The sensitivity of a piezoelectric crystal may be represented either by its charge sensitivity or by its voltage sensitivity. Charge sensitivity is defined as
where, q denotes the generated charge and F denotes the applied force. For a crystal with surface area A,
where, p is the stress (normal or shear) or pressure applied to the crystal surface.
Voltage sensitivity Su is given by the change in voltage due to a unit increment in pressure (or stress) per unit thickness of the crystal.
Thus, in the limit, we have
where d denotes the crystal thickness. Now, since
for a capacitor element, the following relationship between charge sensitivity and voltage sensitivity is obtained:
Note that k is the dielectric constant (permittivity) of the crystal capacitor
The overall sensitivity of a piezoelectric device can be increased through the use of properly designed multielement structures (bimorphs).