Symmetric Lamb waves on plates exhibit anomalies for certain regions of frequency. The phase velocity appears to be double-valued [M. F. Werby and H. Überall, J. Acoust. Soc. Am. 111, 2686–2691 (2002)] with one of the branches having a negative group velocity relative to the corresponding phase velocity. The classification of the symmetric plate modes for frequencies appearing to have a double-valued phase velocity is reviewed here. The complication of a double-valued velocity is avoided by examining mode orthogonality and the complex wave-number spectra. Various authors have noted an enhancement in the backscattering of sound by elastic shells in water that occurs for frequencies where symmetric leaky Lamb waves (generalized to case of a shell) have contra-directed group and phase velocities. The ray diagram for negative group velocity contributions to the scattering by shells [G. Kaduchak, D. H. Hughes, and P. L. Marston, J. Acoust. Soc. Am. 96, 3704–3714 (1994)] is unusual since for this type of mode the energy on the shell flows in the opposite direction of the wave vector. Circumnavigation of the shell is not required for the leaky ray to be backward directed.
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May 05 2003
Negative group velocity Lamb waves on plates and applications to the scattering of sound by shells
Philip L. Marston
Philip L. Marston
Department of Physics, Washington State University, Pullman, Washington 99164-2814
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J. Acoust. Soc. Am. 113, 2659–2662 (2003)
Article history
Received:
July 19 2002
Accepted:
February 10 2003
Citation
Philip L. Marston; Negative group velocity Lamb waves on plates and applications to the scattering of sound by shells. J. Acoust. Soc. Am. 1 May 2003; 113 (5): 2659–2662. https://doi.org/10.1121/1.1564021
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