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

Influence caused by placement of a bone-conducted vibrator onsound transmission
Xiuyuan QIN1; Sho Otsuka2; Seiji NAKAGAWA3
1,2,3

Chiba University, Japan

ABSTRACT
Bone conduction (BC) is another way of sound transmission for our hearing perception besides of airconduction. It conducts sound through skin and skull, bypassing part of the ear canal. The position where thevibrator is put on one’s head makes a difference in BC transmission. Although standards of bone conduction
were established on the position of the mastoid (the prominence of the skull behind the ear), nowadays
manufacturers favor bone-conducted headphone whose vibrator is put on the position of the condyle (thearticular process of the ramus of the mandible bone, in front of the ear). However, there are not enough data indetails for the condyle. In this research, hearing thresholds and ear-canal sound pressure on the condyle andthe mastoid were measured, and a comparison between them was made to investigate the difference of BC
transmission between the two positions, followed by a discussion on the influence of placement on boneconduction.
Keywords: Bone conduction, Mastoid, Condyle

1. INTRODUCTION
Bone conduction (BC) is the conduction of sound by human’s skin and skull (1). It is difficult toestimate the frequency characteristics of BC since many linear and nonlinear mechanisms are involvedin the BC transmission(2), and various factors affect people’s hearing perception of BC, such as thestatic force added to vibrators or different placements of a vibrator on one’s head. There is priorresearch which has studied the influence of placements on three positions: the f orehead, the mastoid,
and the vertex (3, 4). They showed that if a vibrator is put on the mastoid, the lowest mean thresholds
could be obtained. This discovery makes the positions of the mastoid become a normal position used inBC measurements, especially for testing of BC hearing aids. Standards of bone conduction
measurements also were established on the basement of the mastoid by radioear B -71. Nevertheless,with the development of investigation of BC, manufacturers of BC headphones who tend to place thebone-conducted vibrator on the position of the mandibular condyle increasingly appeared because of abetter design for easily using. Compared to the position of the mastoid which was studied for several
years and almost became the basis for standards, there are not enough data of the condyle. In 2008, Mc
Bride (5) used a bone-conducted vibrator Oticon 20 to make a comparison of the perception
performance among different locations on people’s head, such as the condyle, the mastoid, theforehead, the vertex and other positions which were commonly measured for electroencephalograph. It
showed the vibrator on the condyle generated the lowest overall threshold levels in their experiments.
Additionally, speech intelligibility of bone conduction transducer was studied (6, 7), but it is still
controversial that if significant differences in performance between the mastoid and the condyle exist
or not. Since it is unclear that where should be the best position for placing a bone -conducted vibrator,
and there are not enough data of the condyle, it is necessary to investigate characteristics ofbone-conducted transmission on this position. In this paper, hearing thresholds and ear-canal soundpressure (ECSP) of bone conduction on the condyle and the mastoid were measure d by using radioear
B-71 which is a bone transducer used as a reference in international standards such as ISO 389 (8). By
comparing the data of the two positions, the influence of placements on bone -conducted transmission
was investigated, and the difference between conditions with/without earplugs in the experiments wasalso discussed.

1

qinxiuyuan@chiba-u.jp
Otsuka.@chiba-u.jp
3s-nakagawa@chiba-u.jp26519

2. EXPERIMENT
2.1 Fundamental Setup
Six subjects with normal hearing (three males and three females) with an average age of 22 years
participated in the threshold measurements. Their hearing thresholds were better than 20 dB hearing
level for AC stimulation in the frequency range of 0.125 kHz to 8 kHz. All measurements wereperformed in an anechoic room. As stimuli, tone bursts that were sampled at 48 kHz, and had 16 -bit
resolution with 0.5 s duration were used. In this experiment, six frequencies from 0.25 kHz to 8 kHz
were measured. The measured bone conduction vibrator was radioear B -71.
2.2 Hearing threshold measurement
Each subject was equipped with a headband for fixing the bone-conducted vibrator, and the staticforce was controlled to about 3±0.5 N. Since bone-conducted transmission will be influenced by air
conduction (AC), which is leaked from vibration, the condition whether subjects' ear canals are open
or not becomes integral to BC threshold measurements. In this experiment, both open-canal andoccluded-canal conditions were measured. Subjects were required to use earplugs to block their earcanal when the data on an occluded condition need to be measured. On both two conditions, thethreshold on the condyle and the mastoid were measured in order to estimate the influence fromplacements of the bone-conducted vibrator.
For obtaining the BC thresholds, an adaptive method was used. The estimation was made by athree-alternative forced choice and a two-down one-up method (9). Subjects were given three options
in a sequence and asked to choose which of them was indicated as a pure tone stimulus.
2.3 Ear-canal sound pressure measurement
Ear-canal sound pressures were measured using probe microphones (ER-7C, Etymotic Research).
Subjects were also required to use earplugs to block their ear canal when the data on a n occluded
condition need to be measured. On both two conditions, the ear-canal sound pressure on the condyleand the mastoid were measured in order to estimate the influence from placements of thebone-conducted vibrator. The settings of stimuli were the same as the measurements of BC hearing
threshold.

