Some properties of surface acoustic waves in anisotropic-coated solids, studied by the impedance method

The impedance method provides a convenient framework for analysing the velocity spectrum of surface acoustic waves, propagating in anisotropic-coated substrates within the subsonic range where there is no energy leakage into the substrate. The dispersion equation determines subsonic wave velocities as zeros of eigenvalues of the aggregate impedance which are real monotonic functions of velocity at any fixed wavenumber. The impedance method lends insight into the properties of a coated substrate spectrum by elucidating its connection to the individual acoustic parameters of the substrate and the coating layer materials. By using this approach, certain bounds and basic trends for the dispersion of velocity branches in the coated substrates are revealed. The low-frequency and high-frequency dispersion is explored in detail. The criterion is examined for either speeding or slowing trend of the low-frequency linear dispersion of the velocity branch evolving from the Rayleigh velocity of the substrate, Specific behaviour of this branch due to the higher-order dispersion in substrates coated by a relatively light or dense, "slow" or "fast" layer is interpreted. The impedance method facilitates a similar analysis for various layered structures, for instance, substrate with multilayered coating, a plate coated on both sides. and a layer embedded between two substrates. (C) 2002 Elsevier Science B.V. All rights reserved.