Speech Transmission Measurement

Speech transmission measurement evaluates the degree to which a room supports clear speech communication, quantified by the Speech Transmission Index (STI) per IEC 60268-16:2020. Full STI measurement uses a modulated noise test signal covering 7 octave bands and 14 modulation frequencies to determine the modulation transfer function. The simplified STIPA method uses a single modulated noise signal designed to sample the same frequency-modulation space with fewer measurements. Measurement requires a standardized loudspeaker source with known directivity placed at the talker position and a measurement microphone at the listener position. Results range from 0 (unintelligible) to 1 (perfect clarity), with minimum requirements of 0.60 for classrooms (ANSI S12.60) and 0.50 for most general spaces (BS 8233). STI measurements account for the combined effects of reverberation, background noise, and the room’s frequency response on speech intelligibility.