Effects of reverberation and masking on speech intelligibility in cochlear implant simulations

Abstract

Two experiments investigated the impact of reverberation and masking on speech understanding using cochlear implant (CI) simulations. Experiment 1 tested sentence recognition in quiet. Stimuli were processed with reverberation simulation (T=0.425, 0.266, 0.152, and 0.0 s) and then either processed with vocoding (6, 12, or 24 channels) or were subjected to no further processing. Reverberation alone had only a small impact on perception when as few as 12 channels of information were available. However, when the processing was limited to 6 channels, perception was extremely vulnerable to the effects of reverberation. In experiment 2, subjects listened to reverberated sentences, through 6- and 12-channel processors, in the presence of either speech-spectrum noise (SSN) or two-talker babble (TTB) at various target-to-masker ratios. The combined impact of reverberation and masking was profound, although there was no interaction between the two effects. This differs from results obtained in subjects listening to unprocessed speech where interactions between reverberation and masking have been shown to exist. A speech transmission index (STI) analysis indicated a reasonably good prediction of speech recognition performance. Unlike previous investigations, the SSN and TTB maskers produced equivalent results, raising questions about the role of informational masking in CI processed speech.

Effects of source-to-listener distance and masking on perception of cochlear implant processed speech in reverberant rooms

“…The listening difficulties observed by Poissant et al ͑2006͒ are not unexpected, given the placement of the listener in the room's reverberant field. However, the degree of difficulty observed was remarkable considering the small size of the room ͑volume= 79.2 m 3 ͒ and short source-tolistener distance ͑SLD͒ of 4 m, and was far greater than the difficulties normally associated with the magnitude of STI values computed in that study.…”

“…However, when the channels' envelope modulations are low-pass filtered with cutoff frequencies in the 8-16 Hz range, intelligibility decreases ͑Fu and Shannon, 2000;Xu and Zheng, 2007͒. Reverberation, which can act as a low-pass filter for envelope modulations in this frequency range ͑Houtgast and Steeneken, 1985͒, has similarly been shown to degrade intelligibility for listening through actual or simulated implants in rooms that listeners with normal hearing would find acceptable ͑Iglehart, 2004; Poissant et al, 2006͒. Studies of intelligibility for implant users in reverberant spaces have typically focused on the use of frequency modulation ͑FM͒ or sound field devices to improve intelligibility ͑Crandell et Iglehart, 2004;Anderson et al, 2005͒, with mixed results. Few studies have focused on the specific effects of reverberation and noise on implant processed speech.…”

Use of Amplitude Modulation Cues Recovered from Frequency Modulation for Cochlear Implant Users When Original Speech Cues Are Severely Degraded

“…Previous studies by Poissant et al (2006) and Whitmal and Poissant (2009) controlled the spectral resolution of normal-hearing listeners and showed a systematic degradation in speech identification as the number of vocoder channels decreased.…”

“…In general, if the space is surrounded by materials that are highly reflective (e.g., concrete, tile, wood flooring, bricks, and glass), RT 60 values are high. The effects of reverberation on speech identification for CI users has recently received extensive attention (e.g., Poissant et al 2006;Whitmal and Poissant 2009;Zheng et al 2011;Tillery et al 2012). In the present study, when the effect of reverberation was introduced, speech identification performance remained constant until RT 60 = 0.6, and then decreased significantly at RT 60 =1.0.…”

“…As a result, CI users get confused when important speech cues such as formants are presented (Kokkinakis and Loizou 2011; ). Compared to normal-hearing listeners, CI users have been shown to be highly susceptible to the effect of reverberation because their primary source of speech information is based on AM cues (Poissant et al 2006;.…”