Backscattering enhancements associated with antisymmetric Lamb waves on circular plates in water: Observations and imaging of the plate surface using acoustic holography

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When a tilted circular plate is illuminated with high-frequency sound in water, significant enhancements occur when the angle of incidence corresponds to a Lamb wave coupling angle. Previously, scattering associated with the lowest-order symmetric Lamb wave, s0, was studied using acoustic holography and modeled using quantitative ray methods [B. T. Hefner and P. L. Marston, J. Acoust. Soc. Am. 107, 2847 (2000)]. The present investigation focuses on backscattering associated with the antisymmetric Lamb waves. Above the cutoff frequency of the first-order antisymmetric wave, a1, enhancements are observed which are associated with both the a1 wave and the a0 wave. For these enhancements, however, mode conversion between these modes plays a significant role. To examine the mode conversion and to identify the scattering mechanisms, acoustic holography was used to image the plate surface when the plate is ensonified at the a1 wave coupling angle. From the imaged wave field, the enhancement at this angle involves a wave which moves around the circumference of the plate: a ‘‘whispering gallery’’ mode involving the a1 wave or a flexural edge wave. Both of these possibilities are considered as well as a possible scattering mechanism at the a0 wave coupling angle. [Work supported by the Office of Naval Research.]