A single modulation-based index predicts speech intelligibility across most forms of degradation.

Evidence

Another objective measure is the speech transmission index (STI) (IEC, 2003; Houtgast and Steeneken, 1985), which uses a modulation transfer function approach to measure the effect of a given situation on speech intelligibility. It works for most forms of speech degradation and shows good correlation with subjective tests (Steeneken and Houtgast, 1994). It also has the advantage of being relatively easy to calculate, and so can be used in simulations to predict likely improvements in performance. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2558

Categorization

• DEFINE: Defines STI as a modulation transfer function measure of how an environment degrades speech intelligibility.
• XREF: Connects to earlier chapters’ objective-versus-subjective measurement distinction, and to intelligibility indices used in room acoustics.