Background: Wind noise can be a nuisance or a debilitating masker for cochlear implant users
in outdoor environments. Previous studies indicated that wind noise at the microphone/hearing
aid output had high levels of low-frequency energy and the amount of noise generated
is related to the microphone directionality. Currently, cochlear implants only offer
either directional microphones or omnidirectional microphones for users at-large.
As all cochlear implants utilize pre-emphasis filters to reduce low-frequency energy
before the signal is encoded, effective wind noise reduction algorithms for hearing
aids might not be applicable for cochlear implants.
Purpose: The purposes of this study were to investigate the effect of microphone directionality
on speech recognition and perceived sound quality of cochlear implant users in wind
noise and to derive effective wind noise reduction strategies for cochlear implants.
Research Design: A repeated-measure design was used to examine the effects of spectral and temporal
masking created by wind noise recorded through directional and omnidirectional microphones
and the effects of pre-emphasis filters on cochlear implant performance. A digital
hearing aid was programmed to have linear amplification and relatively flat in-situ
frequency responses for the directional and omnidirectional modes. The hearing aid
output was then recorded from 0 to 360° at flow velocities of 4.5 and 13.5 m/sec in
a quiet wind tunnel.
Study Sample: Sixteen postlingually deafened adult cochlear implant listeners who reported to be
able to communicate on the phone with friends and family without text messages participated
in the study.
Intervention: Cochlear implant users listened to speech in wind noise recorded at locations that
the directional and omnidirectional microphones yielded the lowest noise levels.
Data Collection and Analysis: Cochlear implant listeners repeated the sentences and rated the sound quality of
the testing materials. Spectral and temporal characteristics of flow noise, as well
as speech and/or noise characteristics before and after the pre-emphasis filter, were
analyzed. Correlation coefficients between speech recognition scores and crest factors
of wind noise before and after pre-emphasis filtering were also calculated.
Results: Listeners obtained higher scores using the omnidirectional than the directional microphone
mode at 13.5 m/sec, but they obtained similar speech recognition scores for the two
microphone modes at 4.5 m/sec. Higher correlation coefficients were obtained between
speech recognition scores and crest factors of wind noise after pre-emphasis filtering
rather than before filtering.
Conclusion: Cochlear implant users would benefit from both directional and omnidirectional microphones
to reduce far-field background noise and near-field wind noise. Automatic microphone
switching algorithms can be more effective if the incoming signal were analyzed after
pre-emphasis filters for microphone switching decisions.
Key Words
Cochlear implants - hearing aids - pre-emphasis filter - sound quality - speech intelligibility
- speech recognition - wind
