Effect of single-channel compression on temporal speech information.

Although syllabic compression systems are available in many currently marketed hearing aids, the benefit of such systems continues to be debated. The goal of this study was to determine the effect of single-channel amplitude compression on use of speech information were considered, with particular focus on temporal cues. Speech recognition scores were assessed under four conditions, using two types of speech materials. Both temporal and spectral information were provided in the FullSpeech or natural speech stimuli. Unlike previous studies in which use of temporal information may have been confounded by spectral changes, in this study the listener's ability to use temporal information was tested directly using signal-correlated noise (SCN) stimuli, which consisted of a time-varying speech envelope modulating a broad-band noise carrier. Two conditions (FullSpeech and SCN) retained their natural amplitude characteristics. In two corresponding conditions the same stimuli were compressed digitally, using compression parameters appropriate for syllabic compression. This compression reduced the range of time-intensity variations. Recognition scores were obtained from listeners with mild-to-severe sensorineural loss and from a control group of listeners with normal hearing. All stimuli were presented at high levels to reduce potential effects of reduced audibility. Additionally, short-term speech level measurements were compared to individual listener's hearing thresholds to determine the range of speech levels in each frequency region that was audible for each stimulus condition. Results demonstrated no effect of compression on recognition scores for either the FullSpeech or SCN signals for listeners with normal hearing or listeners with hearing loss. This suggests that single-channel syllabic compression processing, of the type used in this study, does not alter the temporal (or spectral) signal sufficiently to cause decreased recognition for VCV syllables when speech audibility is maintained.