PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Sound image localization by bone conduction soundKazuki TAKAI 1; Takumi ASAKURA1
1

Tokyo University of Science, Japan

ABSTRACTAn efficient voice-guidance technique by using bone conduction sound. By adopting binaural reproduction
using the head-related transfer function, the playback sounds for guidance are reproduced to the users in order
to indicate the traveling direction. However, the binaural reproduction technique by the bone conduction
sound has not been established yet compared with the air-conducted sound. Therefore, in this study, a soundimage localization experiment was carried out using a bone conduction sound reproduction method in which
the intended direction of the reproduced sound images are quickly panned to left and right in order to enhance
the perception of the sound direction. The results of the sound localization experiment with the panning angleof 10, 20, 30 degrees were parametrically discussed.
Keywords: Bone conduction sound, Head-related transfer function, Sound image localization

1. INTRODUCTION

Recently, the bone conduction sounds have been applied in various kinds of fields, such as the hearing aid
technology [1] and human navigation [2, 3]. When a person utilizes bone conduction sound techniques, theexcited vibration directly or indirectly transmits to the inner ears via complex path including the airconducted and bone conduction transmission, and the person finally recognize the stimulation given by theactuator as sound. As for the bone conduction sound, many kinds of researches are performed [1-7, 10].
Tjellström et al. [1] investigated the effectiveness of an implanted bone conduction actuator (BAHA, thebone-anchored hearing aid). Such an implanted device contributes to efficient bone conduction transmission,
however, it is hard to take the advantage of the bone conduction reproduction from the viewpoint of a person
who has no difficulty in hearing. Walker et al. investigated navigation system for virtual environments byusing bonephones [2]. From the viewpoint of more physical aspects, Purcell et al. [4] estimated the bone
conduction transfer functions using the optoacoustic emissions. Chang et al. [5] developed a whole-head
human finite-element models, and simulated the transmission of the bone conduction sound. McBride et al.
[6] investigated the head sensitivity mapping for the bone conduction reception by a human head. Qin et al.
[7] performed experimental study on the frequency characteristics of loudness of some kinds of bone
conduction actuators.
On the other hand, from the viewpoint of subjective perception of sounds, various knowledge concerning
three-dimensional sound field reproduction technology, especially in the field of air-conducted soundreproduction, has been constructed [8, 9]. In contrast, as for the bone conduction sound reproduction, such athree-dimensional reproduction technique has not yet been established whereas subjective evaluation for
loudness of the bone conduction sounds are performed [10].
One of the advantages of using the bone conduction sound is that it can reproduce sound without blocking
the ear canals, such as the reproduction using earphones or headphones. By taking such an advantage, it ispossible to provide various kinds of voice information such as the route navigation for indoor/outdoor
movement of a person without preventing smooth verbal communication with surrounding persons.
Furthermore, it is also important not to hinder listening to environmental sounds such as the sounds ofapproaching vehicles by blocking the ear canal. Under such a situation with usage of bone conduction soundtechnique, reproduction of not only easy-to-hear clear sounds but also localized sounds which can indicate
the direction of the navigation may enhance usefulness of the sound guidance technology.
In this research, for the purpose of constructing three-dimensional sound reproduction technique by bone
conduction sound reproduction, a fundamental study on sound localization accuracy in bone conduction
sound reproduction using the head related transfer functions (HRTF) measured by general air-conductedsound incidence into a head and torso simulator was performed.
17518533@ed.tus.ac.jp, t_asakura@rs.tus.ac.jp7887

