Transducer finite aperture effects in sound transmission near leaky Lamb   modes in elastic plates at normal incidence
  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. 
Magne Aanes, Kjetil Daae Lohne, Per Lunde, Magne Vestrheim
physics.ins-det