Dense Evergreen Research Articles

Experimental Study of the Propagation of Sound over Ground

The attenuation of sound propagated out-of-doors is conveniently separated into attenuation due to spherical divergence and excess attenuation due to atmospheric and terrain effects. This excess attenuation is principally caused by sound absorption in the air, the refractive effects of temperature and wind gradients, by turbulence and the effects of terrain and ground cover. To investigate these effects the propagation of sound over open, level ground, through dense evergreen forests, and between hilltops was studied experimentally in the frequency range between about 300 cps and 5000 cps. Extensive micrometeorological instrumentation was utilized to measure and record the relevant micrometeorological parameters simultaneously with the acoustic data for a wide variety of weather conditions. Data on the attenuation of the mean received sound pressure level as well as on the fluctuations about the mean were obtained and correlated with the state of the atmosphere. Over open level terrain, the excess attenuation upwind was found to exceed that for downwind propagation by as much as 25–30 db for source and receiver heights of 12 and 5 ft, respectively. Temperature and wind gradients near the ground-air interface largely account for this difference. In hilltop-to-hilltop propagation, wind direction is of secondary importance, and in dense woods absorption and scattering control. Empirical functions were derived for the purpose of estimating the mean excess attenuation as a function of frequency and distance, for a given set of micrometeorological conditions. These charts have been found useful in many practical problems involving the propagation of sound over open level ground.

Some Experimental Results Concerning the Propagation of Sound over Ground

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.