Residential Smoke Detector Signal Requirements

The National Fire Protection Association (NFPA) is a non-profit agency responsible for developing and issuing codes and standards related to fire safety. The standard for residential smoke detector signal requirements is referred to as the National Fire Alarm Code or NFPA 72.  As of July 1996 and per NFPA 72, residential smoke detectors must generate a distinct audible alarm for a minimum amount of time at a defined intensity level and that specific alarm may not be used for any purpose other than alerting individuals of a fire and the subsequent need to evacuate. This specific audible signal is what is known as the Audible Emergency Evacuation Signal as outlined by ANSI S3.41-1990 more commonly referred to as the Temporal Three or T3 pattern.  The figure below provides a visual illustration of the T3 pattern:   

Distinct Audible Alarm Characteristics

In the original recording with the blue background, one complete cycle of the T3 pattern is boxed off in the red box with an exaggerated version of that cycle appearing to the right of the original recording. As illustrated by the three neon green spikes, the T3 signal consists of three successive 1/2 second ON phases with each ON phase followed by a 1/2 second OFF time as represented by the black horizontal lines appearing after each ON phase.  At the completion of the third and final OFF phase, an additional 1 second OFF time occurs prior to the T3 signal cycling from the beginning. One full T3 pattern takes a full 4 seconds to complete (0.5 seconds ON, 0.5 seconds OFF, 0.5 second ON, 0.5 seconds OFF, 0.5 seconds ON, 0.5 seconds OFF plus 1.0 seconds OFF). 

Minimum Amount of Time

As outlined by NFPA 72, once a residential smoke detector is activated, it must be able to emit a T3 signal pattern continuously for no less than 180 seconds.  This is a duration of 3 minutes.

Defined Intensity Level

The intensity level of the T3 alarm must be reliably heard.  In the United States, this translates to an alarm that measures 75 dBA at pillow level. Note, since fire codes require installation of smoke detectors immediately outside of bedrooms and since the distance from this location to the actual pillow level will vary based on the size of the bedroom. As such, the NFPA outlined an additional performance requirement whereby the measured intensity signal emitted by the smoke detector must minimally measure 85 dbA at a distance of 10 feet.  generated by a residential smoke In the original recording with the blue background, one complete cycle of the T3 pattern is boxed off in the red box with an exaggerated version of that cycle appearing to the right of the original recording. As illustrated by the three neon green spikes, the T3 signal consists of three successive 1/2 second ON phases with each ON phase followed by a 1/2 second OFF time as represented by the black horizontal lines appearing after each ON phase.  At the completion of the third and final OFF phase, an additional 1 second OFF time occurs prior to the T3 signal cycling from the beginning. One full T3 pattern takes a full 4 seconds to complete (0.5 seconds ON, 0.5 seconds OFF, 0.5 second ON, 0.5 seconds OFF, 0.5 seconds ON, 0.5 seconds OFF plus 1.0 seconds OFF). 

Missing Requirement of Smoke Detectors Affecting Individuals With Hearing Loss

While the NFPA 72 requires residential smoke detectors to emit a T3 pattern signal capable of continuous emission for at least three (3) minutes at an intensity level of 85 dBA measured at a distance of 10 feet (3 meters) in a laboratory setting, the 1996 standard DOES NOT outline or stipulate any frequency requirements.  Taking into consideration hearing loss demographics and traditional audiometric configurations commonly seen with presbycusis or noise-induced hearing loss, the ability for individuals with hearing loss to effectively hear the smoke detector in a timely fashion is of concern. As outlined in the Residential Fire Facts section, the majority of residential fire fatalities occur between the time stamp of 11:00 pm and 7:00 am when individuals are asleep and therefore, not wearing properly fit hearing instruments. Furthermore, sleep research has shown that awareness thresholds to auditory signals during different stages of sleep are significantly elevated. It has been estimated that an auditory signal may need to be as much as 40 dB louder for it to be detected during sleep compared to detecting the same signal while the individual is alert and awake. For a summary of research addressing the effectiveness of smoke detectors in waking individuals with and without hearing loss, click  Smoke Detector & Hearing Loss Research Summary.