STI (Speech Transmission Index)

The Speech Transmission Index (STI) is the primary objective measure of speech intelligibility in a room, quantifying how faithfully the temporal modulation patterns of speech are preserved from talker to listener. Defined in IEC 60268-16:2020, STI ranges from 0 (completely unintelligible) to 1 (perfect transmission). The calculation is based on the modulation transfer function (MTF), which measures the reduction of 14 modulation frequencies across 7 octave bands (125–8000 Hz) due to reverberation and noise. STI accounts for both reverberation time and background noise effects simultaneously. Qualification ratings are: <0.30 bad, 0.30–0.45 poor, 0.45–0.60 fair, 0.60–0.75 good, >0.75 excellent. Minimum requirements include 0.50 for general rooms (BS 8233), 0.60 for classrooms (ANSI S12.60), and 0.50 for emergency voice alarm systems (BS 5839-8). STI is the gold standard metric for speech-critical space design.