A fire alarm is an electromechanical or electronic bell , klaxon , chime , horn , speaker , strobe light , or other device which warns people in a building of a possible fire or other condition requiring evacuation . Some horns can produce several different kinds of sounds, including the Code-3 temporal pattern (0.5 second pulse, 0.5 second pause, 0.5 second pulse, 0.5 second pause, 0.5 second pulse, 1.5 second pause), which was designed to be a distinct pattern used only for evacuation purposes. Other sound patterns include march time (0.25 second pulse, 0.25 second pause, repeat), a continuous tone, hi-lo (0.25 seconds alternating between two tones of differing frequency), siren (up-and-down sweep in frequency), slow-whoop (slow rising sweep upwards in frequency), and an electronic bell sound. Fire alarms are often very loud, sounding at roughly 90 decibels. Typically, when a fire alarm is sounded, emergency responders are summoned, the building is evacuated , people gather at predetermined assembly points, and a roll call is held.

Fire alarms may be triggered automatically by smoke detectors , heat detectors , sprinkler flow switches, or manually. Manual pull stations and manual call points are sometimes protected by glass which must be smashed to set off the alarm. Protective covers may also be placed over the station to help prevent false alarms.

The nerve center of a fire alarm system is the Fire Alarm Control Panel , where building personnel and emergency responders are able to locate the source of an alarm, pinpoint trouble or supervisory conditions, silence alarms, and reset the system.