The central aim of this thesis was to assess to what extent users of Bimodal stimuli can benefit from binaural hearing effect with low frequency acoustic information when listening to speech in noisy environment. In order to shed some light on this issue, several simulation experiments were conducted. The results of these experiments can draw the following conclusions.
First, speech perception for Taiwanese Mandarin sentences and bisyllabic words in both multi-talker babble interference and speech shaped noise, significantly improved as unprocessed low frequency acoustic information was added to simulation CI users. Furthermore, the benefit in speech intelligibility of this Bimodal stimuli with different spatial direction of sound source was evaluated. The binaural summation and squelch effect were not found in this study. Thus we suggested a monaurally based glimpsing and grouping mechanism rather than a binaurally based mechanism for the observed advantage with the combined acoustic and electric hearing. However, the head shadow effect was observed. In addition, the binaural hearing effect can keep the improvement of speech intelligibility to an average level, particularly when noise was interfered CI significantly. This outcome indicates that when vocoder stimuli was impoverishment, having access to the acoustic landmarks provided in low frequency region enables listeners to segregate speech from noise effectively.
Second, a tone carry either the dynamic variation of target speech‟s F0 or the amplitude envelope of low pass target speech at 500 Hz, provided little to no benefit in simulated CI users. On the other hand, a tone present as the F0 information of target speech which had modulated both in frequency to track the dynamic variation of target speech‟s F0 and in amplitude envelope of low pass target speech at 500 Hz, significant improved the speech intelligibility in Taiwanese Mandarin sentences. Although the low pass speech at 500 Hz was enhanced the performance more than the F0 information, it was not statistical significance between each other. This result indicated the F0
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information was a major component in low frequency region. However, the low frequency acoustic information was believed to contain more acoustic landmarks than F0 information that boost speech perception in simulated CI users, particularly when target information was impoverishment. This outcome reveled the important of preserving residual low frequency hearing for CI users even was severe to profound hearing loss.
Third, the addition of low frequency acoustic information increased the masking release in Taiwanese Mandarin bisyllabic words with gated speech shaped noise. The CI users often show limited improvement from temporal gaps in fluctuating masker and do not demonstrate successful fuse interrupted speech signals into a coherent speech stream. The lack of masking release, then, may be attributable to general characteristics of the implant processing, including the lack of spectral information in the processed signal and a disruption in the envelope cues extracted by the devices and used by the listeners. Improved the low frequency acoustic spectral information of speech seems to improve simulated CI users‟ ability of glimpsing and grouping interrupted speech stream and resilience to fluctuations in background noise.
Finally, the current study clearly demonstrated that overall Bimodal stimulation enhanced the performance of speech recognition in noise in Taiwanese Mandarin. These results have important implications for the debate about unilateral CI users with even very limited residual hearing has the potential to provide substantial speech recognition benefits, when combined with a hearing aid in contralateral ear. The preserving residual low frequency acoustic information may greatly enhance both the speech intelligibility of tonal language by improved the pitch perception and the ability of speech fusion with fluctuating masker by increased the masking release.
In current study, for speech perception in noise, the important role of low frequency acoustic information as well as F0 information in simulation CI users was demonstrated. Further research should therefore be directed at exploring other strategies to introduce this information into CI
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devices. On the other hand, according to the results of this study, the ITD and ILD cue may be disorder in Bimodal stimulation, due to the different processing strategies between hearing aid and CI. It‟s worthwhile to further examine these cues and regulate the level of difference to restore the squelch effect in Bimodal users. Furthermore, the low frequency information is a major component within low frequency region. However, the low frequency information seems to contain other acoustic phonetic cue which could enhance the performance in more deficit environment. Further research is required to address which cue within low frequency region besides F0 information, aid listener to perception speech. Last but not least, since the findings have important implications for the development of future processing techniques that aim to improve the speech intelligibility in noise, it is important to extend whether the results of this simulation study to clinical data, by replicated the subject to actual Bimodal users.
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