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Transcript
S24-5
Impact of reverberation on speech perception in cochlear implant users
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Grahlmann H.-L. , Rader T. , Weiìügerber T. , Baumann U.
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University Hospital Frankfurt, Audiological Acoustics, ENT Department, Frankfurt am Main, Germany
Speech perception of cochlear implant (CI) users decreases in reverberant environments compared to free-field
conditions. Previous studies showed exponential decrement in word recognition with increasing reverberation
time (RT). Another detrimental factor is the logarithmic direct-to-reverberant ratio (DRR), which decreases with
increasing distance to the sound source. This study evaluates the influence of the DRR on speech perception in
CI users as well as the impact of beamforming on speech perception. Speech test material consisted of the
Oldenburg Sentence Test (OLSA), which was recorded with DRRs between -18 and 21 dB. Recordings were
carried out with a dual microphone hearing aid held by a dummy head. The diffuse reverb tail of the impulse
responses (IR) of a classroom without acoustic treatment (RT 0.75 s) and a church (RT 4.1 s) were extracted
from a library of monaural IRs. The final stimulus presentation consisted of a direct speech sound source in
frontal position (0°) and the reverberated signal, which was generated by 20 equally-distributed surrounding
plane waves created by wave field synthesis (WFS, 128 loudspeakers). The speech reception threshold (SRT)
depending on DRR was assessed in quiet and in noise (10 dB SNR). Tests were carried out in 8 normal-hearing
(NH) subjects with monaural headphones presentation. Therefore, speech test material was processed by a 6channel sine-vocoder to simulate CI listening. Furthermore, 8 users of Advanced Bionics CIs were tested
monaurally in two conditions: (1) direct input to a Harmony speech processor, and (2) preprocessed by means of
an advanced beamforming algorithm. DRR calculated in dB or speech score percentage difference measured at
50% DRR level was assessed respectively. The mean SRTs in classroom reverberation condition were about 18 dB DRR for both groups. With additional noise, the CI subject group showed a slight mean decrement of 2
dB. Mean SRTs in church reverberation condition in the NH group were -13.7 dB DRR (quiet) and -12.5 dB DRR
(noise). The CI group thresholds were -13.1 dB DRR (quiet) and -10.7 dB DRR (noise). On average, speech
scores improved about 28.3% in the advanced beamformer condition compared to standard processing. The
SRTs in quiet showed no statistically significant difference between subject groups. However, a detrimental
effect of noise in addition to reverberation in the church reverberation condition was present in the CI subject
group. The ability of NH subjects to listen to CI-simulating speech in adverse conditions consisting of
reverberated speech and additional noise is superior compared to the performance of CI users. While often
reporting difficulties in reverberated conditions, the performance of CI users in terms of DRR was better than
expected. Hence, the DRR seems not to be the main parameter to signify degraded speech intelligibility.
Beamforming algorithms can significantly improve speech perception in diffuse reverberant environments.
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