Machinery & Energetics

The results of the experimental research of frequency dependency of the phase velocity of backward Lamb wave in CdS crystal plate are presented. Those waves have opposite directions of the phase and group velocities. Under wave propagation along axis of sixth order the symmetrical Lamb mode of first order was investigated. The one length of shear wave fit the plate thickness at cutoff frequency of this mode. For this mode the frequency dependence of phase velocity of backward wave, and also of forward wave (its directions of phase and group velocities 301 coincide), were measured. Detecting of waves was carried out with using piezoelectric properties of the plate material. Excitation of backward Lamb wave was excited with piezoelectric ceramic transducer. The determination of the phase velocity of the waves was performed by measuring of the wavelength with the phase method of variable base. The measurements were carried out in plate with thickness 0,66 mm at frequencies ~ 2,7 MHz. The observed value of phase velocity of backward wave was varied in region 15·10³ ÷ 29·10³ m/s. The existence of backward Lamb wave in CdS plate is proved. This fact may be used to realize the interaction of charge carriers with backward elastic waves and elastic waves with zero group velocity.

Lembe wave, reverse waves