3. RESULTS
3.1 Hearing threshold
Figure 1 shows the average results of hearing threshold measurement of bone conduction. The
horizontal axis represents frequency, and the vertical axis is the relative level of hearing thresholdreferring to interior voltage in MATLAB. When the measured vibrator is put on the condyle, the lowest
thresholds were found at 1 kHz both on the open-canal condition and the occluded-canal condition,and the thresholds were descending from 0.25 kHz to 1 kHz while the thresholds were ascending above
1 kHz. About the difference between open and occluded condition on the condyle, at 0.25 kHz thethreshold on an occluded condition was lower than it was on open condition but from 4 kHz to 8 kHz
the threshold on an open condition became lower. It shows the difference between open and occluded
condition on the condyle depends on the change of frequency.
The influence of earplugs insertion also appeared on the mastoid. When the measured vibrator isput on the mastoid, the lowest threshold was found at 1 kHz when the ear canals were occluded but
when the ear canals were open the lowest threshold was at 4 kHz. It was similar with the frequency
characteristics on the condyle, which shows a decreasing threshold below 1 kHz and a reversingtendency above 1 kHz on the occluded condition on the mastoid. About the difference between open
and occluded condition on the mastoid, from 0.25 kHz to 2 kHz the threshold on an occluded condition
was lower than it was on open.
Comparing the results of the mastoid to the condyle, on the open-canal condition the thresholds
obtained on the condyle were lower than the thresholds on the mastoid at all frequencies. AC may play
an important role on a lower threshold of the condyle. On the occluded-canal condition, the thresholds
obtained on the condyle were also lower than the thresholds on the mastoid from 0.5 kHz to 2 kHz. Itshows the possibility that there are other factors besides AC which influence the BC transmission.

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3.2 Ear-canal sound pressure
Figure 2 shows the results of BC ECSP on the four measured conditions. The horizontal axis
represents frequency, and the vertical axis is sound pressure level. When the vibrator was put on thecondyle, the ear-canal sound pressure on the open condition was higher than it was on the occluded
condition above 1 kHz. A sharp decline was observed above 1 kHz on the occluded condition,
demonstrating an attenuation at high frequency. On the mastoid, the results of the ear-canal soundpressure of an open condition were lower than its occluded condition below 1 kHz. It may be caused bya weaker effect from AC when the vibrator was put on the mastoid. As the comparison between thecondyle and the mastoid, on an open condition the ear-canal sound pressure of the condyle was higher
than it was on the mastoid while the similar results could be observed on the occluded conditions
below 2 kHz. Nevertheless, at high frequency, the ear-canal sound pressure was higher on the mastoid.

4. DISCUSSION
The data showed lower threshold on the condyle on both open/occluded conditions except of 0.25
kHz while higher ECSPs were obtained on the condyle. When a bone-conducted vibrator is put on thehead, not only is the signal transmitted by bone conduction, a part of air conduction is al so transmitted
through the ear canal and affects the results of thresholds. Nevertheless, because the pinna is on theway of signal transmission as a barrier, the influence from air conduction when vibration is reproducedon the mastoid will not as much as it is on the condyle, just as the results of ECSP on the condyle andthe mastoid showed. It is not difficult to hypothesize this is one of the factors which cause lower
thresholds on the condyle when ear canals are open.
When subjects use earplugs in the measurements, the thresholds on the condyle were still lower.

