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Low Frequency Requirements: When, Where, and Why?

Did you know that, effective January 1, 2014, a low frequency audible fire alarm signal is required in all sleeping areas of newly constructed hotel guest rooms and dormitory sleeping rooms?

That’s right: Section 18.4.5.3 in the 2010 and 2013 editions of NFPA 72 requires a frequency signal with a fundamental frequency of 520 Hz to awaken people in occupancies with a protected premises fire alarm system. Even though this new requirement was published in 2009, a lot of confusion remains since the implementation date isn’t until January.

The intent of the new requirement is to improve waking effectiveness, because smoke alarms and fire alarm systems are most valuable when occupants are asleep. This observation is illustrated in a 2010 U.S. Fire Administration study, which reports that 50% of residential fire fatalities occur between 10 PM and 6 AM. And according to a 2008 Dorothy Bruck study, most unimpaired adults wake up quickly to the so-called standard audible signal, even at levels well below 75 dBA. The majority of fire alarm horns will signal with a frequency range between 2 KHz and 4 KHz. Also, integral sounders in nearly all smoke alarms produce a 3 KHz audible alarm signal.

However, how effective is that standard signal at waking up high-risk population segments such as school-age children, the elderly, and the hearing impaired? A U.S. Fire Administration study revealed that 13% of residential fire fatality victims are less than 10 years old and it’s suspected that over 27% of civilian fatalities in residential building fires are linked to alcohol, drug, or chemical influence. Additionally, more than 34.5 million people in the United States are hard of hearing.

In 2006, the NFPA petitioned two research projects from the Fire Protection Research Foundation (FPRF) to study audible fire alarm signal effectiveness in high-risk groups: Waking Effectiveness of Alarms for Adults Who Are Hard of Hearing, and Waking Effectiveness of Alarms for the Alcohol Impaired. The studies reached the following conclusions:

  • The low frequency, 520 Hz signal is the most effective. It woke up 92% of hard of hearing participants when presented at or below 75 dBA for 30 seconds.

  • In comparison, the standard 3 KHz signal woke up 56% at or below 75 dBA.

  • The low frequency signal is superior to bed and pillow shakers and strobe lights.

Read more about low frequency sounder requirements, including sleeping areas affected by the updated code, in Life Safety magazine by clicking the link:  http://lifesafetymagazine.com/2013/11/low-frequency-requirements-when-where-and-why/

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