Cartilage Conduction Is Characterized by Vibrations of the Cartilaginous Portion of the Ear Canal

Abstract

Cartilage conduction (CC) is a new form of sound transmission which is induced by a transducer being placed on the aural cartilage. Although the conventional forms of sound transmission to the cochlea are classified into air or bone conduction (AC or BC), previous study demonstrates that CC is not classified into AC or BC (Laryngoscope 124: 1214–1219). Next interesting issue is whether CC is a hybrid of AC and BC. Seven volunteers with normal hearing participated in this experiment. The threshold-shifts by water injection in the ear canal were measured. AC, BC, and CC thresholds at 0.5–4 kHz were measured in the 0%-, 40%-, and 80%-water injection conditions. In addition, CC thresholds were also measured for the 20%-, 60%-, 100%-, and overflowing-water injection conditions. The contributions of the vibrations of the cartilaginous portion were evaluated by the threshold-shifts. For AC and BC, the threshold-shifts by the water injection were 22.6–53.3 dB and within 14.9 dB at the frequency of 0.5–4 kHz, respectively. For CC, when the water was filled within the bony portion, the thresholds were elevated to the same degree as AC. When the water was additionally injected to reach the cartilaginous portion, the thresholds at 0.5 and 1 kHz dramatically decreased by 27.4 and 27.5 dB, respectively. In addition, despite blocking AC by the injected water, the CC thresholds in force level were remarkably lower than those for BC. The vibration of the cartilaginous portion contributes to the sound transmission, particularly in the low frequency range. Although the airborne sound is radiated into the ear canal in both BC and CC, the mechanism underlying its generation is different between them. CC generates airborne sound in the canal more efficiently than BC. The current findings suggest that CC is not a hybrid of AC and BC.

Audiogram in Response to Stimulation Delivered to Fluid Applied to the External Meatus

“…Tabuchi, et al [22] conducted a "liquid test" in order to differentiate between ossicular fixation and ossicular discontinuity. Nishimura, et al [23] applied water to the ear canal in order to analyze what they refer to as "cartilage conduction." In both studies, pure tone stimuli were delivered to fluid in the canal, and lower (better) thresholds were found at the lower frequencies, similar to the results of the present study.…”

Cartilage conduction as the third pathway for sound transmission

“…As shown in Fig. 2, we assumed that there are three possible pathways for CC [12][13][14][15][16][17][18][19][20]. The first possibility is a direct air pathway.…”

“…After Hosoi discovered CC, we developed a piezoelectric bimorph type of transducer with a ring-shaped attachment dedicated to effectively vibrating the aural cartilage [11]. Since then, we have attempted to identify the specific characteristics of CC using the new transducer [11][12][13][14][15][16][17][18][19][20][21][22][23]. As the result of our studies, we have proposed that CC could be a third independent pathway.…”

Cartilage conduction is the third major pathway for sound transmission

“…With regard to the occlusion effect, it is a common characteristic of BC and CC. However, these two types of conduction have many different characteristics caused by different mechanisms, such as lateralization of sound [3] , influence of earplugs on the hearing threshold [4] , and a threshold shift by water injection in the ear [5] .…”