Atmospheric Attenuation of Sound from an Elevated Source

Abstract

Experimental data were given in a previous paper at the October, 1957, meeting of the Acoustical Society of America on the effects of weather parameters on the attenuation of sound propagated from an airborne source to ground. These results have been analyzed in greater detail and improved procedures have been developed for predicting attenuation. The residual attenuation, defined as that in excess of spherical divergence and molecular absorption, does not vary systematically with any of the measured weather parameters for source elevations of 10° or higher and for altitudes above a few hundred feet. Its average value in this region may be expressed by α0=0.11f12 db/1000 ft, where f is the frequency in cps. At lower source elevations and altitudes, the attenuation is increased by an amount depending on frequency, source elevation, wind velocity and direction, and temperature gradients. Comparison of upwind with downwind propagation for source elevations of 2°, 5°, 10°, and 15° shows a significant difference only for 2°. The statistical data for upwind propagation are compared with theory applicable to shadow formation. (This research was supported by Wright Air Development Center.)