Three beeps and a pause (smoke alarm)

How to read it

The pattern is called temporal-3, or T3. Each beep lasts half a second, with half a second between beeps, and after the third beep there are 1.5 seconds of silence before the next three, so the pattern repeats every 4 seconds. NFPA 72 allows each part 10 percent either way. ANSI S3.41 and ISO 8201 define it as the audible emergency evacuation signal, and UL 217, the standard smoke alarms are tested to, requires a smoke alarm, or a combined smoke and carbon monoxide alarm, to sound it when it detects smoke. Gentex’s manual for its S1209 smoke alarm puts it as “3 beeps, a pause, 3 beeps, a pause”, and NFPA’s Fire Prevention Week tip sheet says three loud beeps mean smoke or fire. The carbon monoxide pattern has four shorter beeps and a longer pause.

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Smoke alarms and building fire alarms

A building’s fire alarm system sounds the same rhythm. NFPA 72 requires it wherever the intended response is to evacuate the building, as Gentex’s manual quotes the code, and New York City’s building code requires it of new fire alarm systems in buildings whose fire safety plan is a total evacuation, the FDNY says. The pattern is the rhythm, not the sound: the FDNY notes that bells, horns or speakers can all sound it.

The pitch of the beeps

No standard sets it. CPSC staff found no requirement in UL 217 for the frequency of a smoke alarm’s horn and say a typical one sounds between 3,000 and 4,000 Hz; the alarm they tested sounded at 3.2 kHz. UL 217 sets the loudness instead: at least 85 dBA, 10 feet from the horn. The clip on this page is synthesised at 3.2 kHz from the standard’s timing, and plays far more quietly. A real alarm may sound higher or lower; the rhythm is what identifies it.