Single- and Multi-Station Low-Frequency Audible Signaling

Carson Cook, PE

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Dec 27, 2023

The 2010 edition of NFPA 72® National Fire Alarm and Signaling Code® introduced the requirement for a new low-frequency public mode audible alarm signaling in sleeping areas. This new low-frequency signal required a fire alarm notification appliance to produce a square wave with a fundamental frequency of 520 Hz +/- 10 percent or provide some equivalent awakening ability. This requirement was introduced based upon research that demonstrated that a low-frequency signal provides a superior awakening ability for persons with hearing loss and alcohol-impaired adults when compared to the typical 3000+ Hz audible alarm signal of standard fire alarm audible notification appliances. A similar requirement was also introduced in the same edition of NFPA 72® for single- and multiple-station smoke alarms, however, it only applied to dwelling units designed for persons with mild to severe hearing loss.

As a result of this requirement, fire alarm manufacturers developed low-frequency tone-based notification and sounders for system detectors as well as low-frequency .wav files for systems with one-way digital voice communication. However, since this requirement was introduced in the 2010 edition of NFPA72®, a listed 120VAC smoke alarm that produces a low-frequency alarm has not been developed due to difficulties with the design of the battery backup and due to the fact that only “sleeping rooms and guest rooms for those with hearing loss” were required to comply with the requirement.

More recently, the 2021 edition of the International Building Code (IBC) introduced a new requirement for audible alarms activated by single- and multiple-station smoke alarms in sleeping rooms of Group R-1 occupancies (transient occupancies such as hotels and motels) and Group R-2 occupancies (permanently occupied occupancies such as apartments and condominiums) that are required to be equipped with a fire alarm system per Chapter 9 of the IBC to produce a low frequency (520 Hz) alarm signal. The code section further states that where a sleeping room smoke alarm is unable to produce a low-frequency signal, a listed low-frequency notification appliance or a system smoke detector with an integral 520 Hz sounder shall be provided. Per the expert commentary of the 2021 IBC, the reason for this requirement is that more recent studies have concluded that the low-frequency alarm is six times more effective than the standard 3000+ Hz alarm in waking people over 65, people who are hard of hearing, school-aged children, and alcohol-impaired persons.

Because there are currently no listed 120VAC smoke alarms that produce a low-frequency alarm, this poses a challenge to single- and multiple-station smoke alarm installations in buildings constructed in accordance with the IBC. Until a low-frequency smoke alarm is developed, this requirement will necessitate utilizing the building’s fire alarm system under one of the following methods:

System Smoke Detectors with Low-Frequency Sounder Bases

In lieu of 120VAC smoke alarms, system smoke detection connected to the fire alarm system with integral low-frequency sounders can be provided. All of the major fire alarm manufacturers currently offer low-frequency sounder bases that can be equipped with their addressable smoke detection. Under this design, the sounder bases can be used for notification of a dwelling unit smoke detector and for public mode notification in buildings with tone-based (non-voice) fire alarm systems. This design is typical for hotels/motels and dormitories where the building management or institution maintains the detectors and wants to be aware of detector activation in a given unit.

System Smoke Detectors with Addressable Notification

Another solution is to leverage a fire alarm system with addressable notification (i.e., the ability to actuate individual or select groups of appliances on a given circuit). Under this option, system smoke detectors would be provided without sounder bases and standalone addressable low-frequency tone-based or voice notification appliances would be provided in the unit that would be programmed to actuate either upon a dwelling unit detector or a building fire alarm.

System Smoke Detectors with Dedicated Notification Appliance Circuits

The last option would be to provide system smoke detection without sounders with either dedicated notification appliance circuits for each unit or addressable control modules per dwelling unit off a dedicated dwelling unit notification appliance circuit. Inside the units, low-frequency toned-based notification appliances or voice-based notification appliances with a low-frequency .wav file would be provided. The control module per unit would be programmed to actuate either upon a building fire alarm or activation of detection in a given unit.

With the last two options, standard 120VAC smoke alarms with 3000+ Hz audible signaling could be provided in lieu of system detection and be supervised by the building fire alarm system via addressable monitor modules. The drawback is that activation of a smoke alarm would result in simultaneous activation of the integral high-frequency alarm and the audible notification appliances connected to the building fire alarm system.

All of the above options assume an addressable fire alarm system. Integration with the building’s fire alarm system results in various drawbacks such as the building Owner being notified anytime a detector activates in a unit. While the detector’s activation can be treated as a Supervisory condition as opposed to a Fire Alarm condition, they are still notified regardless. This also exposes the floor’s signaling line circuit to tampering or troubles (e.g., tenants removing detectors from their bases or attempting to disconnect them from the wiring).

Until listed 120VAC smoke alarms with integral low-frequency signals are developed, the new requirement in the 2021 IBC for audible alarms initiated by single- and multiple-station smoke alarms will necessitate the involvement of the building’s fire alarm system. While this requirement will result in a design that will provide a superior waking ability to the occupants, it will drastically change the way fire alarm systems are designed for non-transient occupancies.

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About the author

Carson Cook
Carson is a Senior Fire Protection Engineer with over 10 years of experience in the fire protection engineering industry.