Low-frequency signal

Ontario Building Code Sentence 3.2.4.18.(6) requires audible signal devices in sleeping rooms in a building of residential or care occupancy to emit a low frequency signal. That is the entire requirement: the Code states no frequency, no waveform and no tolerance. The only figure anywhere near it is a non-mandatory Appendix A note saying devices in the range of 470 Hz to 570 Hz have been shown to be more effective in waking people. The 520 Hz square wave quoted throughout the trade comes from American guidance and from manufacturer documentation, not from Ontario law, and the requirement is new in the 2024 Code with no counterpart in the revoked O. Reg. 332/12.

Last updated: September 2026

What the Code actually requires

Sentence 3.2.4.18.(6) of Division B of Ontario's 2024 Building Code says that audible signal devices in sleeping rooms in a building of residential or care occupancy shall emit a low frequency signal.

That is the whole provision. Read it again and notice what is absent. There is no frequency. There is no waveform. There is no sound pressure level attached to it, no tolerance band, and no reference to a listing standard inside the sentence itself.

This is unusual for a code requirement and it is the reason this term causes so much confusion. The obligation is real and it is enforceable, but the Building Code does not tell you what hardware satisfies it.

The only number Ontario prints, and it is not binding

Volume 2 of the 2024 Building Code Compendium carries the Appendix A explanatory note, A-3.2.4.18.(6), which says that audible signal devices emitting a low frequency signal in the range of 470 Hz to 570 Hz have been shown to be more effective in waking people.

Appendix A is explanatory material. It is published with the Code and it tells you what the drafters had in mind, but it is not the requirement. Quoting it as though it were the rule is a mistake, and so is ignoring it, because on a job where nobody can produce a better basis, a device inside that band is the defensible choice.

Where 520 Hz comes from, and why we will not print it as the Ontario rule

520 Hz appears nowhere in Ontario's Building Code. It is the figure used in American guidance under NFPA 72, and it happens to sit at the centre of the 470 Hz to 570 Hz band in Ontario's appendix note, which is why the two get conflated. NFPA 72 is a different standard operating inside a different code system, and it is not the basis of a requirement in Canada. See NFPA 72 versus CAN/ULC-S524.

Canadian manufacturer documentation does use the figure. Mircom's published page on the Ontario Building Code change describes the requirement as a 520 Hz square wave tone and says it became mandatory for projects submitted after 31 March 2025, the Code itself having come into force on 1 January 2025. We fetched and read that page. Two things about it are worth knowing: it is manufacturer documentation rather than a code source, and it cites no Building Code sentence number of its own.

So if 520 Hz is binding on your project, it is binding because a specification, a product listing or your authority having jurisdiction says so. It is not binding because the Building Code says so, because the Building Code does not say it.

What it means on a job

A conventional horn will not do. A standard fire alarm horn produces a much higher pitched tone, so in-suite sounders in residential and care buildings become a device selection question at the point of ordering, not a substitution you can make on site.

Watch the scope mismatch. Sentence (6) covers residential or care occupancy. The 75 dBA rule for sleeping rooms in Sentence (5) covers residential only in the printed Ontario text. Same article, two different scopes, and the sleeping room requirement entry sets that out.

This is genuinely new. The phrase does not appear in the revoked O. Reg. 332/12. If you learned in-suite audibility on the old code, this is a new obligation rather than a renumbered old one.

What we could not verify

We could not establish from a free Canadian source what the device listing standard requires of a low frequency sounder, because that detail sits inside the paywalled CAN/ULC series and we do not reproduce or guess at it. We also could not establish whether, or when, the requirement reaches existing buildings, which is a Fire Code question rather than a Building Code one: see Building Code versus Fire Code. Confirm both with your AHJ, and read the levels in fire alarm audibility in Ontario.

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