LAMB WAVES GENERATED BY COMPLEX HARMONIC INHOMOGENEOUS PLANE-WAVES

This paper presents a theoretical investigation of the generation of L amb waves inside an immersed plate by complex harmonic inhomogeneous p lane waves. Whereas free modes of a fluid-loaded plate are classically assumed to be formed by inhomogeneous plane waves with real frequency (leaky waves), the nature of Lamb waves is searched in this study in terms of combinations of transient inhomogeneous plane waves character ized by a complex slowness vector and a complex frequency. It is shown that dispersion curves of transient Lamb waves (only the frequency is complex) can be very different from the dispersion curves of permanen t leaky waves (only the slowness is complex), especially for the A(0) and S-0 modes for which there exists a cut-off frequency beyond which these modes do not exist. In the general case, when both the frequency and the slowness are complex, connections between modes clearly appea r from the calculation of dispersion curves. (C) 1997 Acoustical Socie ty of America.