On the low-frequency oscillation of a fluid layer between two elastic plates

In 1965, Lloyd and Redwood predicted the existence of a new guided mode of wave propagation in solid–fluid–solid trilayers @Acustica 16, 169–173 ~1965!#. In this so-called fluid mode, the two solid plates flex sideways in a symmetric way while the fluid in the thin gap between them is squeezed forward and backward parallel to the direction of wave propagation. This mode is slower than the lowest-order bending mode of the plates and asymptotically approaches the Stoneley– Scholte mode of the solid–fluid interface at high frequencies. While the other modes of the solid– fluid–solid trilayer are very similar to the free vibrations of the free solid plates which are only slightly perturbed by the fluid coupling between the plates, the fluid mode cannot be related to any classical Lamb mode in the plates. It is shown in this paper that at low frequencies the fluid mode is truly a coupled vibration of the solid and the fluid, which contribute the stiffness and the inertia, respectively. The highly dispersive phase velocity of this mode was measured over a frequency range of one decade by using a laser interferometric technique. The experimental results were found to be in good quantitative agreement with the theoretical predictions. To the best of our knowledge, these measurements constitute the first conclusive experimental evidence of the existence of the dispersive fluid mode predicted by Lloyd and Redwood. © 1997 Acoustical Society of America. @S0001-4966~97!05512-4#