teckel:
I believe a piezo only acts like a capacitor when "charged" in one direction (for example, with the tone library). The deflection of the disc is what generates the current. However, with toneAC it can't charge in this way as an alternating current is being sent. Therefore, the piezo is never charged and never acts like a capacitor. From my understanding of how a charge on a piezo works, supplying an alternating current eliminates the need for a resistor. But, maybe I'm mistaken.
Yes, completely mistaken. The very same materials used in high value ceramic capacitors are used in
piezo elements because of their very high dielectric constant is due to the ferroelectric effect, where
charge movement is caused by atomic displacements within the crystal lattice. Piezo elements are all
capacitive.
And like all capacitors the impedance goes down as the frequency goes up, so the current will be higher at
high frequencies. And being mechanical devices they can also resonate, leading to the possibility of
higher voltages being generated than are fed in...