Diffraction of evanescent wave tone bursts on membrane in air and related observations of bursts guided by plates in water.

Abstract

A previously described electromagnetic acoustic transducer (EMAT) was used to generate subsonic wave tone bursts on membranes [T. J. Matula and P. L. Marston, J. Acoust. Soc. Am. Suppl. 1 88, S167–S168 (1990)]. In the present research, the evanescent wave field in the air surrounding the membrane was studied. Diffraction of the burst by a sharp edge in the air was also observed. The diffracted signal was measured using a microphone as a function of the gap h between the membrane and a razor edge. The diffracted pressure decreases exponentially with increasing h as expected from an approximate analysis of edge diffraction of evanescent waves [L. B. Felsen, J. Opt. Soc. Am. 66, 751–760 (1976)]. The importance of such diffraction is that it produces far-field radiation of energy that otherwise would be trapped to a membrane or plate in the absence of scattering. In related work an EMAT is used to generate tone bursts of bending waves on an aluminum plate. The bursts propagate down the plate into water where the surrounding wave field is probed. [Work supported by ONR. The author acknowledges the advice of P. L. Marston.]