Speech transmission index

Speech Transmission Index (STI) is a measure of speech transmission quality. The absolute measurement of speech intelligibility is a complex science. The STI measures some physical characteristics of a transmission channel (a room, electro-acoustic equipment, telephone line, etc.), and expresses the ability of the channel to carry across the characteristics of a speech signal. STI is a well-established objective measurement predictor of how the characteristics of the transmission channel affect speech intelligibility.

The influence that a transmission channel has on speech intelligibility is dependent on:

• the speech level
• frequency response of the channel
• non-linear distortions
• background noise level
• quality of the sound reproduction equipment
• echos (reflections with delay > 100ms)
• the reverberation time
• psychoacoustic effects (masking effects)