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Piezoelectric Impedance
The electrical impedance is a distinguishing characteristic for piezoelectric elements. It differs substantially from the impedance of non-piezoelectric dielectric elements when driven at high-enough frequencies. The difference stems from the coupling of electrical energy input to mechanical motion output.

Recall that the electrical impedance is defined as the voltage drop across an element divided by the current through the element. For a (simple geometry) piezoelectric element, the electrical impedance over a given frequency range will appear similar to that shown here:

Impedance Plot
The impedance for a non-piezoelectric element (of the same shape and dielectrical properties) is also shown in blue.

The presence of electrical resonances and anti-resonances make the piezoelectric impedance unique. The resonances result from the electrical input signal exciting a mechanical resonance in the piezo element. For each mechanical resonance in the piezo element, a resonance/anti-resonance pair will exist in the impedance.

This characteristic piezoelectric impedance can be modeled by an equivalent circuit.