NFPA 72 code sections · Devices & Signal Flow: How the Panel Hears and Speaks
Horn (audible appliance): the panel's voice when there is no voice system
A horn is an output. It does nothing until the panel energizes its notification appliance circuit, which the panel does when its programming calls for occupant notification — within 10 seconds of an alarm. The horn must sound in the three-pulse temporal pattern that means 'evacuate', and loud enough: at least 15 dBA above the average ambient sound level in public mode, or 75 dBA at the pillow in sleeping areas. When an authorized person silences the alarm at the panel, the horns stop; a new alarm from another device must re-sound them.
NFPA 72 reference
Chapter 18 — Notification Appliances / Chapter 10
18.4 Audible Characteristics (18.4.2 Temporal-3 evacuation signal; 18.4.4 public mode ≥ 15 dBA above ambient; 18.4.5 sleeping areas 75 dBA); 10.11.1 activation within 10 s; 10.12 alarm signal deactivation (re-sound on a subsequent alarm)
References use NFPA 72 2019/2022 numbering — section numbers can shift between editions, so confirm in the edition your AHJ has adopted.
On the job, step by step
- The panel declares an alarm from the second-floor pull station.
- Within 10 seconds the panel energizes the notification appliance circuits for the whole building (general evacuation).
- Every horn sounds Temporal-3: three half-second pulses, a pause, repeated, at a level 15 dBA above the office background.
- The fire department arrives, confirms the pull was accidental, and an authorized person presses Alarm Silence — the horns stop; the strobes and the panel's alarm indication remain until reset.
- If a detector elsewhere alarms while silenced, the panel re-sounds the horns automatically.
Quick check
1. A horn sounds when:
2. The audible evacuation pattern is:
3. In public mode the horn must be at least:
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