When noise vocoding can improve the intelligibility of sub-critical band speech.
James A. Bashford, Richard M. Warren, Peter W. Lenz · The Journal of the Acoustical Society of America · 2010
This study examined the redundancy of spectral and temporal information in everyday sentences, which were reduced to 16 rectangular spectral bands having center frequencies ranging from 250 to 8000 Hz, spaced at 1/3-octave intervals. High-order filtering eliminated contributions from transition bands, and the widths of the resulting effectively rectangular speech bands were varied from 4% down to 0.5% (Exp. 1) and from 40 Hz down to 5 Hz (Exp. 2). Intelligibility of these sub-critical bandwidth stimuli ranged from nearly perfect in the 4%- and 40-Hz bandwidth conditions, down to nearly zero in the 0.5%- and 5-Hz bandwidth conditions. However, a large recovery of intelligibility was obtained under adverse filtering conditions when the speech bands were used to vocode broader noise bands that approximated critical bandwidths (ERBn) at the 16 center frequencies. For example, the 0.5%- and 1%-bandwidth speech stimuli were only about 1% and 20% intelligible, respectively, whereas scores of about 26% and 60%, respectively, were obtained for the ERBn-wide modulated noise stimuli derived from the speech bands. These large intelligibility increases occurred despite elimination of spectral fine structure and the addition of stochastic fluctuations to the speech-envelope cues. Additional findings and their implications will be discussed. [Work supported by NIH.]