Role of spectral and temporal cues in restoring missing speech information.

This study measured the role of spectral details and temporal envelope (E) and fine structure (TFS) cues in reconstructing sentences from speech fragments. Four sets of sentences were processed using a 32-band vocoder. Twenty one bands were either processed or removed, leading to sentences differing in their amount of spectral details, E and TFS information. These sentences remained perfectly intelligible, but intelligibility significantly fell after the introduction of periodic silent gaps of 120-ms. While the role of E was unclear, the results unambiguously showed that TFS cues and spectral details influence the ability to reconstruct interrupted sentences.

Discrimination of release time constants in hearing-aid compressors.

We examined the ability of twenty-five hearing-impaired and eight normal-hearing listeners to discriminate between release time constants used for compression in hearing aids. The compressor was a standard three-channel system. The stimuli were normal and 'vocoded' sentences from a male and female database. In agreement with other studies looking at different outcomes, performance varied greatly across individuals. This variation was greater in hearing-impaired listeners, for whom the discriminability of a release time of 5 ms from one of 5000 ms (with the attack time fixed at 5 ms) ranged from chance to perfect. This variability was not significantly related to hearing impairment nor to individuals' compression ratios.

Effects of periodic interruptions on the intelligibility of speech based on temporal fine-structure or envelope cues.

The intelligibility of speech signals processed to retain either temporal envelope (E) or fine structure (TFS) cues within 16 0.4-oct-wide frequency bands was evaluated when processed stimuli were periodically interrupted at different rates. The interrupted E- and TFS-coded stimuli were highly intelligible in all conditions. However, the different patterns of results obtained for E- and TFS-coded speech suggest that the two types of stimuli do not convey identical speech cues. When an effect of interruption rate was observed, the effect occurred at low interruption rates (<8 Hz) and was stronger for E- than TFS-coded speech, suggesting larger involvement of modulation masking with E-coded speech.

Speech perception problems of the hearing impaired reflect inability to use temporal fine structure.

People with sensorineural hearing loss have difficulty understanding speech, especially when background sounds are present. A reduction in the ability to resolve the frequency components of complex sounds is one factor contributing to this difficulty. Here, we show that a reduced ability to process the temporal fine structure of sounds plays an important role. Speech sounds were processed by filtering them into 16 adjacent frequency bands. The signal in each band was processed by using the Hilbert transform so as to preserve either the envelope (E, the relatively slow variations in amplitude over time) or the temporal fine structure (TFS, the rapid oscillations with rate close to the center frequency of the band). The band signals were then recombined and the stimuli were presented to subjects for identification. After training, normal-hearing subjects scored perfectly with unprocessed speech, and were approximately 90% correct with E and TFS speech. Both young and elderly subjects with moderate flat hearing loss performed almost as well as normal with unprocessed and E speech but performed very poorly with TFS speech, indicating a greatly reduced ability to use TFS. For the younger hearing-impaired group, TFS scores were highly correlated with the ability to take advantage of temporal dips in a background noise when identifying unprocessed speech. The results suggest that the ability to use TFS may be critical for "listening in the background dips." TFS stimuli may be useful in evaluating impaired hearing and in guiding the design of hearing aids and cochlear implants.

The ability of listeners to use recovered envelope cues from speech fine structure.

Recent work has demonstrated that auditory filters recover temporal-envelope cues from speech fine structure when the former were removed by filtering or distortion. This study extended this work by assessing the contribution of recovered envelope cues to consonant perception as a function of the analysis bandwidth, when vowel-consonant-vowel (VCV) stimuli were processed in order to keep their fine structure only. The envelopes of these stimuli were extracted at the output of a bank of auditory filters and applied to pure tones whose frequency corresponded to the original filters' center frequencies. The resulting stimuli were found to be intelligible when the envelope was extracted from a single, wide analysis band. However, intelligibility decreases from one to eight bands with no further decrease beyond this value, indicating that the recovered envelope cues did not play a major role in consonant perception when the analysis bandwidth was narrower than four times the bandwidth of a normal auditory filter (i.e., number of analysis bands > or =8 for frequencies spanning 80 to 8020 Hz).