When Spectral Smearing Can Increase Speech Intelligibility.

Sentences were reduced to an array of sixteen effectively rectangular bands (RBs) having center frequencies ranging from 0.25 to 8 kHz spaced at ⅓-octave intervals. Four arrays were employed, each having uniform subcritical bandwidths which ranged from 40 Hz to 5 Hz. The 40 Hz width array had intelligibility near ceiling, and the 5 Hz array about 1%. The finding of interest was that when the subcritical speech RBs were used to modulate RBs of noise having the same center frequency as the speech but having bandwidths increased to a critical (ERBn) bandwidth at each center frequency, these spectrally smeared arrays were considerably more intelligible in all but the 40 Hz (ceiling) condition. For example, when the 10 Hz bandwidth speech array having an intelligibility of 8% modulated the ERBn noise array, intelligibility increased to 48%. This six-fold increase occurred despite elimination of spectral fine structure and addition of stochastic fluctuation to speech envelope cues. (As anticipated, conventional vocoding with matching bandwidths of speech and noise reduced the 10-Hz-speech array intelligibility from 8% to 1%). These effects of smearing confirm findings by Bashford, Warren, and Lenz (2010) that optimal temporal processing requires stimulation of a critical bandwidth. [Supported by NIH].