Vickers, Deborah Anne; (2003) The encoding of fricatives for people with severe and profound hearing loss. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The aim of this work was to determine the potential for encoding voiceless fricatives for listeners with a severe-profound sensorineural hearing loss. The first experiment determined whether normal hearing listeners could label band-pass filtered noises as fricatives. Flat spectrum noise bands varying in centre frequency, bandwidth and level were presented between two sinusoids to simulate a vowel-consonant-vowel syllable. Some combinations of noise centre frequency, bandwidth and level were consistently labelled as fricatives /f, s and [/. Labelling was consistent with the acoustic features used to label natural fricatives. The second and third experiments investigated the residual auditory abilities of profoundly hearing-impaired subjects. The second experiment determined the duration required to discriminate the centre frequency of bands of noise for several bandwidths, levels and centre frequencies. Most subjects could discriminate at least four, 250 Hz wide, spectrally adjacent noise bands within their residual hearing range at durations of 80ms or less, which is less than the duration expected for fricatives in running speech. In the third experiment psychometric functions were measured for discrimination of the bandwidth of noise stimuli spectrally centred (linear scale) at 300 Hz. In task 1, either the wider (500 Hz) or the narrower (124 Hz) of two bandwidths was fixed, and the other was varied. All subjects were capable to some extent of discriminating bandwidth. In task 2, either the upper or lower frequency difference between the stimuli was eliminated. Subjects tended to use high-frequency edge cues to discriminate the noises when the wider noise band was fixed. When the narrower noise band was fixed, the noise edge used varied across subjects. Together the second and third experiments indicated a potential for using noise bands to encode fricatives within the low-frequency hearing range of hearing-impaired listeners. The final experiment investigated the ability of hearing-impaired listeners to label noise bands as fricatives when presented within their residual hearing range. The stimuli were encoded VCV syllables, where the vowel was represented by a sinusoid and the consonant was an encoded version of either /f, s, [/. All subjects were able to some extent to label the encoded stimuli as fricatives and most subjects performed best when listening to noises with the spectral envelope simplified to one spectral peak and lowered by a factor of 8 relative to the natural token.

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