The combined effects of reverberation and noise on speech intelligibility by cochlear implant listeners

Abstract

Objective: The purpose of this study is to assess the individual effect of reverberation and noise, as well as their combined effect, on speech intelligibility by cochlear implant (CI) users. Design: Sentence stimuli corrupted by reverberation, noise, and reverberation + noise are presented to 11 CI listeners for word identification. They are tested in two reverberation conditions (T₆₀ = 0.6 s, 0.8 s), two noise conditions (SNR = 5 dB, 10 dB), and four reverberation + noise conditions. Study sample: Eleven CI users participated. Results: Results indicated that reverberation degrades speech intelligibility to a greater extent than additive noise (speech-shaped noise), at least for the SNR levels tested. The combined effects were greater than those introduced by either reverberation or noise alone. Conclusions: The effect of reverberation on speech intelligibility by CI users was found to be larger than that by noise. The results from the present study highlight the importance of testing CI users in reverberant conditions, since testing in noise-alone conditions might underestimate the difficulties they experience in their daily lives where reverberation and noise often coexist.

Key Words::

• Cochlear implants
• ACE
• channel selection
• reverberation

Acknowledgements

This work was supported by Grant R01 DC010494 awarded from the National Institute of Deafness and other Communication Disorders (NIDCD) of the National Institutes of Health (NIH). The authors would like to thank the CI patients for their time. The authors would also like to thank Dr. Arlene C. Neuman of the NYU Langone Medical Center for providing the RIRs. Thanks also go to the two anonymous reviewers and Dr. Chris Brown for providing valuable feedback to this manuscript.

Notes

1. 1. The ACE strategy implemented in the SPEAR3 processor is very similar to that implemented in the Nucleus 24, Nucleus 5, and Freedom systems and most coding parameters of the SPEAR3 ACE strategy matched those used in the commercial systems. In addition, all parameters used (e.g. stimulation rate, number of maxima, frequency allocation table) were matched to the individual CI user's clinical settings.

2. 2. Although adding noise to the clean anechoic signal and then convolving with the room impulse response may reflect a more realistic scenario (e.g. Helms et al, 2012), the method taken in the present study to add noise to the reverberant speech is considered to be common practice in the engineering literature (e.g. Habets, 2010). Nonetheless, there is an approximately 3 dB difference in SNR levels between the reverberation + noise stimuli created by the two methods. For example, the R + N (T₆₀ = 0.6 s, RSNR = 5 dB) condition used in the present study is equivalent, according to speech-transmission index (STI) values (Houtgast & Steeneken, 1985), to adding noise to the anechoic signal at a higher SNR (SNR = 8 dB) and then convolving with the room impulse response (T₆₀ = 0.6 s); this was confirmed by computing the mean of the STI values of 20 reverberation + noise stimuli produced by the two methods (the modulation transfer function in the STI is used to quantify the degree that reverberation and/or noise reduce the modulations). This suggests that convolving both signal and noise with the room impulse response would create stimuli that would be more difficult (by 3 dB in SNR) to recognize than the R + N stimuli used in the present study. Consequently, we would expect the reverberation effects to be larger and the conclusion to be the same in as far as the reverberation degrading intelligibility more than additive noise.

3. 3. The volume control provides a means for adjusting the overall loudness. It acts at the output stage (envelopes) of the signal-processing stage and not at the input gain.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.