Figure 1 – The hearing thresholds of the four conditions measured by B71.

Figure 2 – The ECSP results of the four conditions measured by B71.

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The results may be related to the pathways of BC transmission. BC transmission mainly includes threemechanisms, inertial BC, compressional BC and osseo-tympanic BC (10). The osseo-tympanictransmission is related to the sound radiated by the ear canal wal ls. An occluded ear canal causes anincrease in the level of sound in the ear canal by bone conduction, called the occlusion effect. At low
frequencies the occlusion effect will have much more effects on thresholds rather than it is at highfrequencies (11). The results showed even though on the occluded condition, ECSPs on the condyle
were higher than the mastoid, probably causing a lower threshold. If compared with mastoid, thevibrator which is put on the condyle is closer to the ear canal. When the sound signal is transmitted by
BC, the vibrator on the condyle may transmit much more vibration on the ear canal, making a
difference on osseo-tympanic transmission while the vibrator on the mastoid is not close enough tomake another influence on the ear canal. It could be a possible explanation for the results that higher
ECSPs were obtained on the condyle.

5. CONCLUSIONSIn conclusion, the threshold on the condyle is lower than it is on the mastoid whatever the ear canalis open or occluded, and the ear-canal sound pressures obtained on the condyle was higher than it wasobtained on the mastoid when inputs at the same level were given, showing a possible explanation ofa lower BC threshold on the condyle at low frequencies. The difference between the results of thecondyle and the mastoid may be caused by the prevention of pinna on the way of air-conductedtransmission on an open condition. On an occluded condition, it may come from the difference causedin osseo-tympanic transmission between the condyle and the mastoid. Additionally, by themeasurement of BC hearing threshold and ear-canal sound pressure, occlusion effect influenced moreon the mastoid, since a larger effect from AC will compensate for the occlusion effect on the condyle.
Although the influence caused by placement on BC transmission at low frequency was discussed, theinfluence at high frequency still needs more measurements in further experiments.

ACKNOWLEDGEMENTS
This research was partially supported by a Grant-in-Aid for Scientific Research (B) Number
17H02079 provided by the Japan Society for the Promotion of Science (JSPS) and Research Grant in
the Natural Sciences from the Mitsubishi Foundation for SN.

REFERENCES
1. Bekesy G. Note on the definition of the term: hearing by bone conduction. J Acoust Soc Am.
1954;26:106-107.
2. Stenfelt S. Transcranial attenuation of bone conducted sound when stimulation is at the mastoid and at
the bone conduction hearing aid position, Otol.Neurotol, 2012; 33:105-114.
3. Studebaker G. Placement of vibrator in bone-conduction testing. Journal of Speech and Hearing
Research.1962; 5(4):321-331.
4. Frank T. Forehead versus mastoid threshold differences with a circular tipped vibrator. Ear and Hearing.
1982; 3(2):91 - 92.
5. McBride M, Letowski T, and Tran P. Meteorological measurements for use in sound propagation
calculations. Proc 7th Long Range Sound Propagation; 24-26 July 1996; Ecully, France 1996. p.
137-47.
6. Osafo-Yeboah B, Gripper M and McBride M. Effect of bone con- duction vibrator placement on speech
intelligibility using the callsign acquisition test - a pilot study. Proc. Annual industrial engineering
management systems conference; 2006.
7. Stanley R, and Walker B. Intelligibility of bone-conducted speech at different locations compared toair-conducted speech. Human Factors and Ergonomics Society Annual Meeting Proceedings; 2009.
8. ISO 389-3: Acoustics-Reference zero for the calibration of audiometric equipment-Part3: Reference
equivalent threshold force levels for pure tones and bone vibrators.
9. Levitt H. Transformed up-down methods in psychoacoustics. J.Acoust.Soc.Am. 1970; 49(2):467 - 477.
10. Stenfelt T, etc. Factors contributing to bone conduction: The outer ear. J. Acoust. Soc. Am. 2003;
113(2): 902 - 913.
11. Stone M, etc. A technique for estimating the occlusion effect for frequencies below 125 Hz. Ear Hear.
2014; 35(1):49 - 65.

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