Diffraction assisted rough ground effect

Abstract

Ground effect results from the interference between sound travelling directly from a point source to a receiver and that reflected coherently from the ground. The associated excess attenuation spectra are determined by the source-receiver geometry and the ground impedance. If the ground is hard and smooth then at grazing incidence the first destructive interference may occur at too high a frequency to be useful in noise control. The effective impedance of a rough hard surface is finite and destructive interference can occur at much lower frequencies than if the surface is smooth. If the roughness is spaced periodically then the phenomenon of diffraction assisted rough ground effect occurs. Instead of the single broad destructive minimum observed typically with randomly-spaced roughness elements, a cluster of two or three relatively narrow destructive interference peaks are observed. This phenomenon has been explored through laboratory experiments using semi-cylindrical, rectangular and wedge-shaped strips placed on a glass sheet. The resulting ground effect depends on the number of roughness elements and their spacing. Good agreement with data has been obtained using multiple scattering theory for semi-cylindrical roughness and either BEM or FEM simulations for other roughness shapes. © 2010 Acoustical Society of America.