On the number of channels needed to understand speech

Recent studies have shown that high levels of speech understanding could be achieved when the speech spectrum was divided into four channels and then reconstructed as a sum of four noise bands or sine waves with frequencies e qual to the center frequencies of the channels. In these studies speech und erstanding was assessed using sentences produced by a single male talker. T he aim of experiment 1 was to assess the number of channels necessary for a high level of speech understanding when sentences were produced by multipl e talkers. In experiment 1, sentences produced by 135 different talkers wer e processed through n (2 less than or equal to n less than or equal to 16) number of channels, synthesized as a sum of n sine waves with frequencies e qual to the center frequencies of the filters, and presented to normal-hear ing listeners for identification. A minimum of live channels was needed to achieve a high level (90%) of speech understanding. Asymptotic performance was achieved with eight channels, at least for the speech material used in this study. The outcome of experiment 1 demonstrated that the number of cha nnels needed to reach asymptotic performance varies as a function of the re cognition task and/or need for listeners to attend to fine phonetic detail. In experiment 2, sentences were processed through 6 and 16 channels and qu antized into a small number of steps. The purpose of this experiment was to investigate whether listeners use across-channel differences in amplitude to code frequency information, particularly when speech is processed throug h a small number of channels. For sentences processed through six channels there was a significant reduction in speech understanding when the spectral amplitudes were quantized into a small number (<8) of steps. High levels ( 92%) of speech understanding were maintained for sentences processed throug h 16 channels and quantized into only 2 steps. The findings of experiment 2 suggest an inverse relationship between the importance of spectral amplitu de resolution (number of steps) and spectral resolution (number of channels ). (C) 1999 Acoustical Society of America. [S0001-4966(99)01810-X].