2. DETAIL OF BONE CONDUCTION SOUND REPRODUCTION SYSTEM
2.1

Utilized device and contact condition between the actuator and the surface of thehead
Figure 1 shows (a) adopted actuator utilized to the bone conduction reproduction, (b) theinvestigated contact positions between the actuator and the surface of each part of the head, and (c)the hairband-type fixing device of the actuator to the surface of the head. In this study, two actuators
are attached to the left and right side of the face, respectively. The contact position between theactuator and the head surface were varied into four conditions including the auricular cartilage (P1),
two points around the zygomatic arch (P2, P3), and the mastoid (P4) as shown in Fig. 1 (b-1) and (b2), respectively. These points were chosen because they were often used as the excitation points ofbone conduction reproduction [6]. As a fixing device of the actuators, four kinds of hairband-type
fixing devices as shown in Fig. 1 (c) were adopted. These devices may have different contact pressures
between the actuator and the surface of the face, because they have different widths of hairband  asshown in Fig. 1(c), whereas they have similar dimensions and are made of similar resin-treatedmaterials. The measured results of the contact pressures for each hairband-type device are describedin the next section.

(b-1)

(a)

P1

Bone conduction

Bone conduction

exciter1

exciter2(COM-10917)(HDR 9350-010020)

(b-2)

(c)

P2

P3
P4

J1

J2

α

α

J3

J4

α

α

Fig. 1 Utilized device and contact condition.
2.2 Measurement of the excitation characteristics of the actuator
The contact pressure between the device of actuator and the surface of the subject’s head may
affect the frequency characteristics of the excitation force by the actuator. To investigate thetransmission efficiency of vibration between the device and the contacting head part, the contactpressures caused by wearing the four kinds of fixing devices from J1 to J4 were measured by using asimple acrylic model imitating the head as shown in Fig. 2. The distance between the acrylic plates inthis model was set to 15.7 cm of the average of young male and female face widths [11]. A thin gel
sheet (thickness 1 mm, hardness 15 deg.) imitating human skin was sandwiched between the actuatorand the acrylic plate. The contact pressure was measured using a dial tension gauge (TECLOCK,
DTN-300). The measured results of the contact pressures for each hairband-type device are shown in
Fig. 3. As shown in the figure, the contact pressures between the actuator and the surface of the acrylic
plate were increased in accordance with the narrowness of the hairbands.
Next, the actuator was fixed to the surface of the abovementioned gel by the hairband-type devicesof J1, J2, J3, and J4, and a white noise was generated with the same driving force acted to the actuator
fixed by each hairband-type device. The vibration acceleration transmitted to the acrylic plate wasmeasured by an acceleration pickup (Bruel & Kjaer, Type-4518) attached to the backside of the acrylic
plate. The contact pressure and vibration characteristics measured in the condition from J1 to J4 areshown in Fig. 4. Although these frequency characteristics have peak and dip tendencies including theresonant mode frequencies of the contacting acrylic thin plates, the general tendencies of thedifference between the excitation components by each bone conduction device can be discussed fromthe obtained results. In the conditions of J3 and J4 with relatively higher contact pressures, a higher
vibration acceleration level was observed in relatively high frequency ranges compared to theconditions of J1 and J2 with relatively lower contact pressures.

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(a)

Skin gel

Hairband-type fixing deviceAcc.
pickup15.7 cm(b)

Acc.
pickupActuator

Actuator(b)

Fig. 2 (a)Simple acrylic model imitating the head, (b)Details.

J1
0.8 N

J2
1.3 N

J3
2.3 N

J4
2.5 N

Vibration acceleration level

Fig. 3 Measured results of the contact pressures for each hairband-type device.

0.1k

1k
Frequency

10k

Fig. 4 Results between contact pressures and vibration characteristics.

3. EXAMINATION OF SUITABLE CONTACT POSITION OF ACTUATOR

To investigate the ease of hearing which is considered to be largely influenced by the contact
position between the actuator and the face, a subjective experiment for evaluation of the ease ofhearing of white noise, octave band noises, and female voices was performed by changing the contact
positions from P1 to P4. It should be noted this experiment adopted the fixing device of J4 as shownin Fig. 1(c). As the participants in this experiment, seven males in their twenties were adopted. The
experiment is composed of two parts. The first part investigates the subjective influence by the white
noise and the octave band noises to the ease of hearing, whereas the second one investigates that by
the female voice and the filtered sounds of that voice into each oct. band. The ease of hearing was

