Sound Propagation through the Atmosphere

Abstract

The propagation of sound from an airborne source to the ground has been studied. The source was a propeller type airplane flown at altitudes up to 4800 ft and distances up to 9600 ft. Angles of elevation of the source with respect to the earth's surface were 2°, 5°, 15°, 30°, and 90°. The weather varied to include typical year-round conditions in the Chicago, Illinois, area and winter in Phoenix, Arizona. The noise propagated to the earth was analyzed to determine the variation in attenuation as a function of frequency and the relative position of the noise source. A statistical analysis was made to determine the effects of various weather parameters on propagation of the sound. Average values of sound attenuation were obtained. In addition, the effects of temperature, temperature gradient, humidity, wind, and wind gradient are given in an empirical formula. Propagation does not vary significantly for elevation angles between 5° and 90°, but for the 2° angle there is evidence of ground surface effects. The most important factor for minimizing the noise on the ground on a particular day was wind direction. Other meteorological conditions do not appear to be significant for controlling noise from flight operations around an airport. (Sponsored by the Aero Medical Laboratory, WCRDB, Wright-Patterson Air Force Base, Ohio.)