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 listeners.

7-18. 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 aircraft.

• 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 amplitude).
• When objects reach their resonant frequencies, they create a momentum, which increases in intensity with each oscillation.
• 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.
• Disorientation.
• Pain.
• Microcirculatory effects.
• Visual problems.

7-28. The 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 individual’s responsibility.

• Use of personal protective measures.
• Isolation or distancing of crew members from the noise source.

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.

7-36. 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 inserted properly.

• 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.

• 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 during turbulence.
• 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 vibration.
• 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.