Let's consider a 10N force (F), applied along the thickness axis with a blue curve time profile, over a 1cm² of 100µm thick piezo film with a d33 of 25 pC/N.

C = 100pF.
Q = 250pC.

So we get, measured with an infinite input impedance device :
 

V = Q / C = 2.5V

When measured over a 10MΩ resistance, we would measure the signal represented in red, proportionnal to dF/dt.
The Q value can be computed by :
 

Q = V / R .dt

Three dimensional strains can strongly modify this theoretical signal.

For example, shock gauges are only calibrated for monodimensional stresses and great care should be taken to observe this condition.