing protection and speech-reception
communication. The device includes a miniature
transducer that reproduces speech signals from
the internal communication system. The foam tip
acts as a hearing protector, similar to the
yellow-foam earplugs that pilots wear for double
hearing protection. A miniature wire from the CEP
connects to the ICS through the mating connector
mounted on the rear of the helmet. The CEP has
recently been issued its AWR for all U.S. Army
aircraft using the SPH-4B or HGU-56P helmets and
for the M45 ACPM for all U.S. Army aircraft using
the M24 mask. The tested pilot population has
enthusiastically received this communication
device. This product is not yet in the federal
stock system. For more information on this
product, contact the U.S. Army School of Aviation
Medicine at DSN 558-7680.
Figure 7-3. Communications Earplug
7-35. Insert-Type Earplugs. Insert-type
earplugs are among the most common types of
hearing protection now in use. Earplugs need to
be comfortable if they are to do their job. All
earplugs tend to work loose because of talking or
vibration and need to be reseated periodically to
prevent inadvertent noise exposure. With properly
fitted earplugs, users voices will sound
lower and muffled, as if they were talking inside
a barrel. Noise protection with earplugs is 18 to
45 decibels across all frequency bands. Earplugs
may come in three different types: the E-A-RŪ
foam earplug, the V-51R single-flange earplug,
and the SMR triple-flange earplug. Wearing
earplugs for the first time in the cockpit may
diminish the ability to hear communications in
the cockpit. Crew members may feel that they have
to concentrate and listen more closely to the
transmissions. Once they get used to listening
with the earplugs in place, crew members will
find it easier to hear speech communication.
E-A-RŪ Foam Earplug. The E-A-RŪ
yellow-foam earplug has three qualities: it
excels in noise attenuation, comfort, and ease of
maintaining a seal. To ensure maximum
attenuation, these plugs should be kept clean and
7-37. V-51R Single-Flange Earplug. The V-51R
single-flange earplug comes in five different sizes for better
fit. Different sizes (extra small, small, medium, large, and
extra large) provide a suitable fit for more than 95 percent of
all Army aviation personnel. About 10 percent of aircrew members
need a different size of earplug for each ear. The single-flange
earplug may be cleaned with soap and water.
7-38. SMR Triple-Flange Earplug. The SMR provides about
the same attenuation as the V-51R. Triple-flange earplugs come in
three sizes (small, medium, and large). This earplug is
comfortable for most individuals. This earplug may be cleaned
with soap and water.
Combined Hearing Protection
7-39. The polymeric foam (E-A-RŪ) hand-formed
earplugin combination with the SPH-4B,
HGU-56, and IHADSS helmetswill provide
additional protection from noise generated by all
aircraft in the U.S. Army inventory. Table 7-7 shows exposure
levels for various aircraft when the pilot wears
the SPH-4 helmet with each of the three types of
earplugs at the pilots station.
Table 7-7. Attenuation Levels for Protective
Helmets and Earplugs
7-40. Several types of earmuffs (Figure 7-4) provide adequate
sound protection for ground-support aviation
personnel. Most earmuffs that are in good
condition and properly adjusted will attenuate
sound as well as properly fitted earplugs. The
earmuffs tend to give slightly more
high-frequency protection and slightly less
low-frequency protection than earplugs.
Figure 7-4. Earmuff
7-41. Vibration cannot be eliminated, but its
effects on human performance and physiological
functions can be lessened. Various preventive
measures can be taken to reduce the effects of
- Maintain good posture during flight.
Sitting straight in the seat will enhance
blood flow throughout the body.
- Restraint systems provide protection
against high-magnitude vibration
experienced in extreme turbulence.
Body supports, such as lumbar inserts and added seat
cushions, reduce discomfort and can dampen vibration;
however, during a crash sequence they may increase the
likelihood of injury because of their compression
characteristics. Do not modify the aircraft seats for the
sake of comfort.
- Maintain your equipment. Proper aircraft
maintenance, such as blade tracking, can
reduce unnecessary vibration exposure.
- Isolate the aircrew members or
passengers. When loading patients on
MEDEVAC aircraft, remember that patients
placed on the floor will experience more
vibration than those in the upper racks.
- Limit your exposure time. Make short
flights with frequent breaks, rather than
one long flight, if the mission permits.
- Let the aircraft do the work. Do not grip
the controls tightly. Vibration can be
transmitted through control linkages
- Maintain excellent physical condition.
Fat multiplies vibration while muscle
dampens vibration. Strong muscles act to
reduce the magnitude of oscillations
encountered in flight (damping). An
overweight aircrew member is more
susceptible to decrements in performance
and the physiological effects of
- Maintain good physical condition to
lessen the effects of fatigue. Being in
good physical condition permits you to
continue to function during extended
combat operations with minimum rest.
Energy and alertness keep you alive.
- Maintain sufficient hydration. Drink
plenty of fluids, even if you do not feel
thirsty: a minimum of two quarts of water
in addition to fluids taken with meals.
Dehydration, coupled with vibration, can
cause fatigue twice as fast and double
the time needed for recovery.