RESONANT TRANSMISSION OF A 3-DIMENSIONAL ACOUSTIC SOUND BEAM THROUGH A SOLID PLATE IN AIR - THEORY AND MEASUREMENT

A quantitative model has been developed to explain the prominent featu res of the resonantly enhanced transmission of a three-dimensional air borne sound beam through a solid elastic plate. A quantitative model o f this phenomenon is important in order that air coupling can be explo ited for applications, such as the determination of elastic properties of plate-shaped materials. In our model, the beam axis is aligned to exploit resonant coupling with the AO Lamb mode of the plate, and a fi nite-sized transducer is used to measure transmission of the beam thro ugh the plate. The interaction of airborne and liquid beams with a sol id plate differs significantly. Because of the difference in densities between air and the solid plate material, the resonant coupling mecha nism for airborne beams is very sensitive to the incidence angle of in dividual plane waves in the angular beam spectrum, approximately 1000 times more sensitive than if the coupling fluid is water. It was found that the elastic waves excited in the plate by the incident beam prop agate in a sector limited by a half-angle determined from the incidenc e angle of the beam and the beam ka product. Measurements are presente d that validate the model predictions of transmission loss caused by t he presence of a plate. Specifically, over the range of frequencies 16 0-180 kHz, measured transmission loss caused by the presence of a 0.8- mm-thick aluminum and 1.40-mm-thick PMMA plate were within 1-2 and 2-5 dB of model predictions. (C) 1995 Acoustical Society of America.