Diffraction of evanescent wave tone bursts on a membrane in air

Abstract

A novel electromagnetic acoustic wave transducer (EMAT) was used to generate plane subsonic wave tone bursts on aluminized Mylar membranes. The evanescent wave field in the air surrounding the membrane was studied. Diffraction of the burst by a sharp edge in air was also observed. The diffracted signal was measured using a microphone as a function of the gap h between the membrane and the knife 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.