Recently Introduced Acoustic Sensors

1 Thin-Film Compressional Bulk-Wave Sensor 

Because the membrane vibrates in a compressional mode, rather than shear as in the TSM device, this sensor is most likely to be useful in a gaseous rather than liquid ambient. A gravimetric sensitivity, Sm. of 555 cm g has been reported [83] for one of these 1 GHz devices this value is in good agreement with what one would predict from the analysis cited in Section 3.5.2 below. 

In this device, a thin film or periodic grating on the surface of the crystal slows the wave and prevents radiation of energy into the interior of the crystal [84]. With proper choice of crystal orientation, a purely transverse particle motion at the surface can be obtained, permitting the sensor to operate successfully in liquids. 

Another approach to obtaining a sturdy sensor substrate and a transverse particle motion to permit operation in liquids is the use of a structure that supports Love waves [85,86]. These waves are guided in a layer that is thin compared with the wavelength on a thick substrate. With the proper choice of materials one may also be able to achieve a significant degree of temperature compensation in this device. 

Just as flexural waves can propagate at low speeds in a plate whose thickness is much less than the wavelength, a low-speed flexural wave can propagate in a cylindrical rod whose diameter is much smaller than the wavelength [87]. Because of the low wave speed, operation as a gravimetric sensor in liquids is possible, as with the flexural plate-wave sensor. The gravimetric sensitivity for this sensor is typically S = — l/(2p ), where a is the radius of the rod. 

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