United States Army School of Aviation Medicine

Fort Rucker, Alabama

August 1997

  STUDENT HANDOUT

TITLE: NOISE AND VIBRATIONS IN ARMY AVIATION

FILE NUMBER: 2/5/9/9E/UEA/UEC/UEE/4507-2
 

PROPONENT FOR THIS LESSON PLAN IS:

United States Army School of Aviation Medicine

ATTN: MCCS-HAF

Fort Rucker, Alabama 36362-5000

FOREIGN DISCLOSURE RESTRICTIONS: The materials contained in this lesson plan have been reviewed by the instructor/training developer and determined to be public domain materials. This product is releasable to military students from all requesting foreign countries without restrictions.
 

NOISE AND VIBRATION

TERMINAL LEARNING OBJECTIVE (TLO):

ACTION: Identify how sound, frequency, intensity, and duration contribute to a hazardous noise and vibration exposure.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.

A. ENABLING LEARNING OBJECTIVE (ELO) #1:

ACTION: List the physical characteristics of sound.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.

a. The nature of sound.

(1) Sound is produced when an object or surface vibrates rapidly enough to generate a pressure wave or disturbance in the surrounding air.

(2) Colliding molecules produce vibration pressure increases or a "compression"; molecules rebounding away from each other produce a lower pressure or a "rarefaction".

(3) The high speed of the rotor blade compresses the air faster than the high pressure disturbance can flow away.

b. To understand why Army aviation personnel lose hearing, it is important to understand the mechanism of hearing. Hearing is the perception of sound. Sound and pressure waves must pass through three areas of the ear before they are perceived by the brain.

(1) External ear. The visible portion of the ear and the external auditory canal which ends at the eardrum. Sound is transmitted by air in this portion of the ear.

(2) Middle ear. This small, air-filled cavity separates the external and inner ear. Three small bones or ossicles, the malleus, incus and stapes (hammer, anvil, and stirrup), link the eardrum to the inner ear and mechanically carry sound to the hearing receptors. The Eustachian tube connects the middle ear with the node and permits drainage and ventilation of the middle ear. It also equalizes pressure between the outside ear and the middle ear.

(3) The inner ear is the third component which lies deep within the temporal bone and consists of an auditory section and a vestibular section involved in balance. The auditory portion known as the cochlea, is comprised of a fluid-filled chamber where hair-like receptors for hearing are situated. Hydraulic movement of the fluid is caused by the movement of the stapes. The hair cells detect this fluid movement and transmit electrical impulses to the brain where sound is interpreted.

B. ENABLING LEARNING OBJECTIVE (ELO) #2:

ACTION: Select the physical characteristics of noise.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.

a. Definition. Noise is defined as sound that is loud, unpleasant, or unwanted. In aviation noise could cause annoyance, speech interference, fatigue and hearing loss.

(1) Nonauditory effects of noise can cause fatigue and can be annoying.

C. ENABLING LEARNING OBJECTIVE (ELO) #3:

ACTION: List the three measurable characteristics of noise.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.

a. Noise measurable characteristics.

(1) Frequency is the physical characteristic of sound that gives sound its subjective quality of pitch. Frequency of periodic motion is the number of times per second the air pressure oscillates. The number of oscillations, or cycles per second, is measured in hertz.

b. Army noise exposure criteria. The Surgeon General has established 85 decibels as the maximum permissible sound level of continuous unprotected exposure to steady-state noise for 8 hours.

(1) The following table shows the recommended allowable sound intensities for the various duration of exposure:

WARNING: Unprotected exposure to noise levels in excess of 85 dB can result in temporary or permanent hearing loss. Air crew members must use hearing protection to prevent hearing loss.

D. ENABLING LEARNING OBJECTIVE (ELO) #4:

ACTION: Select the types of hearing loss associated with prolonged exposure to noise.

CONDITION: While performing as an aircrew member.

STANDARDS: In accordance with (IAW) FM 1-301, FM 8-2, and DA PAM 40-501.

a. Types of hearing loss, depends on several factors such as age, one’s health, and the environment one finds themselves in.

(1) Conductive loss occurs when there is a defect or impediment in the external or middle ear. This may be caused by wax, fluid, or calcification build up or birth defects which impedes the mechanical transmission of sound to the inner ear. In most cases it can be treated medically.

(2) Presbycusis loss occurs due to old age. Cells are less resilient as age increases.

