  Electrical hearing by cochlear implants (CIs) may be fundamentally different
from acoustic hearing by normal-hearing (NH) listeners, presumably showing
unequal speech quality perception in various noise environments. Noise
reduction (NR) algorithms used in CI reduce the noise in favor of
signal-to-noise ratio (SNR), regardless of plausible accompanying distortions
that may degrade the speech quality perception. To gain better understanding of
CI speech quality perception, the present work aimed investigating speech
quality perception in a diverse noise conditions, including factors of
speech/noise levels, type of noise, and distortions caused by NR models.
Fifteen NH and seven CI subjects participated in this study. Speech sentences
were set to two different levels (65 and 75 dB SPL). Two types of noise
(Cafeteria and Babble) at three levels (55, 65, and 75 dB SPL) were used.
Sentences were processed using two NR algorithms to investigate the perceptual
sensitivity of CI and NH listeners to the distortion. All sentences processed
with the combinations of these sets were presented to CI and NH listeners, and
they were asked to rate the sound quality of speech as they perceived. The
effect of each factor on the perceived speech quality was investigated based on
the group averaged quality rated by CI and NH listeners. Consistent with
previous studies, CI listeners were not as sensitive as NH to the distortion
made by NR algorithms. Statistical analysis showed that the speech level has
significant effect on quality perception. At the same SNR, the quality of 65 dB
speech was rated higher than that of 75 dB for CI users, but vice versa for NH
listeners. Therefore, the present study showed that the perceived speech
quality patterns were different between CI and NH listeners in terms of their
sensitivity to distortion and speech level in complex listening environment.
