CIS: Common Intelligibility Scale

Last updated: September 2026

What CIS is

The Common Intelligibility Scale is a way of putting a single number on a question that is otherwise argued about: can people actually understand what the loudspeakers are saying. It runs from 0 to 1, higher is better, and it is produced from an acoustic measurement taken in the finished space rather than from a design calculation.

It is a measurement scale, not a rule. Like any scale, it only becomes an obligation when some document adopts a threshold and makes it binding. The whole of the confusion around this term in Canada comes from people assuming that document is the Building Code.

What Ontario's Building Code actually requires

It requires intelligibility. It does not quantify it.

Sentence 3.2.4.22.(1)(b) and Sentence 3.2.4.23.(2)(b) both require that voice messages be audible and intelligible in all parts of the building. That is the whole performance standard.

Appendix note A-3.2.4.22.(1)(b) in Volume 2 of the Compendium explains what the drafters meant. Intelligible means a person with average hearing and cognitive abilities can understand the transmitted messages. The note then says something more useful than a number: there is no absolute measure that will predetermine the effect of loudspeakers, and it may be necessary, once the building is furnished and occupied, to add loudspeakers to improve message quality. It refers to ISO 7731 as suggesting a level at least 15 dBA above ambient.

That is a qualitative standard assessed in a finished building, backed by an appendix note that openly says there is no absolute measure. It is close to the opposite of a CIS threshold.

There is no Common Intelligibility Scale value anywhere in Ontario's Building Code. Both Compendium volumes, 2 261 pages, were searched for "Common Intelligibility" and for "CIS". Zero hits.

Where 0.70 comes from

You will hear 0.70 quoted on Canadian projects as though it were the code number. It is not.

The figure is associated with American emergency communication guidance under NFPA 72, which is a different standard inside a different code system and is never the basis of a requirement in Canada. See NFPA 72 versus CAN/ULC-S524. It may also appear in CAN/ULC-S524 or, very commonly, in the project specification, which is where most intelligibility targets on Canadian jobs actually originate.

We cannot tell you what S524 says about it, because S524 is copyrighted and sold and we do not reproduce or guess at paywalled standard text.

What this means when you write a report

Three practical rules follow.

Do not write "code requires 0.70 CIS" in a deficiency report or a design brief on an Ontario project. It is wrong, and it is the kind of wrong that a reviewer who has read Article 3.2.4.22 will catch.

Do find out where the number on your job came from. If the specification calls for a CIS value, that is a contractual obligation and it is enforceable against you. It is just not a Building Code obligation, and the distinction matters when somebody asks who pays for the extra speakers.

Expect the assessment late. Because the appendix contemplates adding loudspeakers once the building is furnished and occupied, an intelligibility problem tends to surface at the end of a project, after the ceilings are in and the furniture has arrived. Budget and schedule for that rather than treating the speaker layout as settled at rough-in.

What we could not verify

We could not open a free source that sets out the mathematics converting an acoustic measurement into a CIS value, because the underlying acoustics standards are themselves sold, so we do not print the formula or a conversion from any other index. We could not confirm whether CAN/ULC-S524 contains a numerical intelligibility threshold. And we could not identify any Canadian building code, federal or provincial, that adopts a CIS number.

Related: the voice communication system entry for when one is required at all, and fire alarm audibility in Ontario for the levels that apply to the speakers regardless.

---

Fire Alarm Academy provides educational content only. It does not confer any certification, licence or credential, and it is not affiliated with, endorsed by, or accredited by the Canadian Fire Alarm Association, the Electrical Contractors Association of Ontario, ULC Standards, UL Standards and Engagement, CSA Group, the Electrical Safety Authority, any authority having jurisdiction, or any manufacturer. Always work from the edition of any standard adopted by your authority having jurisdiction.