Rippled-Boundary Acoustic Experiment

Abstract

A ripple structure on a sandy ocean bottom produces important changes in the reflection characteristics of the boundary. This effect is analogous to the focusing produced by an optical-reflection grating. The acoustics problem has been studied since the time of Lord Rayleigh, with successes being limited to the case of extremely small corrugation amplitudes. This paper presents some results obtained from a model-tank experiment wherein a scale factor of about 20:1 is applied to the acoustic frequency and linear dimensions. The model boundaries were constructed of a plaster-sand mixture and of very fine sand in both flat and corrugated forms. In the experiments, the incidence angle was set while a complete semicircular reflection pattern was recorded for the whole region above the sonified boundary section. The incidence angle was varied from grazing to normal, so that families of reflection patterns were produced for the flat and corrugated-plaster and for the flat and corrugated-sand boundaries. These patterns are shown in the paper, as well as some curves showing the specular and backscatter [toward the sound source] amplitude dependence upon incidence angle and boundary contour. The experimental work is still in progress.