Grazing incidence propagation over a soft rough surface

Abstract

Attenborough has recently proposed [J. Acoust. Soc. Am. 98, 1717–1722 (1995)] that the effects of surface roughness on an otherwise flat surface of finite acoustic admittance can be modeled as a smooth flat surface with an effective acoustic admittance β*. This effective admittance is determined by the normalized admittance βs and wave number ks of the original smooth surface, as well as a surface roughness parameter σν. Attenborough tested his theory by conducting excess attenuation measurements over plywood and Styrofoam surfaces with two-dimensional roughness elements. Specifically, he investigated the interference pattern between the direct and reflected signals over both smooth and rough surfaces. Experiments are conducted here with three-dimensional roughness elements at grazing incidence. Such a configuration is typical for outdoor sound propagation problems. The results presented here generally confirm Attenborough’s model and open up the analogy to a broader class of problems. It was also found that placing the effective admittance plane at the top of the roughness elements rather than at the bottom yielded slightly better results for the higher acoustic frequencies investigated.