Effects of ground cover on the propagation of sound through the atmosphere

Abstract

Measurements of sound amplitude in the vicinity of a ground plane have been made as a function of frequency of the sound source (50–2000 Hz), distance of propagation (5–300 m), and surface conditions. By treating the impedance as an adjustable parameter, the surface impedance as a function of frequency was determined from the measured amplitudes using a theoretical treatment of a spherical wave in the vicinity of a locally reacting surface. The impedance measurements covered the frequency range 50–1000 Hz. In this frequency range, the results for three distinctly different surfaces suggest that the impedance can be computed from the specific flow resistance and that grass has little effect on the surface impedance except for decreasing the flow resistance due to the root structure. The results of the values of impedance calculated from sound measurements and flow resistance measurements will be compared, and the applicability of the single parameter model of Chessell to the data will be discussed. [Work supported by U.S. Army Research Office.]