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

Evaluation of Propagation Characteristics of Bone-conducted
Ultrasound Presented to the Neck, Trunk and Arms
Riki OGINO 1; Sho OTSUKA1, 2, 3; Seiji NAKAGAWA1, 2, 3
1

Department of Medical Engineering, Graduate School of Science & Engineering, Chiba University, Japan
2

Center for Frontier Medical Engineering, Chiba University, Japan
3

Med-Tech Link Center, Chiba University Hospital, Japan

ABSTRACT
High-frequency sound above 20 kHz can be heard clearly via bone conduction (Bone-conducted ultrasound:BCU). Additionally, BCU is perceived even when presented to body parts distant from the head, like the neck,trunk, and arms, and is expected to be applied for novel devices that transmit sound selectively to persons
who touched by a vibrator on the arm or other parts of the body. First, hearing thresholds were measured
when 30-kHz tone-burst was presented to the neck, trunk and arms in normal hearing participants. The results
showed that BCU presented to the distal parts, including the lower arm, can be perceived at least in normal
hearing participants, but threshold increased depending on the distance from the head. Second, the vibrationof the ear canal was measured using an acceleration sensor when a 30-kHz tone was presented to the neck,arms, and trunk. A prominent spectrum peak corresponding to the stimulus frequency was obtained. Further,
the vibration tended to attenuate when the stimulus points were moved away from the head, although someexceptions existed. Third, the vibration of the upper limb was measured at 9 surface points lined up from theforearm to shoulder when BCU was presented near the wrist. The vibration basically tended to attenuate
depending on the distance from the vibrator; however, some points at raised upper-arm muscle showed larger
amplitudes. These results suggest that efficiency of the propagation varies depending on the stimulus points,
and the upper-arm muscle are suitable also on a stimulus location of BCU.
Keywords: Bone conduction, Ultrasound, Upper arm, Trunk, Threshold, Sound propagation.

1. INTRODUCTION

Several studies have shown that high frequency sound above 20 kHz can be heard clearly via boneconduction (Bone-conducted ultrasound: BCU) (1-3). Moreover, BCU hearing in humans has beenfound under various auditory pathological conditions, including sensorineural hearing loss andmiddle-ear disorders (3). In addition, BCU can be perceived even when presented to body parts
distant from the head, like the neck, trunk, and arms (4) and expected to be applied for novel devices
that transmit sound selectively to persons who touched by a vibrator on the arm or other parts of theupper body. However, there are many unclear points in details of perceptual characteristics andpropagation mechanisms in distantly presented BCU.
In this study, in order to acquire useful information on elucidation of perceptual characteristics
and propagation characteristics, hearing thresholds, the vibration in the external auditory meatus, andthe vibrations of the upper limb were measured.

2. EXPERIMENTS
2.1 Participants and Stimulus Locations
Seven participants (male, 21-24 years) who had no history of deficits of hearing functions
participated in the experiment.
BCU stimuli were presented to the 8 locations: (A) the mastoid process of the temporal bone, (B)the sternocleidomastoid muscle, (C) the sternal extremity of the clavicle, (D) the acromial process,1s-nakagawa@chiba-u.jp

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Figure 1 – Stimulus locations(E) the brachialis muscle, (F) the brachioradial muscle, (G) the xiphoid and the thoracic vertebrae(Figure 1), by a piezoelectric ceramic vibrator (Murata Manufacturing MA40E7S). To the mastoid
process, the vibrator was pressed using a head band. For other locations, an elastic band was used tohold the vibrator.
2.2 Experiment 1: Measurement of Hearing Thresholds (5)Hearing thresholds were measured using a 2 up-1 down three-alternative forced-choice (3AFC)
adaptive procedure. A 30-kHz tone-burst was used as the BCU stimulus.
The results showed that BCU presented to the distal parts, including the lower arm, can beperceived at least in normal hearing participants, but threshold increased depending on the distancefrom the head.
2.3 Experiment 2: Measurement of the Vibration in the Ear Canal (6)In order to acquire useful information on elucidation of perceptual characteristics, as in previous
studies (7), the acceleration sensor (Ono Sokki NP-3211) wrapped in a 10-mm diameter urethanematerial was inserted in the left ear canal, and the vibration generated by the 30-kHz BCU tone in theexternal ear canal was measured. The frequency spectrum was calculated from obtained signals.
The peak of the spectrum corresponding to the stimulation frequency (30 kHz) was clearlyconfirmed at all tested body parts. On the other hand, no significant peak other than 30 kHz wasconfirmed. As the stimulation part moved away from the head, the vibration tended to attenuate.
However, although the distance from the thoracic vertebrae and the xiphoid were almost equal, thevibration was larger for the thoracic vertebrae than the xiphoid.
2.4 Experiment 3: Measurement of the Vibration of the Upper Limb (6)In order to investigate only attenuation due to propagation distance, when BCU was presented tonear the wrist, the vibration of the upper limb was measured at 9 surface points lined up straight fromthe next division of the stimulation site (measurement point 1) to the acromial process (measurement
point 9). The acceleration sensor was attached to each measurement point with nonwoven surgical tape.
The frequency spectra were calculated from obtained signals.
At all subjects and measurement points, spectral peaks corresponding to the stimulation frequency(30 kHz) were obtained. Figure 2 shows the spectral peak value corresponding to thestimulus-frequency obtained at each measurement point. The surface vibration decreased as thedistance from the vibrator increased at the lower arm or the oval head point. On the other hand, at theupper arm, although the distance from the vibrator increased, the surface vibration was not attenuated.

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Figure 2 – Acceleration for the 30-kHz component measured at each point on the upper limb.

3. DISCUSSION AND CONCLUSIONSIn this study, to assess basic properties of distal-presented BCU hearing, hearing thresholds, thevibration in the ear canal and the vibration of the upper limb were measured.
The auditory threshold increased as the stimulus locations moved away from the head, and thevibration caused by BCU stimulation in the ear canal attenuated. Also, the vibration on the bodysurface was attenuated as the measurement points moved away from the stimulation point. From theseresults, it is suggested that BCU stimulation is attenuated due to the propagation distance. However, it
was also found that there is a difference in propagation efficiency depending on the stimulus location.
For example, a raised and hard location like the upper arm or the thoracic vertebrae is suitable as a
stimulation point.
These results provide useful information not only for the improvement of the existing BCU hearing
aid, but also for the development of novel “distantly-presented BCU” devices that can provide soundinformation selectively to at specific person who touches the device with their arms or hands.

ACKNOWLEDGEMENTS

This work was supported by JSPS KAKENHI Grant Number 17H02079 and a Research Grant in theNatural Sciences from the Mitsubishi Foundation for SN.

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