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Smoke and CO alarm sounds

What the beeps and chirps from a smoke or carbon monoxide alarm mean, from the standards that set each pattern, with a clip of each.

About smoke and CO alarm sounds

A smoke or carbon monoxide alarm says what it has detected by the rhythm of its beeps, not by their pitch. Three beeps and a pause, over and over, is smoke or fire; four quicker beeps and a longer pause is carbon monoxide. Both rhythms are set by standards. Each page here has a synthesised clip of the pattern, with captions, and the rhythm written out for anyone who can’t hear it. A chirp is different: each maker sets its own chirps, and they report on the alarm itself.

The two alarm patterns

Temporal-3 is the standard emergency evacuation signal of ANSI S3.41: beeps of half a second with half a second between them, three times, then 1.5 seconds of silence. UL 217 requires a smoke alarm, or a combined smoke and CO alarm, to sound it when it detects smoke, and building fire alarms sound the same rhythm. Temporal-4 is the carbon monoxide signal: four beeps of a tenth of a second, a tenth of a second apart, then 5 seconds of silence, from NFPA 720, whose rules are now part of NFPA 72. UL describes it as four spaced tones followed by a five-second silent period.

Chirps

A single chirp every 30 to 60 seconds is not an alarm for smoke or gas. NFPA’s Fire Prevention Week tip sheet reads it as a low battery, and chirping that continues after a new battery as the end of the alarm’s life. Makers’ manuals differ on the details, and the page sets Kidde’s, BRK’s and Gentex’s side by side.

The pitch

No standard sets the pitch of a household alarm. CPSC staff found no frequency requirement in UL 217 and give 3,000 to 4,000 Hz as typical; the clips use 3.2 kHz, the frequency they measured. The exception is the 520 Hz low-frequency signal that NFPA 72 has required since 2014 of alarms provided to wake people in sleeping areas: a square wave in the same three-beep rhythm.

Sources

  1. Possible Technologies for Improving the Audibility of Residential Smoke Alarms, CPSC-ES-0505 (September 2005), U.S. Consumer Product Safety Commission, §2.3 and Figure 5 (“Three Pulse” Temporal Pattern)
  2. The Audibility of Smoke Alarms in Residential Homes, CPSC-ES-0503 (September 2005), U.S. Consumer Product Safety Commission, §3.2 Smoke Alarm Signal; Figure 3
  3. Carbon Monoxide Alarm Considerations for Code Authorities, UL 2034 and UL 217, UL Solutions, CO alarm signal
  4. CO1209 Series Carbon Monoxide Alarm: Installation Instructions and Owner’s Manual, CO1209 owner’s manual, Gentex Corporation, Temporal 4 activation signal
  5. S1209 Series Photoelectric Smoke Alarm: Installation Instructions and Owner’s Manual, S1209 owner’s manual, Gentex Corporation, models; checkout; temporal 3 activation signal
  6. NFPA Moving CO Alarm Requirements into Fire Alarm Code, June 25, 2018, Occupational Health & Safety, news report
  7. Learn the Sounds of Fire Safety: Fire Prevention Week tip sheet, Fire Prevention Week 2021, ©2021 NFPA, National Fire Protection Association (copy published by Bloomfield Township, Michigan), Smoke alarms; Carbon monoxide (CO) alarms
  8. 3.08 Sleeping Area Fire Alarm Requirements (2019), SFFD Bureau of Fire Prevention & Investigation, 3.08, San Francisco Fire Department, 2013 NFPA 72 Section 18.4.5.3; Specific SFFD Requirements
  9. LFH Series Low Frequency Horn, Data sheet 8830068 Rev B (12/21), Potter Electric Signal Company, Application; Description
  10. What Causes Consistent Chirping?, Kidde support article, Kidde, smoke alarms; carbon monoxide alarms

Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research