Finite aperture influence on energy concentration, frequency shift, and signal enhancement, for acoustic transmission in the negative group velocity region of the S1 leaky Lamb mode

Acoustic beam diffraction phenomena for normal beam incidence to a viscoelastic plate, such as resonance frequency downshift, beam narrowing, and axial signal enhancement near certain leaky Lamb modes, have been demonstrated in recent years, as compared to plane-wave analyses. A close correlation between these phenomena and apparantly different guided-wave dispersion phenomena, including negative group velocity, has been shown by [Aanes et al., Proc. IEEE Int. Ultras. Symp., Taipei, Taiwan, 2015]. It was proposed that the beam diffraction phenomena depend on the size of the mode's NGV region being excited. In the present work, such possible connection between the mentioned beam diffraction phenomena in the frequency region of the S1 leaky Lamb mode, and the gradual excitation of this mode's NGV region, is investigated. A detailed study shows a close correlation between these beam diffraction phenomena and the portion of the S
₁
leaky mode's NGV region being excited.