Abstract
Military aircraft noise contouring programs employ an empirical factor describing the change in duration of a flyby with distance. In the past the SEL or EPNL was calculated from the maximum A-weighted level by the use of a duration correction given by 10 times the logarithm of the distance ratio, applicable to spherical divergence without absorption. Recent measurements have indicated that a better factor for a variety of military aircraft is six times the logarithm of the distance ratio (J. D. Speakman, “Effect of Propagation Distance on Aircraft Flyover Sound Duration,” AFAMRLTR-81-28, May 1981). This paper describes the results of an experimental and theoretical study of the duration factors for rotary wing aircraft. The theoretical study investigates the effect of surface conditions, coherence, and atmospheric absorption on the duration correction.
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