Finite aperture effects in the backward wave propagation region of a leaky Lamb mode at normal beam incidence

Studies of ultrasonic transmission through water-embedded viscoelastic plates at normal beam incidence have indicated a close connection between beam excitation of backward wave regions of certain leaky Lamb mode pairs of the plate, and diffraction effects such as resonance frequency downshift, axial SPL increase and beam narrowing. Transmission at normal beam incidence are investigated here on the acoustical axis for a gradual beam excitation of a backward wave region in a steel plate - with the plate in two different depths from the transmitter transducer. Downshifted resonance frequencies are compared to the cut-off and zero group velocity frequencies for the 𝑆−2 Lamb mode in a vacuum-embedded plate, and to the maximum group velocity frequency for the corresponding leaky Lamb mode in a water-embedded plate. The beam diffraction phenomena observed around the fundamental thickness-extensional mode in the plate are found to depend systematically on the portion of the backward wave region being excited. Relocation of the plate to another depth is shown to affect the downshift of the resonance frequency and the SPL increase.