shows the dispersive curves for the multiple modes of the
steel plate used in this work. The group velocity of the elastic wave
modes at certain frequency is essential to locate AE source. The dispersion
characteristics of multi-modes have been studied by
[14,15]. It is convenient from Fig. 3 to obtain the mode velocity
under the frequency range concerned. In our condition, there are
two modes A0 and S0 over the frequency range of 0–500 kHz. Lamb
waves are guided dispersive waves propagation in an elastic plate
with free boundaries with an infinite number of modes for both
symmetric and antisymmetric displacements within the layer.
These waves are formed by interference of multiple reflections
and mode conversion. The normal way to describe the propagation