Analysis of compressional-wave influence on thickness-shear-mode resonators in liquids

The operation of a thickness-shear-mode (TSM) resonator contacting a finite liquid layer has been analysed to investigate the effect of compressional-wave generation. This effect is mainly related to the non-uniform shear velocity profile across the surface of a TSM device, Hydrophone measurements show two coils of longitudinal waves. Their influence on the TSM resonator response is studied with impedance analysis, varying the spacing between resonator and reflector as well as the reflecting conditions on the top side of the liquid layer. A characteristic response with a periodicity of lambda/2 is observed when the spacing of the liquid cavity or the liquid layer thickness is changed. It indicates standing longitudinal waves in the cavity. Their influence can be modelled with an additional complex impedance in the motional arm of the Butterworth-van-Dyke equivalent circuit representing an own (compressional) transmission line.