An experimental investigation of the propagation of sound over a flat rough surface of finite impedance

Abstract

Recent work on rough surfaces by Medwin et al. [ 657–665 (1986)] has indicated the presence of acoustic boundary waves in the fluid directly above rough surfaces with high acoustic impedances. The effect of this boundary wave is that the sound field above the rough surface is louder than it would have been over a similarly smooth surface at close ranges due to multiple coherent scattering. The opposite is true at longer ranges since the scattering becomes incoherent. The ranges of coherent and incoherent scattering depend on the roughness scale and frequency of the sound source. The experimental work described above was performed over surfaces with high acoustic impedances such as wax and steel. The purpose of this present work is to find out if similar trends exist in the propagation of sound over rough surfaces that possess a lower acoustic impedance. Sound signals will be measured above a rough surface and compared to the signal above a smooth surface of the same material. Preliminary experiments using Styrofoam indicate that similar trends exist above the low impedance rough surface as did over the high impedance rough surfaces. [Work supported by USDA.]