This paper presents a theoretical investigation of the generation of Lamb waves inside an immersed plate by complex harmonic inhomogeneous plane 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 characterized 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 permanent leaky waves (only the slowness is complex), especially for the A0 and S0 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 appear from the calculation of dispersion curves.
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Poncelet et al. (1997) studied this question.
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