Introduction
Recipients of hearing implants such as cochlea implants or active middle ear implants (AMEI) are often contralaterally treated with a conventional hearing aid. These are capable of delivering amplified sound exceeding 100 dB SPL into the ear canal. Development of fully implantable devices must take into account a possible crosstalk between the contralateral hearing aid and the implanted microphone. At our clinic, nearly 50 patients were recently implanted with a CARINA fully implantable AMEI – a rare opportunity to study the sound transmission through the skull with an implanted sensor in vivo.
Materials and Methods
18 Participants were recruited from patients with a CARINA implant on one side and an intact outer ear morphology contralaterally. We measured transfer functions from 4 actuators to 4 sensors at various points on the skull. Main focus was the sound transfer from a contralateral in-ear speaker to the ipsilaterally implanted subcutaneous microphone, with sound pressure directly above the microphone position as a reference.
Results
We find that on average, contralateral crosstalk to the subcutaneous microphone was about 30 dB below ipsilateral sound coupling.
Discussion
According to current audiometric practice, interaural attenuation (crosstalk from airborne sound to the contralateral cochlea) is assumed to be about 50 dB above 1kHz. Our findings indicate that this may be exaggerated and may in fact be closer to 30 dB across frequencies. Therefore, these results are of interest not only for the development of implantable microphones but may also bear relevance to audiometric testing practices such as masking and air-bone gap measurements.

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