1. Overview

2. Cartilage conduction in daily life

3. Practical tools utilizing cartilage conduction

4. Cartilage conduction is distinct from bone conduction

5. References

Cartilage conduction is a pathway in which sound signals are transmitted to the inner ear. This pathway is distinct from conventional sound pathways such as air or bone. Therefore, cartilage conduction it referred to as the third auditory pathway.

Overview

Since approximately 450 years ago, two pathways have been acknowledged for transmitting sound to the inner ear; one is air conduction and the other is bone conduction. In 2004, Hiroshi Hosoi, a professor at Nara Medical University, discovered the phenomenon of cartilage vibration in the inner ear.^[1] By attaching a vibrator to aural cartilage, sound was found be transmitted into the inner ear well. He proposed the novel concept that cartilage has a unique characteristic capable of transmitting sound.^[2] Because sound transmission via pathways that use air or bone are respectively called air conduction and bone conduction, the new pathway is referred to as cartilage conduction. This unique pathway has attracted the attention of several researchers and companies such that some hearing aid prototypes using cartilage conduction have been tested for use in smartphone and earphone.

Cartilage conduction in daily life

One can hear a leathery sound when cleaning one’s ear canal using a cotton-tipped swab. This is an example of sound transmitted via cartilage conduction. In cartilage conduction, sound (vibration) energy stimulates the cartilages of the external auditory canal and induces air vibration. This air vibration can be then transmitted into the inner ear via the eardrum and middle ear, which in turn activates the cochlear nerve. Air vibration created from cartilage conduction is combined with sound from air conduction at the ear canal, and thus an amplified sound is perceived. This same phenomenon can be experienced when a telephone receiver is pressed against one’s ear when it is difficult to hear the person on the other end of the call.

Practical tools utilizing cartilage conduction

1) Hearing aid

Cartilage conduction can be particularly useful for patients with disorders of the outer ear, including aural atresia, where conventional air conduction hearing aids cannot be used. The basic premise of cartilage conduction hearing aids is that cartilage located outside the ear canal is vibrated, such that the vibration can be transmitted despite the presence of any outer ear disorders. Because hearing contributes to language development, the practical availability of cartilage conduction hearing aids would be beneficial to children who suffer from disorders of the outer ear. Currently, a cartilage conduction hearing aid is being developed by RION CO., LTD.

2) Smartphone/Cell phone

Recently, a smartphone prototype using the benefits of cartilage conduction has been manufactured by ROHM Co., Ltd.^[3] This type of the smartphone provides clearer sound compared with conventional phones, even under noisy circumstances. Furthermore, the sound pressure level can be easily modified by adjusting the physical pressure level of the smartphone pressed against the outer ear. Importantly, the cartilage vibration, which is a sound source, can be heard only by oneself but not by others because sound from cartilage vibration cannot be leaked outside. This makes talking in public easy. In addition, the smartphone’s surface can be kept clean because one’s face or ear does not directly contact the smartphone’s surface. The cartilage conduction phone can also be used simultaneously with conventional air conduction hearing aids, which is good news for hearing-impaired people.

3) Earphone

The ear canal is not physically occluded by an earphone that operates via cartilage conduction. As such, this type of earphone allows one to hear alarm signals. Similar to the cartilage conduction smartphones, no sound is leaked from this type of earphone. Prototypes of such earphones have been manufactured by ROHM Co, Ltd.

4) Robot

Cartilage conduction has been considered for communicating with robots. Currently, loudspeaker systems are used to convey sounds produced by robots. However, such loudspeakers generate loud noises in a room filled with many robots that simultaneously emit speech sounds. Therefore, a man-to-man communication system is needed to overcome this problem. Cartilage conduction can realize better communication than loudspeaker systems.

Cartilage conduction is distinct from bone conduction

It was initially thought that cartilage conduction was a type of bone conduction. However, several studies demonstrated that the mechanisms of cartilage conduction differ from that of bone conduction.^[4][5] Bone conduction is attributed to bone vibration at the skull, which is transmitted to the inner ear. In contrast, cartilage conduction is accounted for by sound (vibration) energy stimulating the cartilage at the external auditory canal to create sonic waves in the ear canal. Such sonic wave energy passes through the eardrum and the middle ear before finally activating the auditory nerve. These different mechanisms would explain the following observed differences between cartilage and bone pathways:

1. A variety of sound pressure levels can be created by contact pressure in cartilage conduction compared with that in bone conduction.

2. A sound in cartilage conduction is more stereophonic than that in bone conduction.

3. Frequency characteristics are different between cartilage and bone conduction.

References

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