Mass Sensitivity of the APM

An ideal mass layer deposited at the surface results in increased kinetic energy which is offset by a decrease in propagation velocity [54]  

Rayleigh wave devices exhibit sensitivity, when calculated on a relative basis (Av/vo), that is proportional to frequency. In contrast, SH plate mode sensitivity displays no significant frequency dependence. This difference can be attributed to the fact that with surface waves, the acoustic energy becomes distributed closer to the surface as frequency increases. Like the TSM resonator, the energy density for each propagating SH mode is, dependent upon plate thickness rather than frequency. 

It is reasonable to ask how accurately the mass sensitivity in vacuum reflects the sensitivity when the device has liquid contacting the surface. This was investigated by monitoring the frequency shift of a single device both during vacuum deposition of a metal film and removal of the same film in an etching solution. The sensitivity in the liquid was approximately 6% less than the value measured in vacuum, a discrepancy that lies within our estimates of experimental uncertainty in this case [54]. 

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