Volume 118, Issue 3, September 2005
Index of content:
- TRANSDUCTION 
118(2005); http://dx.doi.org/10.1121/1.2000749View Description Hide Description
A small, high performance fiber opticmicrophone has been designed, fabricated, and tested. The device builds on a previous design utilizing a thin, seven-fiber optical probe, but now adds a micromachined thick silicon diaphragm active element. The resulting sensor head is thin (several millimeters) and light, and the overall microphonesystem is less expensive than conventional microphones with comparable performance. Measurements in the laboratory using a standard free-field technique at high frequencies, an enclosed calibrator at lower frequencies, and pseudostatic pressure changes demonstrate uniform broadband response from near dc (0.01 Hz) up to near 20 kHz. The measuredmicrophone internal noise is nearly flat over this band and does not exhibit noticeable levels of noise. Over the audible portion of this band, the minimum detectablepressure is determined to be per root Hz with further reductions possible using lower noise∕higher power light sources and∕or improvements in the diaphragm. In contrast to conventional high-performance microphones, there is no need for preamplifier packages close to the relatively small sensor head resulting in much lower acoustic scattering cross sections. This attribute, together with high performance, low cost, and immunity to emi, makes the microphone ideal for multielement array applications.
118(2005); http://dx.doi.org/10.1121/1.1985076View Description Hide Description
Properties of transducers and substrates for bulk acoustic wave resonators and sensors are described. These resonators utilize one-dimensional thickness vibrations of structures consisting of a low-loss substrate crystal surmounted by a thin active piezoelectric film that drives the composite in resonant modes to achieve gigahertz frequencies. The structures considered include oblique orientations of the substrate, leading to generation of coupled elastic modes in the composite. A modified Christoffel-Bechmann (CB) formalism is presented to calculate acoustic wave speeds and displacements in the piezoelectric film transducer and the substrate. The CB method also yields the piezoelectric coupling coefficients of arbitrarily oriented piezofilms, for electric fields impressed either along the thickness or laterally. The calculations apply generally to transducer and substrate crystals of any symmetry class. The piezoelectric portion is then made specific for films of class (wurtzite structure) with arbitrary orientation on the substrate, and the substrate calculations are specified for class materials, but apply also to any substrate with or 32 symmetry. Zinc oxide and sapphire are used in an example of the acoustic resonator structure.