Excitation of acoustic beams in layered substrates

2014 A mechanism for the launching of acoustic bulk waves in narrow beams into a solid is discussed. The excitation is at the free surface or on the interface of a layered half-space for which the material of the layer has higher characteristic velocities than those of the substrate material. For a range of layer thickness greater than that corresponding to the cut-off value for the Rayleigh-like mode of this fast-on-slow geometry, there is a form of coupled mode which radiates down into the substrate in a narrow range of zenith angle. The properties of this beam are studied for two cases, one in which the Rayleigh velocity of the layer material exceeds the transverse velocity of the substrate but not its longitudinal, here only a shear wave is radiated into the substrate. In the second case the Rayleigh velocity of the layer material exceeds the longitudinal velocity of the substrate so that both shear and compressional waves are radiated. For typical layer thickness the beams radiated are but a few degrees in width. Revue Phys. Appl. 20 (1985) 305-310 JUIN 1985, PAGE Classification Physics Abstracts 68.25 43.35P