Some Experimental Results Concerning the Propagation of Sound over Ground

Abstract

Under contract with the U. S. Army Signal Corps (contract No. DA-36-039-sc-64503), field studies have been undertaken to investigate the physics of outdoor sound. The propagation of sound over ground in the speech frequency range was studied as a function of distance, terrain, ground cover, wind speed and direction, temperature, and humidity, etc. Propagation over open, level ground, through dense evergreen forests, and between mountain tops was studied in the frequency range between 300 cps and 5000 cps. Extensive micrometeorological instrumentation was utilized. Results were generally expressed as attenuation in excess of inverse square law as a function of distance and frequency for a wide range of meteorological and terrain conditions. Over level ground, the excess attenuation was found to be greater for upwind than for downwind propagation by as much as 30 db. This is due to the presence of vertical temperature and wind gradients, and is in general agreement with Ingard's findings. In hilltop-to-hilltop propagation, large, long-period fluctuations in the received sound pressure level were observed; wind direction appears to be of secondary importance. In dense forests, sound absorption and scattering control. Attenuation coefficients measured in evergreen forests are comparable to those obtained by Eyring in Panamanian jungles.