Envelope cues and audibility: A systematic investigation of compression

Date of Award

1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Teaching and Leadership

Advisor(s)

Christopher W. Turner

Keywords

hearing aids, speech recognition, audiology

Subject Categories

Education

Abstract

Although compression systems are quickly becoming integral components of programmable hearing aids, the benefit of such systems continues to be debated. The goal of this study was to examine the effect of amplitude compression on use of speech information by hearing-impaired listeners while quantifying audibility. Two separate experiments were conducted. The first examined single-channel compression and the second examined multichannel compression. Effects on both temporal and spectral information were considered, with particular focus on temporal cues. Speech recognition scores were assessed using two types of speech materials. Both temporal and spectral information were provided in the FullSpeech, or natural speech stimuli. The listener's ability to use temporal information was directly tested using signal-correlated noise (SCN) stimuli, which consisted of a time-varying speech envelope modulating a broad-band noise carrier. The linear conditions (FullSpeech and SCN) retained their natural amplitude characteristics and in the corresponding compressed conditions the same stimuli were digitally compressed. Compression parameters were typical of those used in commercially available single- and multichannel hearing aids. This compression reduced the range of time-intensity variations to varying degrees. In both the single-channel and multichannel experiments, recognition scores were obtained from listeners with mild to severe sensorineural loss and from a control group of normal-hearing listeners. 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. Single-channel results demonstrated no effect of compression on recognition scores for the FullSpeech or SCN signals for either listener group. For the hearing-impaired listeners, a significant advantage was seen for the multichannel compression at low input levels due to the improved audibility provided by this amplification scheme. Multichannel amplification had no significant effect on speech recognition score for the FullSpeech stimuli when audibility of both linear and compressed speech was maximized. However, multichannel compression did result in decreased recognition scores for the SCN signals at the highest input level. This suggests that while single-channel syllabic compression processing which maintains speech audibility does not alter the temporal (or spectral) signal sufficiently to cause decreased speech recognition, multichannel compression may reduce these cues under certain conditions.

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