Speakers, Sirens, & Strobe Lights for Security Systems
The most important part of any security system is how it alerts people that it has been set off. Alarm bells, sirens, speakers, and strobes (collectively called "annunciators") really serve two purposes, they -- of course -- alert the homeowner and neighbors that something is amiss. But even more importantly, they let the burglar know that they have been detected and that someone will soon be coming to investigate. Usually, this prompts a speedy exit.
Sirens, bells, and strobes are usually are powered by 12 VDC and make a single predetermined sound (or flash) when the voltage is applied.
Speakers are driven by the alarm system with an audio signal. They can make whatever sound the alarm system tells them to. Including voice. (This depends entirely on the alarm panel.)
Note that annunciators are usually wired with four conductor cable. Two of the conductors are for the 12 VDC or audio signal that drives the device, the other two are connected to a tamper switch within the device or just shorted together inside the device. This is so that, if the wiring to the speaker is cut, the alarm still goes off (and dials out or sounds another speaker, hopefully.)