Intelligibility of dual rectangular speech bands: Implications of observations concerning amplitude mismatch and asynchrony

Abstract

The present study examines the integration of information present in different spectral regions of speech using two 1/3-octave bands of everyday sentences (center frequencies 1- and 3-kHz). Nearly vertical slopes were employed (4000-order finite impulse response filtering) to avoid the major contribution to intelligibility made even by conventionally steep slopes (e.g., 100 dB/octave) (see [J. Acoust. Soc. Amer. 108 (2000) 1264]). Heard alone at 75 dB, the rectangular band intelligibilities were 5% (1 kHz) and 10% (3 kHz); heard together, their score was 77%. Conformity to the normal spectral profile was not required for this remarkably high degree of synergy: When the 3-kHz band was kept at 75 dB and the 1-kHz band’s level was decreased systematically, intelligibility remained unchanged from 75 to 45 dB (intensity ratio of 1000:1). But when the rectangular bands were kept at their normal levels, and one band was delayed relative to the other, intelligibility dropped to half with a misalignment of only about 35 ms (approximately half the duration of the average phoneme); scores dropped further, approaching that of a single band when asynchrony approximated average phonemic durations.