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Low-frequency fire alarm sounders are now required in sleeping quarters, as first called out in NFPA 72, 2010 edition. This requirement applies to new construction of hotel rooms, college dorm rooms, or bedrooms in apartments or assisted living facilities.
The low-frequency sounders are 520 Hz, square wave. This frequency was found to be more effective in awakening people with hearing loss. The frequency is also more effective at waking children, who tend to have deeper sleep than adults and can sleep right through high-frequency strobes.
Volume levels were not modified at all by the change in the standard. In addition, the low-frequency sounders are required only in the sleeping quarters; common areas such as hallways or lobbies do not require the low-frequency sounders.
Another application of the 520 Hz, square wave frequency is that is the required tone that precedes voice notifications in a an emergency communication system.
To learn more about the considerations that went into the NFPA 72 low-frequency sounder requirement, click here.