Effect of fixation place on airborne sound in cartilage conduction

When a transducer is placed on aural cartilage, relatively loud sound becomes audible in a conduction form termed cartilage conduction (CC). Previous studies have revealed the acoustical differences between CC and conventional air or bone conduction. This study elucidates the working principle of CC through measurements of threshold shifts by water injection into the ear canal under various fixation place conditions. Seven volunteers with normal hearing participated. A lightweight transducer was fixed for three CC conductions (on the tragus, antitragus, and intertragal incisure), and two non-CC conditions (on the pre-tragus and mastoid). Thresholds were measured at 500, 1000, and 2000 Hz in the 0%-, 40%-, and 80%-water injection conditions. Results for the three CC conditions revealed unique features different from those for the non-CC conditions. For the CC conditions, the thresholds increased by the 40%-water injection at all frequencies. However, with additional water injection (80%-water injection), the thresholds decreased at 500 and 1000 Hz; in particular, dramatically at 500 Hz. The results suggest that a direct vibration of the aural cartilage is important to obtaining the significant contribution of airborne sound to hearing above 1000 Hz. Fixation place results in no significant difference in acoustic features among CC conditions.