Effects of atmospheric turbulence on the interference of sound waves near a hard boundary

Abstract

The mean sound levels resulting from the interference between direct waves and those reflected from the ground are strongly influenced, especially at frequencies near interference minima, by fluctuations in phase and amplitude of the sound waves induced by propagation through atmospheric turbulence. Since it was found experimentally that the correlation length (∠1.1 m) of the meteorological fluctuations is comparable to the separation between the interfering sound paths, previous theoretical work by Ingard and Maling [J. Acoust. Soc. Am. 35, 1056–1058 (1963)] has been extended to allow for partial covariance between the two waves. The theory has been further extended to use the calculations of fluctuations in phase and amplitude of spherical waves, and to include the explicit calculation of the fluctuating acoustical index of refraction from the fluctuating values of temperature and wind velocity. Measurements (1–6 kHz) have been made of the interference spectrum at 15, 30, and 45 m from a point source 1.2 m above a large asphalt surface. Simultaneously, the fluctuating values of temperature and horizontal wind velocity (atmospheric turbulence) were measured at two related points close to the sound path. There is satisfactory quantitative agreement between the sound levels calculated from the measured fluctuating meteorological variables (with one adjustable parameter) and those measured experimentally.