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Rude Awakening: Low-Frequency Sounders

For years, there has been concern about the waking ability of fire alarm audible notification appliances. In general, they have gone through numerous changes since the 1980s. Back then, it was common to use bells to signal an alarm. As technology changed, it was found that the bells emitted too much electrical noise back into the system and it caused problems with transistors and integrated circuits.

Next came mechanical horns. They were better than bells, but as operating voltages in panels lowered, they also created problems due to electrical noise in the system. Next came electronic horns. These worked much better, but to reduce current draw, the frequencies of the horns were higher. They did not penetrate walls and doors as well as the older bells and mechanical horns, so more appliances had to be installed.

One problem with the higher frequency is that is the range many aging individuals first start losing the ability to hear. Studies have been done on alerting the elderly with hearing loss as well as young children. Results of some studies have shown that voice messages (especially a parent’s voice for small children) can be more effective than horns for waking individuals. Also, lower frequencies seemed to be better heard by those with hearing loss.

About 10 or 11 years ago, proposals began to be submitted for the 2010 NFPA 72 to require low frequency sounders in areas intended to awaken occupants. At first, the suggested range of these sounders was between 100 and 700 hertz (Hz). There was debate about which chapter should be responsible for the requirements since, technically, the activation of the notification appliances is more of a Protected Premises chapter scope than the Notification Appliances chapter scope.

The Technical Correlating Committee ordered these proposals to be placed on hold to allow a task group made up of both notification appliances chapter and the household chapter to work on solutions, including an effective date. Most of the proposals were based on providing the lower frequency alarms for those with hearing loss.

The requirements for low-frequency sounders using a 520-Hz square wave signal were first added to NFPA 72 in the 2013 edition of the code, with an effective date of Jan. 1, 2014. This frequency was selected based on results of a 2007 NFPA Fire Protection Research Foundation study, “Waking effectiveness of alarms (auditory, visual, tactile) for adults who are hard of hearing,” by Bruck et al., to be the most effective signal (92 percent effective at 75 A-weighted decibels [dBA] in awakening people with mild to moderately severe hearing loss) in awakening most people with partial hearing loss.”

Sounds straightforward, right?

Unfortunately, the requirements differ significantly in Chapter 18, Notification Appliances (system devices), and Chapter 29, Single- and Multiple-Station Alarms and Household Fire Alarm Systems (stand-alone smoke alarms). Chapter 29’s requirements for low frequency smoke alarm sounders only requires them to be installed in sleeping rooms for those with mild to severe hearing loss where required by laws, codes or standards for people with hearing loss, or where provided voluntarily by those with hearing loss. To the best of my knowledge, no manufacturers today produce any smoke alarms with low frequency sounders, so the only way to meet this is to use system smoke detectors with sounder bases (wired or wireless). If you use detectors with sounder bases, you have to meet the requirements for Chapter 18.

Chapter 18 requires low frequency sounders for sleeping areas to awaken occupants. This means they must be installed in all sleeping rooms, but not in corridors outside of sleeping rooms. So, in a hotel, you will need to install low frequency sounders in all sleeping rooms that would activate when the fire alarm system goes into alarm. The low frequency smoke alarms would only need to be installed in the accessible rooms. This creates a different sounding signals depending on whether the smoke alarm activates or the building fire alarm system activates. As I have had it explained to me, the reason Chapter 18 required them in all sleeping rooms was because hearing loss is not apparent, and those occupants could end up in any of the rooms, so it was decided to err on the side of safety and put them in all sleeping rooms rather than just those designated as accessible.

There is still a lot of discussion going on at the committee level regarding low frequency sounders. It will be interesting where this goes from here. If there are any significant changes made during this current cycle, I will update you in a future article.

About the Author
Tom Hammerberg

Thomas P. Hammerberg

Life Safety Columnist

Thomas P. Hammerberg, SET, CFPS is an independent fire alarm presenter and consultant in The Villages, Fla. He can be reached at TomHammerberg@gmail.com.

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