Effects of fluid-loading on Lamb mode spectra

Light fluid-loading of an infinite, isotropic, elastic plate not only leads to the real modes of the unloaded plate acquiring a small, imaginary component, but also to the imaginary modes acquiring a small, real component. This provides a model with which computations of the leaky Lamb modes are seen to be compatible. The imaginary components of the propagating modes form a ‘‘resonance pattern’’ which contains all the information on the plate’s transmittivity. The level of this pattern increases with the ratio of fluid to plate density. When the latter ratio is large enough to cause the resonance pattern to overlap the peaks of the well-known imaginary semiloops, there is interaction leading to transition from the leaky Lamb mode behavior to that of a plate clamped on its surfaces with slip boundary conditions. There is general agreement with the conclusions of S. I. Rokhlin et al. [J. Acoust. Soc. Am. 85, 1074–1080 (1989)] and the present work broadens the parameter range over which those conclusions have been shown to hold.