Electromechanical coupling coefficients of ultrasonic Lamb waves

The electromechanical coupling coefficients of fundamental and higher‐order ultrasonic Lamb wave modes propagating in a piezoelectric plate have been calculated using two different methods. The first approach calculates the coupling coefficient K from the well‐known equation K²=2(Δv/v)=2(v₀−v_m)/v₀, where v₀ and v_m are the wave velocities on a free and a metallized surface, respectively. The second approach, which involves considerably more calculations, is based on the Green’s function method of analysis. In the case of surface acoustic waves, it has been shown that the two approaches give nearly identical results. It is found that, in most cases, this is also true for Lamb waves. The only exception occurs for the lowest‐order symmetric (S₀) and antisymmetric (A₀) modes, if the ratio h/λ (h=plate thickness, λ=acoustic wavelength) is greater than 1. In this case the Δv/v method gives erroneous results. This occurs because, in this case, the surface metallization causes strong perturbation of the mechanical displacements associated with the waves. In all other cases, the surface metallization produces a weak perturbation of the acoustic wave, and the simple Δv/v method can be used to calculate the coupling coefficient.