Average excess attenuation during sound propagation from an isotropic source above grassland

Abstract

Measurements of sound attenuation from an isotropic source have been made over a meadow at propagation ranges between 15 to 300 m and for source and receiver heights within 3 m of the ground. Approximately 750 measurements were made at more than 70 source–receiver geometries under essentially constant ground conditions. The sound source was driven by a pseudo‐random binary sequence signal giving a discrete spectrum in the 100‐ to 2500‐Hz frequency range. The received signal was analyzed and averaged using Fast Fourier Transform techniques in narrow bands of 3 Hz. A separate measurement of the free‐field source levels allowed excess attenuation to be relatively unambiguously calculated from the measured data. The surface used in the tests appears to have a boundary impedance different from grasslands studied by previous investigators. However, the interference phenomenon between direct and ground‐reflected waves, as modified by meteorological factors remains a major feature of the results, often producing excess attenuations of 20 to 30 dB. The effects of propagation range, source height, receiver height, and vector wind in determining excess attenuation are presented in detail. It is evident from the data that the theory of a simple source radiating into a quiescent, isothermal medium above an impedance plane often cannot be used to accurately predict average attenuation to distances much beyond 50 m from the source. Finally, it is shown that the method of averaging propagation data can significantly alter the apparent results.