American Alarms Blog

Voice Evacuation Systems and Square Wave Technology

by Adam Jacobs 1/19/2022

Voice evacuation systems have been around for years, but more types of occupancies are now required to have them.  Voice evacuation systems, since tones are generated by speakers, meet the 520 Hz requirement in NFPA 72 2010 and 2013.

The reason for the change in the code is research performed by Bruck and Thomas in Australia.* In over a decade of research, they found that the 520 Hz square wave signal was the most effective in waking all individuals. With children and young adults, the low-frequency signal is 6 to 10 times more effective than the current high-frequency signal. In adults with hearing loss, it was found to be more than seven times as effective as the standard high-frequency (~3,000 Hz) tone used in most audible appliances today.

Attached is a basic explanation and guide for voice evacuations systems.


Voice Evacuation Manual.pdf (1,004.35 kb)

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What is "intelligibility"?

by Adam Jacobs 1/19/2022

Intelligibility is a word that most people have never used.  Until a few years ago, I'd never heard the word either.  This is a measurement of how "understandable" a fire alarm voice evacuation signal is to the human ear.  To perform a test of intelligibility, we play a special recording into the voice microphone and then measure the output of the speakers on the system.  Unlike a decibel test, we are not measuring the sound level.  We are measuring how well that message can be heard by the test meter.  We test and record our readings at multiple locations in the building to ensure our recorded message can be understood throughout the space.  After we've done all this, and received acceptable readings throughout, we can feel confident that the fire department will be able to use that system to get people out of the building if needed.  This type of system is normally required in high-rise buildings over 7 stories and in places of assembly.  Just another part of the science of life safety.  For more info, see the Society of Fire Protection Engineers website

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