The influence of the modal properties of a stiff layer embedded in a solid medium on the field generated in the layer by a finite-sized transducer

Abstract

This paper is the second of a pair which compare the modal properties of a stiff layer embedded in a solid medium with the minima of the reflection coefficient. In the first paper [J. Acoust. Soc. Am. 97, 1625–1637 (1995)] the modal properties and the plane-wave reflection coefficient are compared. In this paper a finite width incident beam is modeled and the reflected field is studied, the spatially realistic simulation enabling the comparisons to address the issues which will arise in practical measurements of reflection coefficient minima. The finite beam reflection minima can be caused both by minima of the plane-wave reflection coefficient and by interference of the leaking field produced by a propagating mode with the specular reflection. In systems in which there is a large acoustic impedance contrast between the layer and the surrounding medium, these phenomena occur under practically the same conditions. However, when the surrounding medium is similar to the layer, major changes in the reflection behavior are seen. Predictions are presented for single interfaces and embedded layers, assuming both fluid and solid embedding media. For a single interface it is shown that the reflection coefficient minima correlate directly to the modal properties since plane-wave reflection coefficient minima do not occur. For an embedded layer whose properties are similar to those of the embedding medium, the plane-wave reflection coefficient minima dominate the response and the (strongly leaking) modes of the layer are only excited very weakly by the incident beam.