Transmission properties of bone conducted sound: measurements in cadaver heads.

In the past, only a few investigations have measured vibration at the cochlea with bone conduction stimulation: dry skulls were used in those investigations. In this paper, the transmission properties of bone conducted sound in human head are presented, measured as the three-dimensional vibration at the cochlear promontory in six intact cadaver heads. The stimulation was provided at 27 positions on the skull surface and two close to the cochlea; mechanical point impedance was measured at all positions. Cochlear promontory vibration levels in the three perpendicular directions were normally within 5 dB. With the stimulation applied on the ipsilateral side, the response decreased, and the accumulated phase increased, with distance between the cochlea and the excitation position. No significant changes were obtained when the excitations were on the contralateral side. In terms of vibration level, the best stimulation position is on the mastoid close to the cochlea; the worst is at the midline of the skull. The transcranial transmission was close to 0 dB for frequencies up to 700 Hz; above it decreased at 12 dB/decade. Wave transmission at the skull-base was found to be nondispersive at frequencies above 2 kHz whereas it altered with frequency at the cranial vault.