What intelligibility score does NFPA 72 require for voice evacuation?

Where intelligibility is measured quantitatively, NFPA 72 uses a Speech Transmission Index (STI) of at least 0.45 — equivalent to a Common Intelligibility Scale (CIS) score of 0.7. Importantly, NFPA 72 does not mandate quantitative measurement: a documented listen test in each acoustically distinguishable space is an acceptable way to assess intelligibility.

What that means in practice:

• Audibility (can the signal be heard above ambient noise) and intelligibility (can spoken words be understood) are separate requirements — a voice or mass notification system must achieve both
• Quantitative testing uses a STIPA meter with a calibrated talk-box source, recording an STI score per measurement location
• Acoustically difficult spaces — parking structures, atria, hard-surfaced corridors — are where intelligibility typically fails while audibility passes
• Mass notification systems (NFPA 72 Chapter 24) carry the same intelligibility expectations, plus verified signal priority with the fire alarm

CertBox includes a voice evacuation / mass notification test report that records the functional tests, the intelligibility method used (quantitative or listen test) and per-location STI results, with a professional PDF for the owner and AHJ.