What is Fire Alarm Audibility?

The biggest obstacle to getting occupancy of a completed construction project is getting approval from the fire department.  Of all the items checked by the fire department inspector, the biggest concern and hardest to rectify if not compliant is the level of occupant notification in event of a fire alarm activation.  If occupants can’t hear a fire alarm, you’re not moving in.  Adding last minute notification devices to a space where wall are painted, ceiling grids are in and areas are cleaned is a hit on everyone involved’s reputation.  It’s also completely avoidable with a little front end attention from a reputable fire alarm company.

Many design professionals are somewhat familiar with NFPA 72 notification spacing requirements, but may not be familiar with applying output levels to a particular space.  In this article, we’ll address the sound level requirements of NFPA 72 known as audibility.  Audibility is the ability of an occupant to distinguish the fire alarm signal above ambient sound levels anywhere in the building.  Anywhere means every occupy-able space in the building, including the furthest corner of any office with its door closed.  Now everyone’s hearing level is different, so how can you establish a baseline of acceptable sound level that achieves audibility?  The standard published in NFPA 72 is 15 db over ambient sound levels.  Ambient sound levels can either be prescriptively applied by occupancy type through a table provided in NFPA 72 (Table A.18.4.3), or a performance level can be established by measuring the sound level of a particular space for a 60 second duration.  The highest recorded decibel reading of that measurement must be exceeded by the fire alarm system audible signal by 5 decibels (NFPA 18.4.3.1).  To clarify, ambient is not worst case scenario (TV blasting with the shower on), although, at the fire department inspector’s discretion, it may involve having the TV on.  If there is any question as to what ambient conditions might be, identifying (and getting a hard copy) of what the fire department considers ambient is crucial to passing high profile occupancy tests like hotels.  Identifying the required decibel levels achieved by the fire alarm system audible devices in any given space as established by the fire department is the key to gaining occupancy after a single acceptance test.

Thanks for reading and remember that Affiliated Fire Systems has been helping design professionals, electrical contractors, building owners and fire departments ensure fire alarm systems operate as intended since 1983.  Feel free to contact us or consult our data sheet page if you have any questions regarding your fire alarm system needs.

Gene Rowe