Propagation of Sound in the Atmosphere—Time Fluctuations in Intensity

Abstract

Measurements conducted on the propagation of sound over a dry lake in the Mojave Desert revealed large fluctuations in intensity, even in calm air and for short distances The fluctuations increased with a rise in frequency. For a point source on the surface of the lake, the intensity of a 250-cycle tone at a distance of 50 m was almost uniform, whereas a 4000-cycle tone showed rapid time variations of intensity—within only a few seconds the intensity varied as much as thirty-six-fold. An exponential sound source (4000 cycles), 75 cm above the lake and directed horizontally, exhibited not only great intensity fluctuations but also surprising changes of wave form. Even during intervals of calm, late at night, when there was no observable wind or temperature gradient (up to a height of 10 m), many of these extreme fluctuations occurred. These irregularities of the propagation of sound will be shown in a series of oscillograms. The irregularities have been simulated, qualitatively, by producing convection currents of the air in a soundproof, constant temperature room.