Beamformers With Bilateral Cochlear Implants

Kompis, M, Bettler, M, Nopp, P

Better speech understanding in noise is a major common goal of directional multi-microphone systems for cochlear implants and bilateral cochlear implantation. However, the approaches differ in a number of other relevant aspects such as need for a second implantation or the ability to localize sound sources. Different directional multi-microphone systems have been proposed, ranging from simple two microphone systems mounted in a behind-the-ear (BTE) housing to complex adaptive devices using four or more microphones.
In this ongoing research project, the performance of five different signal processing/presentation strategies is compared: (1) monaural presentation using a single omnidirectional microphone, (2) monaural presentation using a simple two-microphone beamformer in a single BTE unit, (3) monaural presentation using a complex 4 microphone adaptive beamformer, (4) binaural presentation using two omnidirectional microphones, and (5) binaural presentation using two simple directional two-microphone beamformer in two separate BTE units mounted close to the ears of the listener.
The speech reception threshold (SRT) and the minimal audible angle (MAA) of adult, bilaterally implanted cochlear implant users is compared. SRT was measured in room featuring a moderate yet realistic reverberation time of 0.34 s using speech presented at 0° azimuth and speech spectrum noise emitted by a loudspeaker at an azimuth of 90°. For all conditions in which microphones at only one side of the head were used, the noise source was placed at the contralateral side of the head.
Preliminary results from experiments with two bilaterally implanted adults suggest that the most favourable SRT can be achieved using the complex 4-microphone adaptive beamformer (average SRT over both subjects -11.9 dB), followed by the monaurally and binaurally used simple beamformer (conditions 2 and 5, SRT –3.4 and –3.2 dB respectively). The conditions involving omnidirectional microphones with either both cochlear implants (condition 4, SRT –1.1 dB) or only one implant (condition 1, SRT +0.3 dB) resulted in the poorest SRT.
MAA were between 16° and 32° under both conditions involving binaural signal presentation (conditions number 4 and 5). Under all other conditions, in which only one cochlear implant was used, MAA was above 128°, i.e. above the highest value considered in the test-protocol.
These preliminary data suggest that even simple two-microphone beamformers can outperform binaural presentation using omnidirectional microphones in cochlear implant users. Bilateral cochlear implants and multi-microphone beamforming systems seem to complement each other favourably. Sound localisation, i.e. measurable MAA values, were observed only if signal presentation was binaural, thus taking advantage of both cochlear implants.

[Supported by the Swiss national Science foundation and Medel GmbH, Austria]

Contact Information

Department of ENT, Head, Neck and Cranio-Maxillo-Facial Surgery
Inselspital, University of Berne
E-mail: martin.kompis@insel.ch 		 
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