Low Frequency Fire Alarm for the Hard of Hearing
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What is 520 Hz?

Since the end of November of 2013, there has been quite a lot of talk about “low-frequency” and “520 Hz” in audible alarms (or sounders), as states and municipalities begin to adopt NFPA 72, 2010 or 2014 editions, for their building codes. Those editions of NFPA 72 require the installation of 520Hz sounders in sleeping areas (§ Not to mention of course that the 520 Hz alarm signal is scientifically demonstrated to be the most effective emergency signal for awakening everyone, with or without hearing loss.

Please be aware that a manufacturer’s claim of an alerting signal with a 520 Hz dominant frequency is not quite enough. The scientific research that led to the 520 Hz requirement demonstrated that a mixed frequency, with the 520 dominant and harmonics, is what is required for maximally effective awakening power. The odd harmonics make it a somewhat discordant sound instead of a clear musical tone. The mixed frequency aspects of the 520 Hz signal and its harmonics are clearly defined in the current requirements of UL 217 and UL 464.

This is what a compliant 520 Hz signal looks like with its harmonics. The first peak on the left is at 520 Hz, and the other peaks are the odd harmonics at the frequencies and amplitudes that are also required by UL 217 and UL 464.

520 Hz

As you can see, the 520 Hz dominant frequency on the left is only part of what a compliant low frequency signal must have. The Lifetone HLAC has been tested by UL and listed by UL. It is a genuine low frequency 520 Hz sounder listed to the current editions of UL 217 and cUL S531, a claim we can happily make and will remind you of at every opportunity.

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