Cartilage conduction is characterized by vibrations of the cartilaginous portion of the ear canal.

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

Threshold-shifts by the seven water-injection conditions in cartilage conduction.

<p>Threshold-shifts from the 0%-water injection condition were described. The water injection first elevated the thresholds at all frequencies. Beyond 60% of the ear canal, the thresholds at 0.5 and 1 kHz were improved. In addition, when the injected water overflowed to touch the transducer, the thresholds at 2 and 4 kHz were improved. Vertical bars indicate standard deviations.</p

Threshold-shifts by the 40%- (A) and 80%- (B) water injections.

<p>Threshold-shifts from the 0%-water injection condition were described. The additional water injection from 40% to 80% of the ear canal remarkably improved the cartilage conduction thresholds at 0.5 and 1 kHz. Vertical bars indicate standard deviations.</p

Sound transmission pathways in cartilage conduction.

<p>Part A shows three possible transmission pathways when the transducer is placed on the cavity of the concha [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120135#pone.0120135.ref008" target="_blank">8</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120135#pone.0120135.ref009" target="_blank">9</a>]. In the first pathway, vibrations of the transducer directly produces air-borne sound, some of which reaches the ear canal and is transmitted to the cochlea via the conventional pathway in air conduction (AC). This pathway is termed “Direct-AC”. In the second pathway, vibrations of the aural cartilage and soft tissue are transmitted to the cartilaginous portion. These vibrations induce an acoustic signal in the canal which is transmitted by AC to the eardrum. This pathway is termed “Cartilage-AC”. In the third pathway, vibrations of the aural cartilage and soft tissue are transmitted via the skull. This pathway is termed “Cartilage-bone conduction”. Part B, C, and D show the change in the sound transmission when the water is injected into the ear canal. When 40% of the ear canal is filled with water, the surface of the water probably levels at the bony potion. In this condition, the Direct-AC and Cartilage-AC are interrupted (Part B). When the water is additionally injected to 80% of the ear canal, the surface of the water probably reaches the cartilaginous portion. The vibration of the cartilaginous portion is efficiently transmitted to the eardrum, which is mediated by the injected water (Part C). Consequently, if the vibration of the cartilaginous portion contributes to the sound transmission, the threshold will first be elevated by the 40%-water injection, and be improved by the 80%-water injection. When the transducer touches the water (overflowing-water injection condition), the vibration is directly transmitted to the water (Part D).</p

Cartilage conduction transducer.

<p>Part A and B show the cartilage conduction transducer and its fixation in the cavity of the concha.</p

Thresholds in the 80%-water injection condition for bone and cartilage conductions.

<p>The thresholds were represented in force level referring to 1 μN. Vertical bars indicate standard deviations.</p