Cochlear implant microphone location affects speech recognition in diffuse noise

Abstract

Background: Despite improvements in cochlear implants (CIs), CI recipients continue to experience significant communicative difficulty in background noise. Many potential solutions have been proposed to help increase signal-to-noise ratio in noisy environments, including signal processing and external accessories. To date, however, the effect of microphone location on speech recognition in noise has focused primarily on hearing aid users.

Purpose: The purpose of this study was to (1) measure physical output for the T-Mic as compared with the integrated behind-the-ear (BTE) processor mic for various source azimuths, and (2) to investigate the effect of CI processor mic location for speech recognition in semi-diffuse noise with speech originating from various source azimuths as encountered in everyday communicative environments.

Research design: A repeated-measures, within-participant design was used to compare performance across listening conditions.

Study sample: A total of 11 adults with Advanced Bionics CIs were recruited for this study.

Data collection and analysis: Physical acoustic output was measured on a Knowles Experimental Mannequin for Acoustic Research (KEMAR) for the T-Mic and BTE mic, with broadband noise presented at 0 and 90° (directed toward the implant processor). In addition to physical acoustic measurements, we also assessed recognition of sentences constructed by researchers at Texas Instruments, the Massachusetts Institute of Technology, and the Stanford Research Institute (TIMIT sentences) at 60 dBA for speech source azimuths of 0, 90, and 270°. Sentences were presented in a semi-diffuse restaurant noise originating from the R-SPACE 8-loudspeaker array. Signal-to-noise ratio was determined individually to achieve approximately 50% correct in the unilateral implanted listening condition with speech at 0°. Performance was compared across the T-Mic, 50/50, and the integrated BTE processor mic.

Results: The integrated BTE mic provided approximately 5 dB attenuation from 1500-4500 Hz for signals presented at 0° as compared with 90° (directed toward the processor). The T-Mic output was essentially equivalent for sources originating from 0 and 90°. Mic location also significantly affected sentence recognition as a function of source azimuth, with the T-Mic yielding the highest performance for speech originating from 0°.

Conclusions: These results have clinical implications for (1) future implant processor design with respect to mic location, (2) mic settings for implant recipients, and (3) execution of advanced speech testing in the clinic.

Figure 1

Physical output of the BTE mic (A) and T-Mic (B) are shown as a function of frequency for the 0 (dashed line) and 90° (solid line) source azimuth. (C) displays the mic response difference, in dB, between the 0 and 90° as a function of frequency for the T-Mic (solid line) and BTE mic (gray line). In (C), a negative value indicates that the mic output was greater for sources originating from 90° as opposed to 0°.

Figure 2

Mean TIMIT sentence recognition (in percent correct) for source azimuths of 0 and ±90° in all three microphone conditions: T-Mic (filled circles), BTE mic (unfilled circles), and 50/50 (shaded circles). Panels A and B of Figure 2 display unilateral (n = 11) and bilateral, best-aided (n = 9) listening conditions, respectively.