Voice Evacuation Intelligibility Helper
A voice evacuation system that is loud but unintelligible fails its purpose and, increasingly, its acceptance test. This tool covers the three questions a designer and an inspector ask: do the measured STI values in each space pass NFPA 72-2022 Annex D, will this room's reverberation and background noise allow intelligible speech at all, and how many ceiling speakers at what tap does it take to get there.
1. Do the measured STI values pass?
NFPA 72-2022 Β§18.4.11 requires voice communications to be intelligible, and Chapter 24 applies it to each acoustically distinguishable space (ADS) where intelligibility is required. Annex D.2.4.1 gives the pass criteria: within an ADS, at least 90 % of the measurement locations have an STI of 0.45 (CIS 0.65) or better, and the average of all locations is at least STI 0.50 (CIS 0.70).
Intelligibility check
- ADS boundaries matter: an ADS is a space with acoustics (size, finishes, noise) distinct from its neighbors β a corridor, an open office, and a stair are separate spaces even without walls between them. Do not average a failing room into a passing floor.
- Where intelligibility is required is set by the designer and AHJ for each ADS; some spaces (mechanical rooms, private restrooms) are commonly designated as not requiring intelligibility while still requiring audibility.
- Stricter local thresholds exist. Some jurisdictions amend the criteria upward (for example, 0.50 at every location and 0.55 average). Check the adopted amendments.
Show the math
Code cited: NFPA 72-2022 Β§18.4.11, Chapter 24, Annex D.2.4.1
2. Will this room allow intelligible speech? (planning estimate)
Room acoustics estimate
Show the math
Code cited: NFPA 72-2022 Β§18.4.4.1 (audibility), Annex D.2.4.1 (STI targets). Reverberation and STI estimate: Sabine equation and simplified STI model (acoustics, not code).
3. How many ceiling speakers, at what tap? (planning estimate)
Speaker layout estimate
- Carry the edge SPL into Panel 2 as the speech level to estimate STI with this room's reverberation.
- Tap down, add speakers: for intelligibility, more speakers at lower taps beat fewer speakers driven hard β direct sound rises relative to the reverberant field.
- 110 dBA maximum (Β§18.4): the combined level at the minimum hearing distance must not exceed 110 dBA β check under a speaker on a high tap in a low room.
- Amplifier headroom and battery: speaker watts Γ count sets the amplifier size and feeds the battery calculation.
Show the math
Code cited: NFPA 72-2022 Β§18.4.4.1, Β§18.4, Β§18.4.11. Coverage geometry and sensitivity equation are acoustics, not code.
Related
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