Acoustic and physiologic aspects of bone conduction hearing.

Bone conduction (BC) is the way sound energy is transmitted by the skull bones to the cochlea causing a sound perception. Even if the BC sound transmission involves several pathways including sound pressure induced in the ear canal, inertial forces acting on the middle ear ossicles and cochlear fluids, alteration of the cochlear space, and pressure transmission through the 3rd window of the cochlea, the BC sound ultimately produces a wave motion on the basilar membrane similar to that of air-conducted sound. The efficiency of the BC stimulation is largely dependent on the skull bone where the skull acts as a rigid body at low frequencies and incorporates different types of wave transmission at higher frequencies. The interaural stimulation difference is determined by the difference between contralateral and ipsilateral BC sound transmission: the transcranial BC sound transmission. To benefit from binaural processing, the transcranial transmission should be low, while the same should be high when using BC hearing aids for unilateral deaf subjects. By appropriately positioning the stimulation, high or low transcranial transmission can be achieved.