(3) Sensorineural hearing loss occurs when the cochlea is damaged. It is most frequently produced by noise, but can also be caused by heredity, disease, and aging. This hearing loss is permanent and usually occurs in the higher frequencies first. Substantial loss may occur before the speech frequencies are affected; there is no medical treatment for this type of loss; hearing aids may be beneficial.

E. ENABLING LEARNING OBJECTIVE (ELO) #5:

ACTION: Select the characteristics of noise induced hearing loss

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.

a. Noise induced hearing loss is insidious and usually undetectable.

(1) Characteristics of noise induced hearing loss include:

F. ENABLING LEARNING OBJECTIVE (ELO) #6:

ACTION: List the general noise characteristics of military aircraft used by Army aviation.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 1-301, FM 8-2, and DA PAM 40-501.

a. General findings. Looking at Army aircraft as a group, both fixed and rotary wings, certain generalizations can be made.

(1) The frequency that generates the most intense level is 300 Hz. Low frequency noise will produce a high frequency hearing loss. Providing adequate hearing protection for lower frequencies is very difficult.

(2) Overall noise levels generally are equal to or exceed 100 dB. This exceeds the average 85 dB damage risk-criteria of DA PAM 40-501.

(3) Exposures to these levels without hearing protection will lead to definite permanent noise induced hearing loss.

b. Noise in Army fixed wing aircraft originates from power plants, propellers, and transmissions.

(1) Noise levels varies between aircraft depending on the location of the engines and their proximity to the cockpit.

(2) Other fixed wing aircraft, depending on their mission, have more insulation than others.

c. Noise in Army rotary wing aircraft originates from power plants, rotor systems, and transmissions that produce significant low frequency pure tone narrow band width noise.

(1) Observation helicopters are small in size but could generate extreme amount of noise with levels exceeding 100 dB.

(2) Attack helicopters noise is in close proximity to the crew members due to the position of the engines and rotor system in relation to the cockpit.

(3) Utility and cargo helicopters noise levels fluctuate with cargo doors and ramps open. Troops being airlifted should wear hearing protection in these aircraft. Air crew members must ensure that passengers wear hearing protection while inside the troop compartment.

d. Noise in Air Force cargo aircraft. Due to our worldwide missions, air crewmember may have to rely on fixed wing aircraft to move their aircraft to distant locations around the world.

(1) Noise levels on cargo aircraft can exceed 100 decibels during air load operations on the ground.

(2) While flying as passengers, crew members could be exposed to noise levels exceeding 95 decibels.

(3) During air load operations, wear hearing protection to minimize the potential for hearing loss.

G. ENABLING LEARNING OBJECTIVE (ELO) #7:

ACTION: Select the most practical and economical method of noise reduction available to aircrew members.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.

a. Noise protection.

(1) A number of methods of protecting human hearing and/or controlling noise are available. Some methods are not economically feasible; others are not suitable for operational requirements. The following major methods of controlling noise must be considered.

(2) Design or plan to eliminate the noise. This is the ideal way of controlling noise.

b. Isolate the noise source. This can be done by increasing the distance between the noise source and the exposed person.

(1) Enclose the noise source. This can be accomplished by using sound and energy absorbent material and baffling.

c. Personal protective measures. A number of devices are available to attenuate (lessen) the noise at an individual's ears. These are the most practical and economical method available for noise protection.

(1) Personal protective measures have certain distinct characteristics.

(a) Attenuation. Attenuation is the amount of noise protection provided by a specific protective device. The attenuation of any given noise protective device is the number of dB it reduces from the total energy reaching the ear drum.

(b) Speech intelligibility. Speech intelligibility and other acoustic signals are better understood in noisy environments when noise protective devices are utilized. This is due to an increase in the signal to noise ratio brought about by a reduction in the masking effect of the noise.

(c) Maximum attenuation. Maximum attenuation for any device is approximately 50 dB. At this point sound is transmitted to the inner ear by bone conduction.

(2) Types of personal protective measures.

(a) Ear plugs. Foam, single flange, and triple flange. These devices are inserted into the external ear canal. They are inexpensive, easy to carry, and effective when fitted properly.

