Sensitivity to bone-conducted sound: excitation of the mastoid vs the teeth

The sensitivity of nine subjects to bone-conducted sound was measured at three positions: osseointegrated percutaneous titanium implants in the temporal bone, the skin-covered mastoid and the teeth. Voltage levels supplied to a bone-anchored hearing aid (BAHA) transducer and to an Oticon bone-transducer were measured and the thresholds obtained at the three positions were compared. Using the mechanical impedance of the teeth, the parameters of a first order model for the vibration transmission through the teeth was calculated. Also, the equivalent force thresholds were calculated from the voltage threshold levels. The sensitivity to bone-conducted sound, for both voltage and force thresholds, conformed fairly well at the three positions for frequencies below 1 kHz; however, above 1 kHz, bone-conducted sound applied at the titanium implant becomes more sensitive than at the two other positions investigated. It was concluded that the teeth can be used for the application of bone-conducted sound, in particular for pre-operative assessment of a BAHA and to facilitate service and quality control of such a hearing device.