A Two-Microphone Noise Reduction System for Cochlear Implant Users with Nearby Microphones—Part II: Performance Evaluation

Abstract

Users of cochlear implants (auditory aids, which stimulate the auditory nerve electrically at the inner ear) often suffer from poor speech understanding in noise. We evaluate a small (intermicrophone distance 7 mm) and computationally inexpensive adaptive noise reduction system suitable for behind-the-ear cochlear implant speech processors. The system is evaluated in simulated and real, anechoic and reverberant environments. Results from simulations show improvements of 3.4 to 9.3 dB in signal to noise ratio for rooms with realistic reverberation and more than 18 dB under anechoic conditions. Speech understanding in noise is measured in 6 adult cochlear implant users in a reverberant room, showing average improvements of 7.9-9.6 dB, when compared to a single omnidirectional microphone or 1.3-5.6 dB, when compared to a simple directional two-microphone device. Subjective evaluation in a cafeteria at lunchtime shows a preference of the cochlear implant users for the evaluated device in terms of speech understanding and sound quality.

A Two-Microphone Noise Reduction System for Cochlear Implant Users with Nearby Microphones—Part I: Signal Processing Algorithm Design and Development

“…We believe that this is a minor problem. Beamforming can be switched off in these situations, adaptation takes a few milliseconds in which the signal is still well audible andexcept under anechoic conditions-signal suppression is rarely great enough to actually miss a signal completely [15]. However, as situations outside of buildings may approach an anechoic environment, we propose a leakage control algorithm to alleviate this problem.…”

“…With the above supporting algorithms, very simple or more complex beamformers can be designed, as needed. In accordance with the aims of this research stated in the introduction, we will concentrate on a small computationally inexpensive version in Section 4 and [15].…”

“…A portable beamforming device implementing the algorithm in Figure 1 was built in order to be able to perform experiments in real-time, with cochlear implant users and in real environments [15]. The system is built around a 16 bit fixed point digital signal processor (Motorola DSP56F826) and uses a Cirrus Logic CS42L50 sigma-delta Stereo CODEC with 24-bit quantization.…”

“…In this paper, a simple adaptive beamformer with two nearby microphones is introduced. In [15], the system is evaluated in simulated rooms and real acoustic environments using a portable real-time prototype of the proposed system. The evaluation includes physical measurement as well as speech understanding test in noise and subjective assessments of 6 cochlear implant users.…”

A beamformer post-filter for cochlear implant noise reduction

“…Using a single, unilateral sound processor with two microphones, large improvements due to adaptive beamforming have been demonstrated for cochlear implant recipients, especially in specific acoustic scenarios involving a single interfering noise. Improvements were 7-16 dB SRT (Wouters and Vanden Berghe, 2001), 10 dB SRT (Spriet et al, 2007), 8-10 dB SRT (Kompis et al, 2008), and 56 percentage points in word recognition (Chung and Zeng, 2009). As the number of noise sources is increased to create a more diffuse noise field, the benefit is somewhat moderated but still significant, e.g., 2-12 dB SRT (Spriet et al, 2007), 3.5 dB SRT (Chung et al, 2006), 6.1 dB SRT (Gifford and Revit, 2010), and 47 percentage points word recognition (Chung and Zeng, 2009).…”