Ultrasonic beam transmission in the backward-and forward-wave frequency-wavenumber bands of a fluid-embedded steel plate

$A$
three dimensional hybrid finite-element angular-spectrum method is used to model sound beams transmitted through a water-embedded steel plate at oblique beam incidence. Simulations are compared with measurements. The characteristics of the transmitted pressure beam field are investigated for normal and oblique angles of beam incidence, in the backward- and forward-wave regions about the zero-group velocity (ZGV) point for lower-order symmetric Lamb modes in the plate. The results indicate that beam excitation in the regions of the backward mode branches of the S
-2
and S
2
Lamb modes are both necessary for nearfield beam enhancement. For an acoustic beam steered in the positive x direction, beam excitation exclusively in the region of the S
-2
backward mode branch and
$\mathrm{S}_{1}$
forward mode branch, gives beam displacement in the negative and positive x-direction, respectively. Beam excitation in the region of the ZGV point (exciting both S
-2
and
$\mathrm{S}_{1}$
) gives minor or no beam displacement.