piezo element with 3 wires

I have a bunch of piezo buzzers that I salvaged from some older smoke detectors they have 3 wires: red, black and white.

If I just use red and black it works but I was wondering what the white might actually be for? I had always assumed it was ground but then why would you need something like this:


the extra wire in that schematic is labeled as "feedback"?

The “feedback” lead permits you to build oscillators that “automagically” run at the ideal frequency (and thus maximum volume) for a particular piezo element without having to have any timing elements in the circuit itself (other than the piezo itself, which is not dissimilar from a giant crystal.)

thanks for that knowledge nugget westfw,

that makes perfect sense for a smoke detector buzzer to play at the loudest possible volume.