Sound, physics acoustic reflection

Lopez-Poveda, E.A., and R. Meddis. 1996. A physical model of sound diffraction and reflections in the human concha. Journal of the Acoustical Society of America 100(5) 3248-3259. 

While the design of such coatings is straightforward, selection of appropriate materials is not. Usually materials with the properties required for a particular application are not readily available, and some custom laboratory fabrication is necessary. This usually involves selecting a polymer composite which somewhat approximates the required physical properties. Then minor alterations to the chemical constituents or fillers are used on a trial basis and the acoustic properties (some combination of Young s Modulus and damping factor, sound speed, attenuation, density, and front-face reflectivity) of these sample formulations are measured. This continues until a suitable formulation is achieved. 

All the effects reflect the physical phenomenon of acoustic or ultrasonic cavitation. With this, to initiate the cavitation one must apply a certain threshold sound pressure designated as a cavitation threshold and determine the cavitation strength of a liquid. 

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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