Low frequency sounders are used to awaken people during a fire alarm event instead of a typical fire alarm horn. Studies have shown the lower sound frequency of 520 hz is more effective at waking up sleeping or inebriated persons so they’ve been required in sleeping areas (i.e. hotel rooms, on call rooms, etc.) in lieu of the standard 3.1 Khz horn since the 2010 edition of NFPA 72.
It seems straight-forward that low frequency sounders would be installed in all designated sleeping areas. However, as usual, it’s not straight-forward. Most sleeping areas are found in a residential type occupancies. Dwelling units have their own fire alarm notification requirements found in Chapter 29 of NFPA 72. General alarm notification requirements for publicly accessible spaces are found in Chapter 18 of NFPA 72. This is where the confusion comes in. Where do we apply public space notification requirements in apartment/condominium buildings that employ general alarm notification for sprinkler or manual station activation?
Let’s first make sure we’re on the same page by defining the areas where sleep regularly takes place. A ‘dwelling’ is a one or two unit building used for living purposes (a house or townhouse). A ‘dwelling unit’ is a single unit within a building providing complete, independent living facilities for one or more persons (an apartment or condo). A ‘sleeping unit’ unit is a single unit providing permanent provisions for sleep and has sanitation or kitchen facilities, but not both (hotel rooms or single room occupancies). Finally, a ‘sleeping area’ is a space within a dwelling or dwelling unit where sleep can reasonably be expected to occur (a bedroom or a living room with a couch).
Low frequency signaling is required in spaces when the intent of notification is to wake the occupants up. This is applied in sleeping units since that is the primary function of the unit. However, it is not the intent of general alarm signaling to wake people up across the board. Per NFPA 72 Chapter 18, the intent of general alarm is to achieve audibility of the signal in all occupiable spaces. Without getting too technical, audibility for public mode signals is achieved at 15 decibels (dB) over average ambient sound levels, not best or worst case levels. The prescriptive level of ambient sound in a residential setting is provided in NFPA 72 Table A18.4.4 as 35 dB, so the general alarm signal for an apartment building is supposed to measure at least 50 dB in all occupiable spaces, including dwelling units. Typical general alarm horns are in the public space of an apartment building (corridors, laundry room, etc.) & has a typical sound level of 95 dB measured at 10′ from the horn. If the general alarm signal is less than 50 dB at any point within a dwelling unit, then a general alarm horn is added. But what kind? Low frequency or a typical fire alarm horn? Our next blog will answer this & go over how to deploy low frequency signaling.
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Gene Rowe, SET
NICET IV, Fire Alarm Systems
Affiliated Fire Systems, Inc.
About Gene Rowe
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