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Transducer beam diffraction effects in sound transmission near leaky Lamb modes in elastic plates at normal incidence

The plane wave theory of fluid-embedded viscoelastic plates is often used in waveguide applications, and to estimate wall thickness and sound velocities in plates and pipe walls. In normal-incidence through-transmission measurements, downward frequency shift, signal amplification, and distorted beam patterns, as compared to plane-wave theory, have been observed for resonant peaks in frequency bands near certain Lamb modes. These phenomena are here studied using three models, accounting for 3D description of the transducer's sound field. Finite element modelling, angular spectrum modelling, and a combination of these, are used to simulate the signal propagation through a system consisting of a piezoelectric transducer and a water-embedded steel plate. This includes description of the transducer (its electro-acoustical coupling, vibration, and radiation in water), excitation of viscoelastic leaky Lamb modes in the plate, and the subsequent transmitted sound field in water. The models, capable of accurately describing the measured sound field phenomena, are used to investigate the observed deviations between measurements and plane-wave theory, by varying Poisson's ratio for the plate over the range 0.01 - 0.49.