Dual band speech intelligibility: Implications of some novel observations concerning amplitude mismatch and asynchrony

Abstract

Two 1/3-octave bands of everyday sentences (center frequencies 1 and 3 kHz) were employed. To avoid the major contribution of slopes (even when 100 dB/octave) upon intelligibility [Warren and Bashford, J. Acoust. Soc. Am. 106, L47 (1999); Warren et al., J. Acoust. Soc. Am. 108, 1264 (2000)], nearly vertical slopes were employed (4000 order FIR filtering producing 2000 dB/octave slopes). Heard alone at 75 dB, intelligibilities were 5% (1 kHz) and 10% (3 kHz); together, the synergistic score was 80%. When the 3-kHz band was kept at 75 dB and the 1-kHz band’s amplitude was decreased systematically, intelligibility remained unchanged (1.4%) from 75 to 45 dB (amplitude ratio of 1000:1). But when both bands were at their normal relative levels, and one band was delayed relative to the other, intelligibility dropped to half with asynchrony of only about 35 ms (approximately half the duration of the average phoneme), decreasing to an intelligibility score of a single band when asynchrony approximated phonemic duration. It is suggested that broadband speech processing relies upon cross-frequency comparison of dynamic changes in local patterns with little regard to differences in pattern amplitude. This comparison appears to be based upon units approximating the duration of the average phoneme. [Work supported by NIH.]