Audibility
Audibility is the requirement that the fire alarm signal actually be heard where people are, and it is a measured performance test rather than a device specification. Ontario Building Code Sentence 3.2.4.18.(7) sets the floor area minimum at not less than 10 dBA above the ambient noise level and not less than 65 dBA, with any intervening doors closed. A sleeping room in a building of residential occupancy is higher, at not less than 75 dBA under Sentence 3.2.4.18.(5). The maximum is unsettled: the free January 2025 Compendium prints Sentence 3.2.4.18.(4) with clause (a) capping the level at 110 dBA measured 3 m from the device, while clause (b) grants relief against a 100 dBA ceiling that clause (a) never states, so confirm the maximum against the amendment document in force.
Last updated: September 2026
What the word actually covers
Audibility is a performance requirement, not a piece of hardware. It asks one question: at the point where a person is standing or sleeping, with the building in its normal state and the doors shut, is the alarm signal loud enough to be noticed. Everything else, the horn you selected, the circuit it sits on, the tap setting on a speaker, is only a means to that end.
Two things follow. Audibility is measured, not drawn. A layout showing a device every 20 m is a proposal, not compliance. And the ambient noise level of each space is an input to the answer, which means somebody has to know what the room sounds like with its equipment running.
Which provision drives it
In Ontario the governing article is 3.2.4.18 of Division B of the 2024 Building Code, O. Reg. 163/24, which adopts NBC 2020 as amended by the Ontario Amendment Document.
| Where | Requirement | Sentence |
|---|---|---|
| Within a floor area | Not less than 10 dBA above ambient and not less than 65 dBA, intervening doors closed | 3.2.4.18.(7) |
| Sleeping room, residential occupancy | Not less than 75 dBA, intervening doors closed | 3.2.4.18.(5) |
| Sleeping room, residential or care occupancy | Must emit a low frequency signal, with no frequency stated | 3.2.4.18.(6) |
| Maximum level | Printed text is internally inconsistent, see below | 3.2.4.18.(4) |
The article number moved. Under the revoked O. Reg. 332/12 audibility sat at 3.2.4.20. Ontario renumbered Subsection 3.2.4 to line up with NBC 2020, and the shift is not a uniform offset, so do not subtract two from every other article you remember. An older document citing 3.2.4.20 for audibility is not wrong, it is just old.
The maximum is genuinely unsettled
The free Compendium prints clause (a) of Sentence (4) as not more than 110 dBA measured at a distance of 3 m from the device, then prints clause (b) as permission to exceed 100 dBA provided the level measured 2 000 mm above floor level stays at or below 100 dBA. The relief in clause (b) has no 100 dBA ceiling in clause (a) to attach to. The revoked O. Reg. 332/12 wording carried 100 dBA in clause (a), which made the pair read coherently. NBC 2020 has neither clause and simply caps the signal at 110 dBA in any normally occupied area.
We are not going to pick one of those readings for you. If a maximum figure matters on your project, request the amendment document actually in force from the ministry and confirm with your authority having jurisdiction. The full working is set out in fire alarm audibility in Ontario.
Two tests, not one
The most common field error is treating 65 dBA as the whole requirement. It is a floor, and the 10 dBA above ambient test runs alongside it. In a quiet office corridor the 65 dBA figure governs. In a mechanical room sitting at 72 dBA with the equipment running, 65 dBA is a failure and you need 82.
The second most common error is measuring with the door open. The measurement condition is written into the sentence itself, and a reading taken in an open doorway is not the reading the Code asks for. That one word, closed, is what pushes sounders out of stairwells, into corridors, and then into suites.
Where the Code stops
The Building Code gives you levels and the conditions they are measured under. It does not give you meter type, weighting, position, dwell time, device spacing, or how much level you lose through a closed door. All of that sits in CAN/ULC-S524, which is copyrighted and sold. We do not reproduce it here, and we do not fill the gap with the American figures, because NFPA 72 is a different standard inside a different code system.
Audibility is proven on site during verification, not signed off from a calculation. The neighbouring entries carry the special cases: the sleeping room requirement for the 75 dBA rule, and the strobe for the situations where sound alone is not accepted at all.
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