1. They provide attenuation from 18-45 dB across the frequency band.

2. They should be worn anytime you are exposed to noise levels in excess of 85 dB. They are very effective when worn in conjunction with the SPH-4, SPH-4B, HGU-56, and IHADSS flight helmets.

a. Roll rather than squeeze each plug into as small a diameter as possible between finger and thumb.

b. Insert quickly into the ear canal by reaching over the head with the opposite hand and pulling ear outward.

c. Hold gently in place with fingertip for 1 minute until expansion is complete a good seal should be accomplished by a vacuum sensation (a back pressure).

d. Keep plug clean by washing in mild soap and rinsing thoroughly in water. Discard if discoloration or disfiguation occurs after cleaning.

e.Do not use where hazardous chemical vapors could be absorbed into plug.

(b) Ear muffs. These devices are worn covering the ear. They provide 10-41 dB protection, across the frequency band, are comfortable, and because they can be readily seen, managerial control for wearing hearing protection is enhanced. Do not forget ground personnel or your passengers. They are subject to hearing loss just as well as the pilots.

(c) Headsets. These devices are worn covering the ears, but also provide for radio communication. Commonly worn in VIP type aircraft, they provide little noise attenuation due to rough handling and abuse, poor construction, improper fits, and deteriorated ear seals. The crash attenuation provided by a helmet is also lacking.

(d) Protective helmets.

1. For aviators and crew members, this is the best means of personal protection. They provide both noise and crash attenuation.The SPH-4B, HGU-56, and IHADSS provide greater protection in the higher frequencies. However, it is low frequency noise in the aviation environment that is the cause for concern.

2. The helmet is an excellent hearing protection device. It will provide optimal protection only if certain guidelines are followed:

a. It must fit properly.

b. It must be worn correctly.

c. The ear cup seals must be soft, unwrinkled, and tear free. When the seals harden they must be replaced.

3. If the SPH-4B, HGU-56, and IHADSS helmets are worn properly, the noise attenuation brings the noise exposure within the confines of the damage risk criteria for every aircraft except the UH-60A, and CH-47D.

4. The polymeric foam (EAR) hand formed ear plug in combination with the SPH-4B, HGU-56, and IHADSS helmets will provide additional protection from all aircraft noise in the US Army inventory.

H. ENABLING LEARNING OBJECTIVE (ELO) #8:

ACTION: List the sources of nonoccupational noise exposure.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.

a. Nonoccupational noise exposure. Noise does not end at the flight line; thus our ears often never get the chance to recuperate from the noise exposure associated with flying. Aviators must be aware of the sources of potentially damaging noise exposure and take appropriate action to minimize these exposures.

(1) General aviation. Many aircrew have civilian private or commercial pilot certificates. They may fly for pleasure or additionalincome. This is an extremely critical source of exposure, since most private aircraft are flown without headsets, relying on speakers for hearing voice communications. Most general aviation single-engine light aircraft have noise levels in excess of 85 dB below 1000 Hz, which requires noise protection. Unprotected, aircrew could suffer noise induced hearing loss.

(2) Weapons firing. This applies to several categories of weapons firing. Unprotected, these impulse noises can result in sustained acoustic trauma. Some high velocity small arms weapons have peak intensities in excess of 168 dB. All small arms in the Army produce impulse noise levels above 140 dB. Noise protection should be used whenever aircrew are engaged in weapons firing. Some sources of indirect or Nonoccupational exposure are:

(a) Hunting.

(b) Skeet or other target shooting.

(c) Annual weapons qualification.

(3) Moonlighting. A variety of off-duty jobs may expose the air or ground crews to additional potentially harmful noise exposures.

(a) Bartending (95-110 dB), in a club where loud music is played.

(b) Member of a "rock" band (110-150 dB).

(4) Contemporary music. Frequently aircrew will innocently expose themselves to extremely loud and sustained levels of noise via music. Surveys in Officer's and NCO clubs have revealed exposures and intensity levels exceeding 130 dB. This exposure has also shown to produce permanent hearing loss. Thus, commanders are obligated to protect their personnel by regulating these exposures.

(a) "Walkman" radios aim high noise levels directly into the ear canal.

(b) Portable stereo systems "boom boxes."

(5) Hobbies and recreation. Often hobbies and recreation result in innocent and thoughtless exposure.

(6) Household chores. Again, exposure to intense noise.

(a) Lawn mowers (95-100 dB).

(b) Vacuum cleaners (90-100 dB).

(c) Blender (93 dB).

