Mass sensitivity of two-layer shear horizontal plate wave sensors

Velocity and mass sensitivity formulae in explicit form for shear-horizontal (SH) plate wave sensors are presented. The sensor geometry consists of an isotropic plate of thickness b and an isotropic thin layer of thickness h. The mass loading layer is assumed to be acoustically thin (h much less than lambda), where lambda is the acoustic wavelength. The mass loading sensitivity, S(m)V, of the sensors used in velocity measurements decreases by a factor of (1-C2) for all the SH modes due to the effects of the elasticity of the mass loading layer, where C2 = S(S2)2/V(S1)2, V(S1) and V(S2) are the shear wave velocities of the two materials respectively. Because of the inertial effect of the mass loading layer, Sv decreases by a factor of (1 + rho2h/rho1b)-1 for the lowest order mode and by a factor of (1 + 2rho2h/rho1b)-1 for the other modes, where rho1 and rho2 are the density of the plate and the loaded mass layer, respectively. We also show that the relation S(m)V = (V/V(g))S(m)f is valid for the composite case, where S(m)f is the sensitivity formula for sensors configured in resonant frequency measurements, and V and V(g) are the phase and group velocity of the SH plate modes respectively.