  The interaction of ultrasonic waves with fluid-embedded viscoelastic plates,
pipes, and shells, have been subject to extensive theoretical and experimental
studies over several decades. In normal-incidence through-transmission
measurements of a water-embedded solid plate using ultrasonic piezoelectric
transducer sound fields, significant deviations from plane wave theory have
recently been observed. To quantitatively describe such measured phenomena,
finite element modeling (FEM), also combined with an angular spectrum method
(ASM), have been used for three-dimensional (3D) simulation of the
voltage-to-sound-pressure signal propagation through the electro-acoustic
measurement system consisting of the piezoelectric transducer, the
water-embedded steel plate, and the fluid regions at both sides of the plate.
The observed phenomena of frequency downshift of the plate resonance, increased
sound pressure level through the plate, and beam narrowing / widening, are
ascribed to the finite angular spectrum of the beam, that excites a region of
negative and zero group velocity for the leaky Lamb mode in question.
