INFLUENCE OF COMPRESSIONAL WAVE GENERATION ON THICKNESS-SHEAR MODE RESONATOR RESPONSE IN A FLUID

Acoustic interferometry was performed with thickness-shear mode (TSM) resonators to investigate the effect of compressional wave generation on the response (resonant frequency and damping), Resonator response was measured while the spacing between the resonator and adjacent solid was filled with fluid and the spacing was varied. A characteristic resonance response was observed whenever the spacing reached a multiple of lambda(c)/2, where lambda(c) is the compressional wavelength in the fluid. Compressional wave generation arises from a gradient in the inplane surface displacement. A model is proposed to predict the resonator response that arises from combined shear wave and compressional wave generation. Experimental data fit to this model determine device coupling to shear and compressional waves. The model also relates resonator response to the surface displacement profile. By measuring this displacement profile, compressional wave generation can be estimated. The effect of surface roughness and device geometry on shear and compressional wave coupling is examined. The results indicate that even in a semiinfinite fluid, compressional wave generation contributes significantly to device damping (motional resistance) but not to the frequency shift.