Piezoelectric sound resonance

A piezoelectric-sound-resonance cavity (PSRC) has been used as a sensing element for hydrogen [39]. The sensing mechanism was based on the acoustic property differences... 

In situations where absorption of the incident radiation by the transducing gas is troublesome a piezoelectric transducer (made from barium titanate, for example) can be attached to the sample (or sample cuvette in the case of liquids) to detect the thermal wave generated in the sample by the modulated light (8,9). The low frequency, critically damped thermal wave bends the sample and transducer thus producing the piezoelectric response. The piezoelectric transducer will also respond to a sound wave in the solid or liquid but only efficiently at a resonant frequency of the transducer typically of the order of 10 to 100 KHz (see Figure 4). Thus neither in the case of microphonic nor piezoelectric detection is the PA effect strictly an acoustic phenomenon but rather a thermal diffusion phenomenon, and the term "photoacoustic" is a now well established misnomer. 

Coherent reflections at the top and bottom boundaries of the plate give way for a set of standing acoustic waves between the two main surfaces of the plate. Due to the piezoelectric nature of quartz two sets of resonance frequencies exist for each mode, depending on the electrical boundary conditions. The first set corresponds to a plate with open-circuit boundary conditions. From the physical point of view charges will be collected on the electrodes building up a potential difference and hence an electrical field from the electrical point of view the electrodes are unconnected. This resonance is termed anti-resonance in the piezoelectric literature and parallel resonance in electronics literature. The second set of resonance frequencies corresponds to a plate with short-circuit boundary conditions. The electrodes are connected and a potential difference cannot be built up. The respective names are resonance in piezoelectric and series resonance in electronics Hterature. The differences arise from piezoelectric stiffening accompanied by differences in the sound velocity. The anti-resonance (parallel) frequencies of each of the three acoustic modes are completely decoupled giving ... 

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