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Cartilage Conduction Hearing
Cartilage conduction (CC) is a form of conduction that allows a relatively loud sound to be audible when a transducer is placed on the aural cartilage. The CC transmission mechanism has gradually been elucidated, allowing for the development of CC hearing aids (CC-HAs), which are clinically available in Japan. However, CC is still not fully understood.
The sound transmission pathway to the cochlea is generally classified into air and bone conduction (AC and BC). For AC, sound generated outside the ear travels to the eardrum through the ear canal and is transduced into vibrations of the ossicles to reach the cochlea. For BC, skull bone vibrations induced by a transducer are transmitted to the cochlea, involving at least five components [1,2,3]. Sound can also be perceived by body part vibrations other than the skull bone [4,5,6], and the transmission mechanisms are unique from one another. When the transducer is placed on the aural cartilage, particularly on the tragus, a relatively loud sound is audible . This form of conduction is referred to as cartilage conduction (CC) . Generally, hearing through non-osseous soft tissue conduction is not as clear as conventional BC. However, a clear sound is audible in CC, and it is perceived louder than when a transducer is placed on the mastoid or forehead .
2. The Theoretical CC Transmission Pathway
３. Sound Pressure Level in the Ear Canal via CC
The measurement of the sound level in the ear canal similar to real-ear measurements  contributes towards understanding the phenomenon. objectively demonstrated the loudness increase by measuring the sound pressure level in the ear canal using a probe microphone (Figure 2) . Conversely, to reproduce the difference in the sound pressure level in the ear canal between the attached and unattached conditions, not only the bony portion of the ear canal but also the cartilaginous portion was necessary to consider . The airborne sound generated by a simulated cartilaginous portion (movable plate) played an important role in the reproduction of the sound pressure level in a simulated ear canal.
4. Hearing Threshold Measurements via CC
4.1. Threshold Shift with an Earplug
4.2. Threshold Shift with Water Injected into the Ear Canal
5. CC in Pathological Ears
6. CC Applications
This entry is adapted from 10.3390/audiolres11020023
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