For any specified mode (e.g., the axial mode) in a piezoelectric transducer, there are two adjacent resonances that depend on the interaction of the ceramics with the mechanical components of the transducer. These resonances are called series resonance and parallel resonance. Series resonance falls somewhat below parallel resonance. The frequency difference between these two resonances will increase as the total ceramic capacitance increases and will decrease as the relative energy storage of the stack increases.
The designer may choose to operate at either series or parallel resonance, depending on the resonance characteristics and power supply characteristics that he desires.