Noise levels found in military helicopters exceed noise exposure limits required by Department of Defense Instruction 6055.12 (1991) and Army PAM 40-501, "Hearing Conservation" (1991). Noise consists of a mixture of random broadband noise and periodic harmonic and high frequencies generated by the machinery contained within the helicopter, including impulse noise burst generated by weapons systems (Wiener and Nagel, 1988). Noise levels in helicopters with higher load capacities such as the CH-47 Chinook and the U.S. Air Force CH-53 Pavlow are extremely intense, and under some flying conditions, will exceed the helmet’s capability to provide adequate hearing protection for crewmembers. Figure 8.1 shows a distribution of noise levels found in today’s helicopters along with expected noise exposure of individuals wearing normal issue helmets with and without foam earplugs. About 15% of the flight conditions in Army aviation exceed protection limits of 85 dBA that are provided by the HGU-56/P or SPH-4B flight helmets. Wearing foam earplugs in combination with the helmet limits the noise exposure to less than 85 dBA for about 99% of the flight conditions.
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