Speech Transmission Index (STI): Measurement and Prediction Uncertainty
Peter A. Mapp · 2020
The speech transmission index (STI) is the most widely used method employed for rating the potential speech intelligibility of sound and communication systems and is used internationally. The technique is detailed and standardised in international standard International Electrotechnical Commission (IEC) 60268–16 (BS EN 60268–16). The potential areas of uncertainty associated with STI measurements and predictions include: limitations imposed by the method itself (e.g. monaural measurement/binaural hearing); and uncertainties naturally inherent within the technique itself (use of a pseudorandom noise–based measurement signal). STI employs a rating scale of 0 to 1.0, where 0 relates to completely unintelligible speech and 1.0 to perfect intelligibility speech. Apart from the potential binaural effect of reducing reflection and reverberation reductions of intelligibility, echoes and strong, discrete late-arriving reflections can also lead directly to an erroneous assessment when using STI. STI accounts for the loss of intelligibility caused by a late arrival or echo.