The Speech Transmission Index (STI) has become the internationally accepted method of testing and assessing the potential intelligibility of sound systems. The technique is standardized in IEC 60268-16, however, it is not a flawless technique. The paper discusses a number of common mechanisms that can affect the accuracy of STI measurements and predictions. In particular it is shown that RaSTI is a poor predictor of STI in many sound system applications and that the standard speech spectrum assumed by STI often does not replicate the speech spectrum of real announcements and is not in good agreement with other speech spectrum studies. The effects on STI measurements of common signal processing techniques such as equalization, compression, and AGC are also demonstrated and the implications discussed. The simplified STI derivative STIPA is shown to be a more robust method of assessing sound systems than RaSTI and when applied as a direct measurement method can have significant advantages over Impulse Response-based STI measurement techniques.
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