Speech Intelligibility Index (SII)

The speech intelligibility index (SII) was developed to predict the intelligibility of the speech signal by weighting the importance of different frequency regions of audibility for a given speech test. To obtain the SII, the frequency spectrum between 100 and 9500 Hertz (Hz) is divided into speech bands, either by octaves, 1/3 octaves, or critical bands. One must calculate the product of the audibility function and the frequency band importance function for each speech band and [Page 1770]add them together. The SII is represented by the following equation:

$$SII=\sum _{i=1}^n{I}_i{A}_i$$

where n is the number of frequency bands, and Ii are the values of the band importance function, and Ai are the values of the band audibility function for each frequency band i. The SII ranges from 0 ...