Method for exciting lamb waves in a plate, in particular a container wall, and an apparatus for carrying out the method and for receiving the excited lamb waves

摘要

In order to excite Lamb waves in a plate or container wall, an IDT transducer (interdigital transducer) is employed. The IDT comprises a layer composed of piezoelectric material, and on one of whose surfaces two electrodes which engage in one another like fingers are applied. The IDT is acoustically coupled to the plate or container wall by means of the surface facing away from the electrodes. If a radio-frequency AC voltage is applied to the connections of the electrodes, then a thickness vibration is initiated in the piezoelectric layer. If the speed of propagation or speed of sound of the longitudinal waves which result in this thickness vibration in the piezoelectric layer is matched to the phase velocity with which a s₀ mode Lamb wave propagates in the material of the plate or of the container wall then only s₀ mode Lamb waves are initiated by the resonance effect in the plate or container wall, by virtue of the acoustic coupling between the piezoelectric layer and the plate or container wall. The matching process can be carried out by selecting the distance (D) between the fingers of the electrodes such that this distance (D) is equal to half the wavelength &lgr;/2 of the thickness vibration to be initiated in the piezoelectric layer.