Deeks, John Michael; (2001) The perception of pitch and timbre of vowel-like stimuli by the profoundly hearing-impaired. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Profoundly hearing-impaired listeners have great difficulty in processing the timbre and pitch of vowel sounds. Psychoacoustic measures suggest that spectral processing is very limited for these listeners, but that temporal processing is relatively well preserved. The role of these residual abilities in vowel perception is not known. Three experiments examined performance using stimuli ranging from very simple to close approximations of natural vowels. For discriminating the centre-frequency (CF) and fundamental frequency (F0) of a single formant, performance was about an order of magnitude worse than in normal hearing listeners. Pure-tone frequency discrimination was better than CF discrimination, but F0 discrimination for the formant complexes was not. The results suggested a spectral mechanism for timbre processing, and a temporal mechanism for pitch processing. The second experiment investigated the discrimination of a single formant contained within 2-formant vowels. A synthesis method which presented the first and second formant in separate halves of the period led to lower DLs for vowels with a formant separation of 440 Hz or less. Mutual masking between simultaneously present formants was shown to limit formant discrimination. Again, a spectral mechanism gave a reasonable account of the results. The final experiment measured vowel and diphthong identification, using a range of fixed and sweeping F0s. Performance was often barely above chance. The primary cue to vowel identity was F1, while the cues used for diphthong identity were not obviously related to the formant transitions. Factors found to be important in discrimination were not important in vowel and diphthong identification. Overall, the results suggest that spectral mechanisms may play a more important role in vowel timbre perception than originally thought in these listeners. The discrimination of pitch was consistent with a temporal mechanism. The results suggest limitations to the complexity of vowel sounds that can be usefully conveyed to these listeners.

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