we have excited the PZT with a swept sinus wave from 1kHz to 10kHz and from 10kHz to 100kHz.
here are the 2 different spectrums:

- the 1kHz-10kHz is a standard spectrum where we see the impedance behavior of the PZT: Zpzt ~ 1/jCw

- the 10kHz-100kHz exhibit several PZT resonances and the 1st one is close to 28kHz.

(without PZT resonances, we should have the same behavior at higher frequencies than in the range 1kHz-10kHz)