Abstract
We study thickness-shear vibration of a piezoelectric crystal plate whose one surface is in contact with a viscous fluid layer of finite thickness. The crystal plate is driven by a lateral electric field. The theory of piezoelectricity and the theory of Newtonian fluids are used. Both free and forced vibration solutions are obtained. An approximate expression for the frequency shift in the crystal plate due to the presence of the fluid is presented. The admittance of the structure is also calculated. The results are useful for acoustic wave fluid sensors.