Estimation of relationships between transducer placements and peripheral propagation in cartilage conduction

Bone-conduction (BC) has been applied to hearing aids for the conductive hearing loss, however, also has some disadvantage especially in wearability of a sound transducer. Therefore, as a solution, "cartilage conduction (CC)" has been proposed and applied to devices such as a hearing aid and smartphones. In CC, a sound transducer is placed on the cartilage of the pinna, and the air-conduction (AC) and osseotympanic BC components are dominantly transmitted. However, even in CC, the vibrating surface often contacts not only with the aural cartilage but also with the osseous parts of/around the pinna, and effects of such transducer placement on perception characteristics and propagation mechanisms remain unclear. In this study, we measured hearing thresholds and vibrations of the head when the transducer was placed on (1) the pinna, (2) the mastoid process of the temporal bone, and (3) the ear-front point (middle of between the tragus and the mandibular condyle). The results suggested that the ratios of the inertial and compressional BC components increases when the transducer is placed on the osseous parts, particularly in high frequency range. These findings provide useful information to optimize CC devices and develop a calibration method of CC.