Surface wave formation at an impedance discontinuity

Abstract

The formation of air-coupled acoustic surface waves over an impedance plane has been studied in a series of model experiments. Surface waves were generated at an impedance discontinuity on a flat surface, one section of the surface being acoustically hard, and the other composed of an array of small cavities with an effective impedance suitable for surface wave propagation. A point source of sound (800 Hz–2 kHz) was located on the hard surface and the vertical sound pressure distribution measured over the impedance plane, at various distances. The formation, propagation, and decay of the surface waves were clearly observed. The measured pressure distributions are consistent with the theoretical formulation of Rasmussen [J. Sound Vib. 84, 598–602 (1982)] for propagation of sound over an impedance jump. An alternative formulation for the case of plane waves incident at grazing angles to a surface has been described by McAninch and Myers [AIAA 26th Aerospace Sciences Meeting, paper AIAA-88-0179 (1988)]. This simpler formulation has been found to be consistent with our measurements, and with the Rasmussen theory, when the distance between source and impedance discontinuity is sufficiently large.