LEAKY PLATE MODES - RADIATION INTO A SOLID MEDIUM

The propagation of ultrasonic waves has been studied for a thin plate (an alumina membrane 60 mu m thick) surrounded by a second medium (hel ium) with much lower density and slower sound speeds. When the helium freezes, there are sudden jumps in the attenuation of both the shear S H(0) and the longitudinal L(1) plate modes which appear to result from radiation of sound into the solid helium. Since there is a large acou stic mismatch between the plate and the helium, the attenuation of the se ''leaky'' modes is relatively small. The expected attenuation has b een calculated for the SH(0) and L(1) modes in the low-frequency limit where the sound wavelength is much greater than plate thickness. When the helium is fluid, neither mode can radiate and the attenuation is small. Once the helium is frozen, however both modes become leaky. For the SH(0) mode, only shear waves are generated in the helium but the L(1) mode can radiate both transverse and longitudinal waves. The magn itude of the observed attenuation changes, as well as their dependence on frequency and pressure, agree quite well with those calculated for leaky modes using the known properties of the alumina plate and of so lid helium. (C) 1996 Acoustical Society of America.