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

Abstract

In order to excite Lamb waves in a plate (8) or container wall (8), it is proposed that an IDT transducer (interdigital transducer), which is known per se and comprises a layer (1) composed of piezoelectric material, and on one of whose surfaces two electrodes (2, 3) which engage in one another like fingers are applied, be acoustically coupled to the plate (8) or container wall (8) by means of the surface facing away from the electrodes (2, 3). If a radio-frequency AC voltage is applied to the connections (6, 7) of the electrodes (2, 3), then a thickness vibration is initiated in the piezoelectric layer (1). If the speed of propagation or speed of sound of the longitudinal waves which result in this thickness vibration in the piezoelectric layer (1) is matched to the phase velocity with which an so mode Lamb wave propagates in the material of the plate (8) or of the container wall (8), then only so mode Lamb waves are initiated by the resonance effect in the plate (8) or container wall (8), by virtue of the acoustic coupling between the piezoelectric layer (1) and the plate (8) or container wall (8). The matching process can be carried out by selecting the distance (D) between the fingers (4, 5) of the electrodes (2, 3) such that this distance (D) is equal to half the wavelength λ/2 of the thickness vibration to be initiated in the piezoelectric layer (1).