Voice Intelligibility Reference

What it takes for a fire alarm voice system to be intelligible, not just audible. NFPA 72 requires intelligible messages and has you map the building into acoustically distinguishable spaces (ADS); Annex D gives the STI/CIS scores a design should hit. The targets are two criteria at once — a per-point floor and a higher average.

Intelligibility targets (Annex D)

NFPA 72 Annex D
The STI and CIS scores an acoustically distinguishable space should meet: a per-location floor at ≥ 90% of points, and a higher average across the space. Annex D is informational guidance unless the AHJ or a project spec invokes it.
MetricPer location (≥ 90% of points)ADS average
STI (Speech Transmission Index)0.450.50
CIS (Common Intelligibility Scale)0.650.70
Above roughly 90 dBA of ambient noise, meeting these may be impossible and alternative notification (such as visible signage) is used instead. Measurements are taken at about 5 ft above the floor.

Audible is not the same as intelligible

A horn only has to be heard. A voice system has to be understood — a message loud enough to hear but garbled by echo tells no one where to go. So intelligibility is measured separately from loudness, using the Speech Transmission Index (STI), a 0-to-1 score of how much a space degrades speech, taken with an STIPA meter. CIS is the same idea mapped onto a common scale. The two-part target — a 0.45 floor at most points and a 0.50 average — lets a design carry a few weak spots as long as the space as a whole is clear.

The ADS concept keeps this practical. Rather than test everywhere, the designer divides the building into acoustically distinct areas and decides which ones genuinely need intelligible voice — a mechanical room at 90+ dBA may be exempt and handled with visible signage instead. Getting there starts with even speaker coverage; see the speaker tap & dB chart.

Common questions

What is the intelligibility requirement for a fire alarm voice system?

NFPA 72 requires that emergency voice messages be intelligible — understandable, not merely loud enough to hear. That requirement is in the body of the code (Chapters 18 and 24). The numeric targets in Annex D are guidance: within each space that requires intelligibility, at least 90% of measurement points should reach an STI of 0.45 (0.65 on the CIS scale), and the space's average should reach an STI of 0.50 (0.70 CIS).

What is an acoustically distinguishable space (ADS)?

An ADS is an area of the building treated as a unit for intelligibility — a space defined by its enclosure or by its acoustic, environmental, or use characteristics (reverberation, ambient noise, how it is occupied). The designer maps the building into ADSs and labels each one as either requiring or not requiring voice intelligibility, so effort goes to the spaces that matter and not to places where it is impractical.

Is STI 0.45 or 0.50 the requirement?

Both — they are two criteria applied together, not a contradiction. STI 0.45 (CIS 0.65) is the floor that at least 90% of measurement points must clear, allowing up to 10% weaker spots. STI 0.50 (CIS 0.70) is the average the whole space must meet. A design passes only if it satisfies both.

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