[Dangers of Piezo] Piezo size/shape vs generated voltage?

[kalel]

I didn't know how to make a proper subject, but I'm interested in some rough knowledge about what type of Piezo element items (e.g. small Piezo buzzer, larger Piezo, etc) makes what kind of voltage under what kind of conditions.

I'm sure this might be difficult to measure with a multimeter (especially a cheap one, although less to feel sorry about if it does generate a high voltage), but I'm essentially trying to figure out if using a certain element such as a Piezo speaker will generate dangerous voltage, and under what conditions (e.g. is it a light tap, a squeeze, a strong hammer hit, a gun shot, etc.).

I don't necessarily wish to start placing optocouplers everywhere I use a piezo buzzer. But I want to understand the danger a bit better (so if something like that is necessary, perhaps I might).

[floobydust]

Piezos can generate a lot of voltage, as a fast pulse. Barbeques, lighters use them for ignitors.

The small piezo speakers, put an LED across one, light taps and they easily flash the LED.  Like when they are used as sensors for electronic drums.
I just tried a neon lamp and a whack with a pen flashes the neon lamp, so over 90V there.

If you hit one hard, it can damage an IC from the voltage spike. Best to use a scope to look at it, but the voltage depends on how hard the crystal is hit.
It's a low energy pulse and resistor/zener is enough for protection.

[Kleinstein]

The piezo element is a usually small capacitor. When stressing it, there will be voltage / charge in it. The energy is limited by the capacitance. The voltage is approximately proportional to the amount of stress. It hit with something this creates a short pulse of high stress and thus a short pulse of high voltage. If stressed slow, one gets a long pulse (if the capacitor does not discharge). However the level of stress and thus the voltage is usually much lower. As the pulse energy is limited, a zener or similar is quite effective to limit the voltage.

The voltage also depends on the dimensions of the piezo: if thin and thus high capacitance, the voltage is relatively small.