Effects of spectral smearing and temporal fine structure degradation on speech masking release.

This study assessed the effects of spectral smearing and temporal fine structure (TFS) degradation on masking release (MR) (the improvement in speech identification in amplitude-modulated compared to steady noise observed for normal-hearing listeners). Syllables and noise stimuli were processed using either a spectral-smearing algorithm or a tone-excited vocoder. The two processing schemes simulated broadening of the auditory filters by factors of 2 and 4. Simulations of the early stages of auditory processing showed that the two schemes produced comparable excitation patterns; however, fundamental frequency (F0) information conveyed by TFS was degraded more severely by the vocoder than by the spectral-smearing algorithm. Both schemes reduced MR but, for each amount of spectral smearing, the vocoder produced a greater reduction in MR than the spectral-smearing algorithm, consistent with the effects of each scheme on F0 representation. Moreover, the effects of spectral smearing on MR produced by the two schemes were different for manner and voicing. Finally, MR data for listeners with moderate hearing loss were well matched by MR data obtained for normal-hearing listeners with vocoded stimuli, suggesting that impaired frequency selectivity alone may not be sufficient to account for the reduced MR observed for hearing-impaired listeners.