The Speech Intelligibility Index (SII)  procedure has been used for many years to determine the intelligibility of passbands heard singly and in combination. The SII is a complex multi-step procedure that initially cancels contributions from filter slopes by combining intelligibility judgments of partially masked high-pass and low-pass speech having a series of cut-off frequencies. This data is used to calculate the band importance values, which then can be used to produce band intelligibility estimates. Studebaker and Sherbecoe (1991)  employed a commercially available broadband recording of W-22 word lists to produce band importance estimates that are listed in Table B.3 of the SII. Recently, J.M. Kates (2013)  used the data reported by Studebaker and Sherbecoe to produce an improved method used to determine SII importance values and intelligibility estimates shown in his Figures 1 and 3.
But rather than trying to improve the SII procedure, Warren, Bashford, and Lenz (2011)  proposed a direct measurement alternative: The Rectangular Passband Intelligibility (RPI) procedure. We employed the same copy of the commercially available recording of W-22 lists employed by Studebaker and Sherbecoe. Instead of generating high-pass and low-pass speech, we used high-order FIR filtering to produce effectively vertical slopes for passbands at different center frequencies. Listeners were then able to use these rectangular passbands to obtain direct intelligibility measurements of the same passbands heard singly and in the combinations reported using the SII procedure. The relative intelligibilities obtained were in general similar using the two different procedures as can be seen in the Warren et al. Figures 5 and 6. Also, the RPI procedure is not limited to word lists: It had been used previously with sentences (Warren, Bashford, & Lenz, 2005) .
The RPI has several advantages over the SII procedure. These include: (1) the intrinsic advantage of direct measurement over estimation; (2) the RPI is simpler to use, eliminating the requirement for measuring intelligibility of high-pass and low-pass speech heard at various signal/noise ratios. In addition, there is no need for calculating the intermediate stage of importance values followed by the requirement of a transfer function in order to obtain intelligibility estimates; (3) the RPI procedure is the only one that can obtain passband intelligibility judgments under quiet conditions. This makes it possible to quantify the decrement produced by extraneous sounds and distortions.
[1.] ANSI S3.5, 1997, Reaffirmed, 2012. “Methods for the Calculation of the Speech Intelligibility Index,” (American National Standards Institute, New York).
[2.] Studebaker, G.A., & Sherbecoe, R.L. “Frequency-Importance and Transfer Functions for Recorded CID W-22 Word Lists,” Journal of Speech and Hearing Research, 1991, Vol. 34, 427-438.
[3.] Kates, J.M. “Improved Estimation of Frequency Importance Functions,” Journal of the Acoustical Society of America, 2013, Vol. 134, EL459-EL464.
[4.] Warren, R.M., Bashford, J.A., Jr., & Lenz, P.W. “An Alternative to the Computational Speech Intelligibility Index Estimates: Direct Measurement of Rectangular Passband Intelligibilities,” Journal of Experimental Psychology: Human Perception and Performance, 2011, Vol. 37, 296 - 302.
[5.] Warren, R.M., Bashford, J.A., Jr., & Lenz, P.W. “Intelligibilities of 1-Octave Rectangular Bands Spanning The Speech Spectrum When Heard Separately and Paired.” Journal of the Acoustical Society of America, 2005, Vol. 118, 3261-3266.
Richard M. Warren
and Distinguished Professor Emeritus
Department of Psychology
University of Wisconsin-Milwaukee
PO Box 413
Milwaukee, WI 53201