The Speech Transmission Index (STI) runs from 0 (unintelligible) to 1 (perfect). Classrooms and meeting rooms should reach 0.6 or higher. Reverberation and background noise both lower STI, so absorption and quiet HVAC go together.
Learning center / Acoustics fundamentals
Speech intelligibility and STI
Measuring how clearly speech comes across in a room.
2 minute read · Updated Nov 2026