Speech Intelligibility

Speech intelligibility is the degree to which speech can be understood by listeners in a given acoustic environment. It is affected by reverberation time, background noise level, direct-to-reverberant ratio, room geometry, and the presence of sound reinforcement systems. Speech intelligibility is quantified using the Speech Transmission Index (STI) per IEC 60268-16, with values ranging from 0 to 1. An STI of 0.60 or above is generally required for adequate communication in classrooms and meeting rooms. Building regulations specify intelligibility requirements: ANSI S12.60 requires STI > 0.60 for classrooms, and BB93 specifies maximum reverberation times that support intelligibility. Factors that improve intelligibility include reducing RT60, lowering background noise, increasing direct sound levels, and ensuring favorable early reflection patterns. Speech intelligibility is the primary design criterion for classrooms, courtrooms, worship spaces, transportation terminals, and emergency PA systems.