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A piezoelectric substance is a crystalline mineral which responds to a mechanical force by generating a voltage. This voltage is proportional to the amount of force applied , and as well as the type of force applied (i.e. tension and compression produce opposite polarities). Furthermore, this effect is reciprocal, so a piezoelectric substance will also respond to an electric field by generating a mechanical response that is proportional to the field's strength and polarity.
The material of the piezoelectric substrate determines the velocity of the acoustic wave, which is in the range of 1500-4800 m/s. This is 105 times slower than the electromagnetic wave velocity, allowing for a longer delay along a shorter delay line. The most common piezoelectric substrate materials are quartz, lithium niobate, lithium tantalate, zinc oxide, and bismuth germanium oxide.
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