Effect of Propagation Distance on Aircraft Flyover Sound Duration.
AIR FORCE AEROSPACE MEDICAL RESEARCH LAB WRIGHT-PATTERSON AFB OH
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In the past, NOISEMAP and other aircraft noise contouring programs incorporating single event, time-integrated measures such as Sound Exposure Level SEL or Effective Perceived Noise Level EPNL assumed that the sound duration for flyovers doubled for each doubling of the distance between the source and a receiver. This simplifying assumption considers only the losses due to the spherical divergence of a sound wave as it propagates over distance. Mathematically this meant that in calculating SEL or EPNL versus distance functions, a duration term was introduced that was proportional to multiplying the logarithm of the ratio in propagation distances between two points by a coefficient of 10. Controlled level flyover noise tests were conducted on A-10, C-135A, C-141, E-3A, F-5E, F-15 and F-18 aircraft to directly measure sound duration as a function of propagation distance. Data were also acquired during a dedicated series of C-130E actual takeoffs and landings. Our findings show the duration coefficient varies between 5 and 7 for different aircraft types. Clearly the old coefficient of 10 is wrong. Data are included that also show that the sound attenuation mechanisms controlling this duration coefficient are basically independent of the frequency content of the aircraft noise.