On the transmission of sound generated by an electromagnetic device from the mastoid process to the petrous bone

An electromagnetic device (EMD) for sound transmission from the mastoid process to the petrous part of the temporal bone was designed, built, and analyzed. A theoretical model was developed in order to predict the EMD acoustical performance and the distortion caused by skull bones. This model enabled the investigation of various technical improvements and calibration options. The EMD was miniaturized by using rare earth magnets in the construction of both external transmitter and internal receiver. In vitro implantation of the internal receiver into the mastoid process was carried out by simple mastoidectomy techniques. The input and output powers of the system, including the EMD and skull bones components, were measured and the power transfer function was calculated. The sound, which was generated by the EMD and transmitted through the dry as well as simulated cadaver skull, had high fidelity in spite of some vibratory distortion at the lower sonic frequencies. Results suggest that further development of hearing aids based on electromechanical sound transmission should take into account the specific acoustic response pattern of the skull bones.