Perturbation method for analyzing mass sensitivity of planar multilayer acoustic sensors

A perturbation method is developed to analyze the mass loading sensitivity of planar composite acoustic gravimetric sensors. The sensitivity formulas are obtained in explicit forms for the two lowest sagittal (D/sub 1/ and D/sub 2/) modes, the lowest shear horizontal (SH/sub 0/) mode and high-order SH/sub m/ modes in a two-layer isotropic composite plate sensor. The composite plate consists of a plate of thickness b coated by a film of thickness h on which the mass loading layer of infinitesimal thickness is deposited. This coating can be a chemically selective film which is assumed to be acoustically thin (h/spl Lt//spl lambda/), where /spl lambda/ is the acoustic wavelength. For Love modes supported by a film coated on a semi-infinite substrate and for Rayleigh modes on a semi-infinite substrate, the sensitivity formulas are expressed in analytical form. These formulas specify the contribution of each material parameter in the substrate and film, and of the elasticity of the mass loading layer for each planar sensor, and provide a general guide for enhancing the sensor sensitivity.