Some remarks on propagation characteristics of ridge guides for acoustic surface waves at low frequencies

Abstract

The plane-stress equations of the theory of elasticity are solved to predict the propagation characteristics of flexural and pseudo-Rayleigh modes of infinitely tall ridge guides for frequencies lower than the lowest thickness-shear frequency of thin plates. The parabolic dispersion characteristic of the former mode shows appreciable departure from the hitherto published results in the above limiting case. The large discrepancy is due to the incompatibility of the flexural type of motion in the ridge and the only Rayleigh type of disturbance that the adjoining free substrate can sustain. This causes undesirable radiation of the surface wave energy into the bulk of the substrate and is responsible for cutoff frequencies obtained by several authors. The consequent implication for the useful operational range of such waveguides is discussed. The nondispersive velocity of the latter mode at dc is found to be 1.5% lower than a microwave network result.