I. ENABLING LEARNING OBJECTIVE (ELO) # 9

ACTION: Identify the physical characteristics of vibrations.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 8-2.

a. Vibration. The motion of an object relative to a reference position (usually the object at rest)involving a series of oscillations resulting in the displacement and acceleration of the object.

b. Frequency. The number of oscillations of any object in a given time. The international standard unit of frequency is the hertz (HZ), which is 1 cycle per second (cps).

c. Amplitude. The maximum displacement about a position at rest.

d. Duration. The amount of time exposed to vibration.

e. Natural body resonance. The mechanical amplification of vibration by the body occurring at specific frequencies.

f. Damping. The loss of mechanical energy in a vibrating system. This causes the vibration to slow down.

(1). When the body is subjected to certain frequencies, the tissue and organs will begin will begin to resonate (increase in amplitude).

(2). The connective tissue that binds the major organs together reacts to vibrations like shock absorption.

(3). The minor amplitudes of the vibration plus the body’s ability to provide some damping provide reasons why humans do not receive life threatening injuries every time they go flying.

J. ENABLING LEARNING OBJECTIVE (ELO) #10

ACTION: Identify the sources of vibration.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 8-2.

a. Vibrations are produced within the aircraft and the environment in which it operates.

(1). Vibrations are generated within the aircraft from the engine, the main rotor, and the rotor systems.

UH-1

(2). Helicopter vibration occurs with similar intensities in all three axes of motion.

(3). The amplitude of the vibration differs in each mode of flight. The highest level of vibration occurs during the transition to flight from a hover.

(4). Vibration also increase significantly with airspeed, internal or external loading of the aircraft.

b. Environmental factors such as turbulence may also cause vibration.

K. ENABLING LEARNING OBJECTIVE (ELO) # 11

ACTION: Identify the effects of vibration on human performance in Army aircraft.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 8-2.

a. Manual coordination and control "touch" is degraded at 4-8 Hz. Pilot induced oscillations occurs when the air crew member over controls during turbulence and/or transition from a hover to flight.

b. Vision. Due to the vibration in the aircraft, visual instruments may be difficult to read. Helmet mount or night vision devices may vibrate at 4-12 Hz.

c. Speech. Can be distorted during oscillations of 4-12 Hz. Above 12 Hz, speech becomes increasingly difficult to interpret.

L. ENABLING LEARNING OBJECTIVE (ELO) # 12

ACTION: Select the method(s) used to reduce the vibrational threat in Army Aviation.

CONDITION: While performing as an aircrew member.

STANDARD: In accordance with (IAW) FM 8-2.

a. Intrinsic factors include:

(1). Anthropometry (body size, mass) of an individual. An overweight aircrew member is more susceptible to decrements to performance and the physiological effects to vibration.

(2). Muscle tone. Strong muscles act to reduce the magnitude of oscillations encountered in flight (damping).

(3). Posture. Sitting straight in the seat will enhance blood flow throughout the body.

b. Extrinsic:
 

(1). Restraint systems provide protection against high magnitude vibration experienced in extreme turbulence.

(2). Body support such as lumbar inserts and seat cushions reduce discomfort: however, during a crash sequence they may increase the likelihood of injury due to their compression characteristics.

c. Reduction of vibrational source.

(1). Maintain your equipment.

(2). Blade tracking.

(3). Isolate the aircrew member or passengers.

(a). When loading patients on MEDEVAC aircraft, remember, that more vibration will be experienced by patients placed on the floor rather than in the upper racks.

(b). Proper wear of the seat belt will reduce the amplitude of vibration during flights of severe and extreme turbulence.

(4). Limit your exposure time.

(a). Make short flights with frequent breaks, rather than one long flight, if possible.

(b). Let the aircraft do the work. Do not grip the controls tightly. Vibration can be transmitted through control linkages during turbulence.

d. Maintain excellent physical condition.

(1). Fat multiplies vibration, while muscle dampens vibration.

(2). Maintaining good physical condition lessens the effects of fatigue, And permits you to continue to function during extended combat operations with minimum rest. Energy and alertness is what keeps you alive.

(3). Maintain sufficient hydration. Drink plenty of fluids, even if you don’t feel thirsty. A minimum of two quarts of water over and above fluids taken with meals. Dehydration coupled with vibration can cause fatigue twice as fast and double the time needed for recovery.

(4). Sit up straight when possible to increase blood flow to the spine, and aid in the reduction of compression injuries.