Low-frequency 520 Hz alarm (sleeping rooms)
A fire alarm in a sleeping room that sounds a low buzz in place of a high-pitched beep. It is the three-beep fire alarm rhythm at 520 Hz, which NFPA 72 requires of alarms provided to wake people in sleeping areas.
The pattern A low buzz in the three-beep fire alarm rhythm: buzz, buzz, buzz, pause; repeating. Each buzz and each gap lasts half a second, and the pause after the third lasts 1.5 seconds. The pattern repeats every 4 seconds.
Synthesised from the standard’s timing and tone; real alarms are much louder. A synthesised clip, not a recording. The sound alone (WAV)
How to read it
Since 1 January 2014, NFPA 72, the National Fire Alarm and Signaling Code, has required audible appliances provided for sleeping areas to awaken occupants to sound a low-frequency signal: a square wave, or a signal with equivalent awakening ability, with a fundamental frequency of 520 Hz, 10 percent either way. The rule was added in the 2010 edition and applies to new construction of sleeping areas such as hotel rooms, college dorms, apartments and assisted living, FacilitiesNet reported as it took effect. It is meant for people with some hearing loss, and others, such as children, who have trouble waking to higher-pitched sounds. The rhythm is the fire alarm’s three beeps and a pause: Potter’s LFH horn sounds a 520 Hz square wave in the temporal-3 pattern, and the four-beep carbon monoxide pattern when wired to Potter’s synchronisation module. This page describes the code and isn’t legal advice.
Also searched as: 520 hz alarm, hotel fire alarm sound, low frequency alarm, low frequency sounder.
These describe what each alarm sound means and which standard sets it. Sounds differ between makers and models, and the alarm’s own manual lists what each of its sounds means. What to do when an alarm sounds is set out by the US Fire Administration at usfa.fema.gov and by your local fire department.
Next to a household smoke alarm
A household smoke alarm’s piezoelectric horn sounds a high tone, typically 3,000 to 4,000 Hz, CPSC staff say, and they found it can’t produce a low-frequency sound at the required level and still meet UL 217’s battery requirements. FacilitiesNet notes that the traditional high-pitched sounders stay in use in many other areas.
Older buildings in San Francisco
San Francisco’s fire department applies NFPA 72’s 2013 sleeping-area rules to existing apartment buildings of three or more units that have a fire alarm system. It accepts existing horns that reach the code’s sound levels even if they don’t sound the low-frequency tone.
Read next
- Next in Smoke and CO alarm soundsThree beeps and a pause (smoke alarm)A smoke or fire alarm sounding for smoke or fire. Three beeps and a pause, repeating, is the temporal-3 pattern, the standard emergency evacuation signal.
- The same shape in Asian flagsFlag of IranIran’s flag has three equal stripes of green, white and red, with the national emblem in red in the centre and the words “Allahu akbar” (“God is greatest”) written 22 times in white along the edges of the green and the red. Iran’s constitution names the colours, the emblem and the motto but gives them no meaning; the art historian Kimia Maleki writes that the 22 marks the day the revolution of 1979 won, the 22nd day of the 11th month of the Iranian calendar.
More smoke and CO alarm sounds
The same shape elsewhere
Browse symbols drawn with letters and numbers or lines.
Sources
- 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
- LFH Series Low Frequency Horn, Data sheet 8830068 Rev B (12/21), Potter Electric Signal Company, Application; Description
- NFPA 72's Low Frequency Sounders Requirement Takes Effect, Casey Laughman, January 2014, FacilitiesNet (Building Operating Management), part 1
- Possible Technologies for Improving the Audibility of Residential Smoke Alarms, CPSC-ES-0505 (September 2005), U.S. Consumer Product Safety Commission, §3 (piezoelectric horn)
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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