Interaction between a Plate and a Sound Field in a Fluid Medium

Abstract

When underwater sound impinges on a steel plate at an oblique angle of incidence, there have been observed strong reflections directed toward the source. An analysis of the interchange of energy between a sound field in a liquid medium and flexural vibrations in a plate immersed in the field indicates the mechanism involved in this type of non-specular reflection. It has been found that for a plate and a liquid of specified properties, the behavior of the system depends on the product of plate thickness and frequency. For a steel plate b inches thick in water at a frequency of f kilocycles per second, the forces exerted by the water on the plate become negligible for values of bf exceeding about 20. For values of bf less than about unity, the effect of the plate in the sound field becomes negligible. It is therefore in the range of values between 1 and 20 for which the results are of particular interest. The velocity of phase propagation and the attenuation of flexural waves having origin in the plate have been calculated as a function of bf. There has also been found the angle between the plate and the wave fronts generated in the liquid. This angle, for the upper range of bf, is substantially equal to the angle of incidence for maximum non-specular reflection. The calculated values are in good agreement with measurements made by W. J. Finney.