The speech intelligibility index standard and its relationship to the articulation index, and the speech transmission index

The history of the Speech Intelligibility Index (SII) Standard (ANSI S3.5-1997) and its relationship to the Articulation Index (AI) and the Speech Transmission Index (STI) are reviewed. The SII Standard defines a method for computing a physical measure (SII) that is highly correlated with the intelligibility of speech. The SII is calculated from acoustical measurements of speech and noise. The standard is a revision of the ANSI S3.5-1969, ‘‘American National Standard Methods for the Calculation of the Articulation Index.’’ In that sense the AI and the SII are one and the same thing. In addition, however, SII incorporates the Speech Transmission Index procedures appropriate for many conditions of use where the traditional AI does not apply [H. J. M. Steeneken, and T. Houtgast, ‘‘A physical method for measuring speech-transmission quality.’’ J. Acoust. Soc. Am. 67, 318–326 (1980)]. Other changes of the Standard are due to new data which have been accumulated since 1969 for various parameters and procedures used in the calculations. These include spread of masking, standard speech spectrum level, and relative importance of various frequencies to speech intelligibility. In addition, the SII calculation details have been adapted to computer implementation rather than to manual chart-type computations.