Effects of ground cover on the propagation of sound through the atmosphere

Bands of noise were propagated over various ground surfaces and sound amplitudes were measured as a function of frequency of the sound source (40 Hz–2kHz), distance of propagation (5–300 m), and surface conditions. By treating the surface impedance as an adjustable parameter, the impedance as a function of frequency was determined from the measured sound amplitudes assuming spherical wave propagation in the vicinity of a locally reacting surface. The surface flow resistance was also experimentally determined in each case. In the frequency range 40 Hz–2 kHz, the results for three distinctly different surfaces suggest that the impedance can be computed from the specific flow resistance [C. I. Chessell, J. Acoust. Soc. Am. 62, 825 (1977)]. Surface vegetation appears to have little effect on the impedance.