A fire alarm system that detects perfectly and cannot be heard has failed. Audibility is a design calculation about sound levels in each space, through closed doors, above the background noise that space actually has, and it is the requirement most often broken by later changes to a building rather than by the original design.
How audibility is specified
The design standard sets minimum sound levels in occupied areas and a higher level in rooms where people sleep, measured with doors closed and against the normal background noise of the space. A workshop with machinery running needs more than an office. That is why sounder positions are a calculation rather than a matter of putting one on each floor.
What breaks it later
New partitions. A door upgraded to a heavier fire door. Acoustic treatment. A change of use that brings in noisy equipment. Each of these can drop the level in a room below the design figure without anybody touching the alarm system. This is why a fire risk assessment after building work should include a check that the alarm is still audible where people are, rather than only that it still sounds.
Beacons and the deaf occupant
Visual alarm devices, beacons, are used where sound alone cannot be relied on: very noisy areas, areas where hearing protection is worn, and areas used by deaf or hard of hearing people. They are a supplement to sounders in most buildings rather than a replacement. For an individual who is deaf, a vibrating pager or pillow device linked to the system is the normal arrangement, and it belongs in the personal emergency evacuation plan for that person.
Voice alarm system
A voice alarm system replaces or supplements tones with spoken messages, which lets different areas be given different instructions and removes the need to train visitors in what the tones mean. It brings its own design requirements around speech intelligibility, because a message nobody can make out is worse than a tone. Where one is installed, intelligibility is what the commissioning should have measured.
Fire horns, and where a horn beats a sounder
A horn is a sounder with a reflex or exponential flare that concentrates its output into a narrower, more penetrating sound, and the reason to choose one is ambient noise rather than volume for its own sake. In a factory, a plant room, a workshop or a loading yard the alarm has to be clearly distinguishable from machinery, and a horn's output and tone cut through where an electronic sounder at the same rating disappears into the background. Outdoors and in very large open spaces the same applies. What a horn does not solve is the two cases where sound is the wrong signal: where people wear hearing protection or the noise is high enough to need it, and where an occupant is deaf or hard of hearing, and both need beacons or a vibrating device rather than a bigger noise. The design figures to hold onto are the general sound level target in occupied areas, a higher one at a bed head, and a margin above the ambient level rather than an absolute number.
Questions people ask about fire alarm sounder
How loud does a fire alarm have to be?
Loud enough to meet the minimum sound levels in the design standard in every occupied space, measured with doors closed and against that space's background noise, with a higher level where people sleep.
Are beacons a substitute for sounders?
Generally no. They supplement sounders where sound cannot be relied on. Relying on visual alarms alone requires that everybody is always in sight of one, which is rarely true.
How is a deaf employee warned?
Usually a vibrating pager or pillow device linked to the alarm system, recorded in that person's personal emergency evacuation plan along with who assists them and by which route.
When is a fire alarm sounder beacon needed rather than a sounder?
Wherever the sound alone cannot be relied on. The recurring cases are a noisy environment such as a workshop or a plant room, a place where hearing protection is worn, an area used by people with a hearing impairment, and single occupancy rooms such as a hotel bedroom or a WC where a deaf occupant may be alone, which is usually answered by a vibrating pad or a dedicated device rather than a beacon alone. The risk assessment decides it, and the common mistake is fitting beacons everywhere as a general upgrade, which adds cost without answering the specific case that needed them.