Totally implantable hearing aids: the effects of skin thickness on microphone function.

With the advent of totally implantable hearing aids, the question of the impact of the choice of implantation site on microphone function has yet to be fully addressed. We investigated the effects of skin thickness on microphone function over a 2-week period in a porcine model. Sound attenuation was found to be directly proportional to skin thickness within the range of 0.5 to 3.0 mm. A decrease in resonant frequency of 500 Hz was noted after implantation under a skin thickness of 3.0 mm. The general shape of the frequency response curves was maintained across all thicknesses tested. Attenuation (dB loss) across the range of 500 to 8,000 Hz appeared to be linear over the various skin thicknesses measured, with a regression coefficient (r = .99 at 1 kHz and r = .99 at 3 kHz). To minimize attenuation and exploit the sound-collecting qualities of the external ear, the deep meatal skin appears to be a favorable position for implantation.