If we draw a similarity from a piezoelectric transducer to a Mass-Spring-Damper (MSD) system, would piezoelectric forcing be akin to base excitation (specified displacement of the base) or force excitation (specified applied force).

Let's assume:

• The movement of the mass is akin to the kinetic energy in the system.
• The stretch or compression of the spring is akin to stored potential energy.

Now let's consider the application of a step voltage and the reaction of the piezo transducer.

• According to the forced excitation model, we equate the applied voltage to the force. In this case, no immediate increase in energy would be present. The mass will take time to move and the spring will thus take time to stretch.
• According to the base excitation model, the base excitation would be equated to applied voltage. If we specify a sudden step-wise displacement, we have immediate energy in terms of the compressed spring. The mass then has to react and gain momentum.

I believe the base excitation model is more accurate. When a voltage is applied suddenly (step response) to a piezoelectric transducer, the natural strain state of the piezo is immediately shifted, elastic energy immediately appears, and the piezo then gains velocity and displaces over time until it settles at its new equilibrium strain. Deep stuff.