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The changes to interaural acoustics imparted by placing circumaural headphones over bilateral cochlear-implant sound processors

Interaural level differences (ILDs) are the primary binaural cue used by bilateral cochlear-implant (BiCI) listeners for horizontal-plane sound localization. As such, factors affecting their fidelity are important to understand. One approach for delivering binaural stimuli to BiCI listeners is to present virtually spatialized signals via circumaural headphones placed over the listener’s sound processors. An assumption of this approach is that the binaural cues presented are relatively unaltered by the transmission process and headphone placement; yet there is a lack of evidence supporting this assumption. Therefore, this study measured the effect of small changes to headphone placement on the ILDs received by BiCI sound processors. The intent was to determine both the extent and frequency range in which ILDs are affected using recordings of sine-sweeps received by the device microphones. Results show that slight changes in headphone placement affect coupling to the sound processor microphones and outer ear, primarily influencing ILDs between 1 and 5 kHz as much as 8.6 dB at the midline. They also show that occluding the listener’s ear canals with earplugs or moldable putty significantly reduces ILD variability from inconsistent headphone placement. In summary, these data suggest ways to improve this presentation method for BiCI research studies.