Sounder

A sounder is essentially the device that makes the loud noise to alert everyone that there's a fire or other emergency. Think of it as the "voice" of the fire alarm.
When a smoke detector or heat detector senses a fire, it sends a signal to the main fire alarm panel. The panel then activates the sounders (and often flashing lights called "beacons" or "strobes") to get people's attention immediately. Electronic Sounders: These are very common and can produce a variety of loud, attention-grabbing tones, sometimes even with pre-recorded voice messages to guide people to safety.