This study investigated the contribution of low-frequency harmonics to identifying Mandarin tones in natural and vocoded speech in quiet and noisy conditions. Results showed that low-frequency harmonics of natural speech led to highly accurate tone identification; however, for vocoded speech, low-frequency harmonics yielded lower tone identification than stimuli with full harmonics, except for tone 4. Analysis of the correlation between tone accuracy and the amplitude-F0 correlation index suggested that "more" speech contents (i.e., more harmonics) did not necessarily yield better tone recognition for vocoded speech, especially when the amplitude contour of the signals did not co-vary with the F0 contour.

This study assesses the effects of adding low-or high-frequency information to the band-limited telephone-processed speech on bimodal listeners' telephone speech perception in quiet environments. In the proposed experiments, bimodal users were presented under quiet listening conditions with wideband speech (WB), bandpass-filtered telephone speech (300-3,400 Hz, BP), high-pass filtered speech (f > 300 Hz, HP, i.e., distorted frequency components above 3,400 Hz in telephone speech were restored), and low-pass filtered speech (f < 3,400 Hz, LP, i.e., distorted frequency components below 300 Hz in telephone speech were restored). Results indicated that in quiet environments, for all four types of stimuli, listening with both hearing aid (HA) and cochlear implant (CI) was significantly better than listening with CI alone. For both bimodal and CI-alone modes, there were no statistically significant differences between the LP and BP scores and between the WB and HP scores. However, the HP scores were significantly better than the BP scores. In quiet conditions, both CI alone and bimodal listening achieved the largest benefits when telephone speech was augmented with high rather than low-frequency information. These findings provide support for the design of algorithms that would extend higher frequency information, at least in quiet environments.Keywords cochlear implant, bimodal hearing, bandwidth effect, electroacoustic stimulation, telephone speech For technological and economic reasons (e.g., to save the bandwidth of communication channels), the public telephone network only passes spectral information between 300 Hz and 3400 Hz. Thus, in telephone speech, lowfrequency information below 300 Hz and high-frequency information above 3400 Hz is mostly severely distorted. This bandwidth was selected according to the study on speech intelligibility by French and Steinberg (1947) for normal-hearing listeners. The band limiting poses only negligible challenges for listeners with normal hearing; however, the reduced spectral information can have negative impact on speech intelligibility for listeners with cochlear implants (CIs). Several previous studies (see, e.g., Cohen, Waltzman, & Shapiro, 1989;Cray et al., 2004;Fu & Galvin, 2006;Horng, Chen, Hsu, & Fu, 2007;Ito, Nakatake, & Fujita, 1999;Milchard & Cullington, 2004) evaluating conventional CI listeners' telephone usage have demonstrated that their recognition of telephone speech is significantly worse than their recognition of wideband speech, and it has been generally established that the reduced bandwidth of the telephone speech accounts for a significant amount of performance deterioration. These studies support the hypothesis that techniques to extend the bandwidth for telephone speech can potentially improve CI listeners' telephone speech perception. So far, only a few studies have examined bandwidth extension techniques for improving CI users' perception of telephone speech. For listeners who wear conventional CI devices, a recent study by Liu, Fu, and Nar...

The smearing effects of room reverberation can significantly impair the ability of cochlear implant (CI) listeners to understand speech. To ameliorate the effects of reverberation, current dereverberation algorithms focus on recovering the direct sound from the reverberated signal by inverse filtering the reverberation process. This contribution describes and evaluates a spectral subtraction (SS) strategy capable of suppressing late reflections. Late reflections are the most detrimental to speech intelligibility by CI listeners as reverberation increases. By tackling only the late part of reflections, it is shown that users of CI devices can benefit from the proposed strategy even in highly reverberant rooms. The proposed strategy is also compared against an ideal reverberant (binary) masking approach. Speech intelligibility results indicate that the proposed SS solution is able to suppress additive reverberant energy to a degree comparable to that achieved by an ideal binary mask. The added advantage is that the SS strategy proposed in this work can allow for a potentially real-time implementation in clinical CI processors.

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This study assessed the effects of binaural spectral resolution mismatch on the intelligibility of Mandarin speech in noise using bilateral cochlear implant simulations. Noise-vocoded Mandarin speech, corrupted by speech-shaped noise at 0 and 5 dB signal-to-noise ratios, were presented unilaterally or bilaterally to normal-hearing listeners with mismatched spectral resolution between ears. Significant binaural benefits for Mandarin speech recognition were observed only with matched spectral resolution between ears. In addition, the performance of tone identification was more robust to noise than that of sentence recognition, suggesting factors other than tone identification might account more for the degraded sentence recognition in noise.