Identification of envelope-expanded sentences in normal-hearing and hearing-impaired listeners.

The present study examined the effects of temporal-envelope expansion on speech perception. Sentence identification performance was measured in normal-hearing and hearing-impaired listeners for stationary and fluctuating noise. Two expansion schemes were used to increase the depth of the slow (< 16 Hz) and fast (< 256 Hz) amplitude fluctuations of the stimuli. In the first scheme, the envelope of the stimuli was raised to the power 2. In the second scheme, the high- and low-level segments of the envelope were compressed and expanded, respectively. When envelope processing was applied to speech before the addition of noise, the first form of expansion generally degraded identification, while the second form generally improved identification. When envelope processing was applied to speech after the addition of noise, both forms of expansion improved or did not affect identification scores in both groups of listeners when applied to the slowest fluctuations. When applied to the broadest range of fluctuations, both forms of expansion degraded identification. However, in hearing-impaired listeners, the second form of expansion yielded an increase in performance for fluctuating noise. This complex pattern of results will be discussed in light of previous studies on envelope expansion.