Fig 1.
Sound transmission pathways in cartilage conduction.
Part A shows three possible transmission pathways when the transducer is placed on the cavity of the concha [8, 9]. 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).
Fig 2.
Subject’s head condition during the measurements.
The subject’s head was fixed in the condition of the entrance of the ear canal facing up to the ceiling. The pillow had the space which enabled a masker earphone to be worn without the interference with the pillow.
Fig 3.
Cartilage conduction transducer.
Part A and B show the cartilage conduction transducer and its fixation in the cavity of the concha.
Fig 4.
Threshold-shifts by the 40%- (A) and 80%- (B) water injections.
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.
Fig 5.
Threshold-shifts by the seven water-injection conditions in cartilage conduction.
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.
Fig 6.
Thresholds in the 80%-water injection condition for bone and cartilage conductions.
The thresholds were represented in force level referring to 1 μN. Vertical bars indicate standard deviations.