PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Effects of cartilage conduction vibrator placement in the pinna onthe detection threshold and the ear canal sound pressure

Toru JITSUKAWA1; Sho OTUSKA1, 2, 3; Seiji NAKAGAWA1, 2, 3
1

Dept. of Medical Eng., Graduate School of Sci. & Eng., Chiba University, Japan
2

Center for Frontier Medical Eng., Chiba University, Japan

3Med-Tech Link Center, Chiba University Hospital, JapanABSTRACT
Bone-conduction has been used as a hearing aids for people with conductive hearing impairment, but there
was a problem in the wearability of the vibrator. To solve the problem, cartilage-conduction (CC) thattransmits the vibrator to the auricular cartilage has been proposed and applied to hearing aids andsmartphones. The characteristics of CC perception vary depending on a number of variables, including theplacement of the vibrator within the pinna, the contact area of the vibrator and the contact pressure, but theeffects of such conditions have not been studied. In this study, we observed variations of detection thresholdand ear canal sound pressure (ECSP) depending on the position of the vibrator within the auricle. Tone bursts
with frequencies from 250 to 8,000 Hz were transmitted to the upper, middle and lower parts of the auricle.
Both experiments were conducted under conditions in which the vibrator did not come into contact with theauricle. The results showed that the threshold decreased and the ECSP increased when transmitted to themiddle and lower auricles at lower frequencies (250 and 500 Hz). These results provide useful information
not only to optimize the devices using CC, but also to elucidate the peripheral mechanism of CC perception.
Keywords: Bone-conduction, Cartilage-conduction, Placement of vibrator, Threshold, Ear canal soundpressure

1. INTRODUCTION
Normally, sound enters the ear canal as a vibration of air and propagates through the middle ear tothe cochlea of the inner ear. Thus, the sound propagating through vibration of air is called as the airconducted sound. On the other hand, the sound transmitted through body structure such as the skin,muscles and skull is called bone-conducted sound. In bone-conduction (BC), the sound is said to betransmitted as 4 components(1): (a) the air-conducted component that is emitted from the vibrator anddiffracted to enter the ear canal, (b) the osseotympanic component which involves sound radiated into
the ear canal, (c) the inertial osteogenic component which is based on the relative motion between the
middle ear ossicles and the temporal bone, and (d) the compressed osteogenic component which
results from compression and expansion of the cochlear shell (Figure 1 ).

t-jitsukawa@chiba-u.jp
otsuka.s@chiba-u.jp
s-nakagawa@chiba-u.jp6595

Figure 1 Sound components transmitted by bone-conduction
Thus, some bone-conducted components are transmitted to the inner ear without passing through theouter and middle ears. Therefore, BC has been used as hearing aids for people with conductive hearing
loss caused by injuries on the outer and middle ears. BC has been applied to devices such as hearing
aids and headphones, however, problems such as pain, discomfort and the difficulty of securing thetransducer, have hindered the spread of its application. Therefore, to address these issues, cartilage
conduction (CC) has been proposed (2), and applied to hearing aids (3) and smartphones (4).
However, only a limited number of studies have been conducted on CC and the details of theperception characteristics and propagation mechanisms remain unclear. Even in CC, the sound isthought to be transmitted as the same components as the ordinary BC, and the osseotympaniccomponent and air-conduction components are predominant in the normal hearing (4). The
characteristics of CC perception vary depending on a number of factors including the placement of
the vibrator within the pinna, the contact area of the vibrator and the contact pressure. Despite this,the effects of such stimulus conditions have not been studied.
In this study, we observed variations of the detection threshold and ear canal sound pressure(ECSP) depending on the position of the vibrator within the pinna.

2. Methods
2.1 Vibrator and placements
A vibrator with a plastic diaphragm attached to a piezoelectric element (Figure 2) was attached tothe upper part (mainly ear rings), center part (mainly paired rings) and lower part (mainly tragus) of
the subjects’ pinnae (Figure 3). In addition, for comparison, we also presented stimuli in the “noncontact” condition in which the vibrator did not touch the pinna.

Elastomer
Acrylic plate

Piezoelectric vibrator
Figure 2A Schematic diagram of the cartilage-conduction vibrator

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Figure 3 Placements of the vibrators in the experiments
2.2 Measurement of the threshold
The detection threshold at each presentation placement was measured for each condition. Tone
bursts of 0.25, 0.5, 1, 2, 4 and 8 kHz (duration 800 ms, including 150 ms rising/falling ramps) werepresented. The detection threshold was estimated using a 3-altanating-forced-choice using a 2 down1 up method. The experiments were coonducted in an anechoic chamber.
2.3 Measurement of the sound pressure in the ear canal
A probe microphone was inserted into the subject's ear canal, and the sound pressure in the ear
canal was measured for each vibrator placement. The tip of the probe microphone was inserted near
the tympanic membrane. Tone bursts of 0.25, 0.5, 1, 2, 4 and 8 kHz (duration 1 s, including 50 ms
rising/falling ramps) were presented. The experiments were conducted in an anechoic chambe

3. ResultAn analysis of variance (ANOVA) showed a significant effect of the vibrator placement on thedetection threshold (p < 0.05). In addition, a Holm’s multiple comparison showed significant
differences between the measurements taken at the “center”-“lower” and “upper”-“non-contact”
placements (p < 0.05). In terms of the ECSP, the vibrator placement had a significant effect (p < 0.05),
and a Holm’s multiple comparison showed significant differences between the measurements taken atthe “center”-“lower” and “upper”- “non-contact” placements (p < 0.05).

4. CONCLUSION
A significant difference was observed between the contact- and the non-contact conditions on both
the detection threshold and the ECSP. It is inferred that the middle bass range was amplified due tothe osseotympanic component. On the other hand, no significant difference was observed between“upper” and “non-contact” placements on both the detection threshold and the ECSP. This result
suggests that the cartilage was not vibrated well and the necessary amount of the osseotympanic
component was not obtained by the stimulus to the upper pinna.

ACKNOWLEDGEMENTS
A part of this research was conducted with the support of JSPS KAKENH Grant Number 17H02079
and a Research Grant in the Natural Sciences from the Mitsubishi Foundat ion for SN.

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