RE: NFPA 72 A.188.8.131.52(2)
I am unaware of any available low frequency smoke alarms.
There are some accessory devices that produce low frequency tones (along with vibratory) etc. For instance, SafeAwake (www.safeawake.com). Full disclosure, SafeAwake is a sister company of my employer, CSE.
Combustion Science & Engineering, Inc.
NFPA made the mistake of allowing signals to be used on the basis of DB levels only rather than adopt the Slow Whoop multiple frequency. The industry had to add additional sounders , boosters to make signals audible as the high frequency sounders could not penetrate wall or be heard by older people, Dr Dorothy Bruck from New Zeeland did a study and found that the lower frequency sounds were more effective in waking people. Acoustical experts all new that human response and wall penetration was always better at lower frequencies.
However the advocates of slow whoop realized that some frequencies could be masked in a building . That is :a frequency that is a regular sound in a building would be automatically shut off by the human brain and not heard. This required the use of Slow Whoop ascending thru multiple frequencies where several would be different and distinquishable .
Because NFPA did not recognize the importance of frequency the audiable signal manufacturers produced the lowest cost devices with the highest DB rating. Unfortunately these devices were at a high frequency that was not audible to many older adults and would not penetrate walls. The lower 520 HZ was found to be the most effective in waking people in Dororothy Bruch's studies and was adopted by NFPA.
IF you want to alert people particulrly those that are sleeping 520 HZ has been proven most effective.In a factory enviroment or one with high ambient noise and multiple frequencies the Slow Whoop Alerting remains the best.
The cadense is of little importance unless it is used for coded signalling for first respionders which has been largely forgotten in recent years.
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