An alternative to the computational Speech Intelligibility Index estimates: direct measurement of rectangular...

The need for determining the relative intelligibility of passbands spanning the speech spectrum has been addressed by publications of the American National Standards Institute (ANSI). When the Articulation Index (AI) standard (ANSI, S3.5, 1969, R1986) was developed, available filters confounded passband and slope contributions. The AI procedure and its updated successor, the Speech Intelligibility Index (SII) standard (ANSI, S3.5, 1997, R2007), cancel slope contributions by using intelligibility scores for partially masked highpass and lowpass speech to calculate passband importance values; these values can be converted to passband intelligibility predictions using transfer functions. However, by using very high-order digital filtering, it is now possible to eliminate contributions from filter skirts and produce rectangular passbands. Employing the same commercial recording and the same one-octave passbands published in the SII standard (Table B.3), the present study compares Rectangular Passband Intelligibility (RPI) with SII estimates of intelligibility. The directly measured RPI differs from the computational SII predictions. Advantages resulting from direct measurement are discussed.