For the piezo horns in most fire alarms, are these piezos active (they make a sound when power is applied to them) or passive (a circuit or micro-controller sends the sound to play to the piezo when power is applied; without the circuit the piezo would make no noise on its own as the sound is not embedded into it)?
some usually are but some are not it depends on the brand for for example simplex the PO are I believe they are wired in and into circuitboard panel but notify I believe a mic controller that’s powered by it.