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judged by using mono-polar five categories from “Considerably easy to hear” to “Considerably
difficult to hear”.
Next, the scheme of the experiment is as follows. In this experiment, the subjects wore both theinner-ear type headphone and the bone conduction device in order to evaluate the relative extent of“ease of hearing” in comparison between the bone conduction and air-conducted sounds. It should be
noted that the subjects keep evaluating all the conditions with both the inner-ear type headphone andthe bone conduction device attached. This scheme was adopted in order to reduce the subjective
influence of the detachment operation on the evaluated scores as much as possible. Firstly, the output
volume of the bone conduction reproduction was determined so that the subjective loudness of the
reproduced white noise including the frequency components from 500 Hz to 8 kHz bands by the bone
conduction device was equivalent to that by the inner-ear type headphone. For this adjustment,
following procedure was performed. The subject hears the reference air-borne sound of theabovementioned band-limited white noise from the inner-ear type headphone. Then, the subject cankeep hearing the sound for enough time to judge the loudness of the sound. After that, the subject
hears the same sound from the bone conduction device, and control the output volume by turning thevolume controller of the audio interface. It should be noted that the subject can repeat theabovementioned adjustment for arbitrary times. The adjusted output volumes into the channels forbone conduction and air-borne actuators were fixed throughout the experiment including the first andsecond parts. Secondly, to familiarize the subjects with this experiment, three randomly chosenconditions were firstly evaluated by the subjects before the main experiment. It should be noted thatthe results of the trial evaluation were excluded from the analysis and discussion. Then, the ease ofhearing of the abovementioned signal of the white noise, and octave band noises of 500 Hz, 1 kHz, 2
kHz, 4 kHz, and 8 kHz oct. bands, which were generated by filtering the white noise, were judged on
the five categories. In each evaluation, the air-conducted sound was firstly reproduced, and after 5
second interval, the bone conduction sound was reproduced. Then, the subject answers the evaluatedscore for the bone conduction sound so that the score of the air-conducted sound is relatively evaluatedas “3”. Lastly, the ease of hearing of a female voice spoken in Japanese with duration of 5 seconds,
and the filtered signal of that voice into 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 8 kHz oct. bands were
judged on the five categories. In this part of the evaluation, the air-conducted sound was firstly
reproduced, and after 5 second interval, the bone conduction sound was reproduced. Then, just thesame as the procedure of the first part, the subject answers the obtained score for the bone conduction
sound so that the score of the air-conducted sound is relatively evaluated as “3”. In these experiments,
the air-conducted test sounds were reproduced with the inner-ear type headphone of ER-4S made by
Etymotic research. It should be noted that the adopted inner-ear type headphone has relatively highsound insulation, and completely blocks the canal of the subject from the incoming sounds to theirears. The results of the first and second parts of the experiment are shown in Fig. 5 (a) and (b),respectively. It should be noted that the results of obtained by reproducing the white noise and thefemale voice including the frequency component from 500 Hz to 8 kHz band are indicated as “all” inthe figure. In both results of Fig. 5 (a) and (b), it was found that presenting the bone conduction soundto the position of P2 is appropriate from the viewpoint of ease of hearing.

Fig. 5 Results of suitable contact position, (a)White noise, (b)Voice.

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4. SOUND LOCALIZATION PERFORMANCE WITH BONE CONDUCTIONREPRODUCTION
4.1 Basic performance of sound localization with binaural bone conduction reproduction
In order to confirm the performance of the sound field reproduction, a sound localization test wascomparatively performed for both air-borne and bone conduction sounds. In this experiment, the sound field
was simulated as follows. The subject was surrounded by 36 discrete virtual sound sources arranged in every
10 deg. As shown in Fig. 6, and the subject had to answer which source was radiating sound by the numberof incident angle. Seven male subjects in their twenties were adopted.
Next, the processes of the binaural signal generation are described as follows. Firstly, white noise soundincluding the frequency components from 500 Hz to 8 kHz oct. bands having a duration of 5 seconds was
adopted as the sound source. The HRTF measured using the head and torso simulator (ACO, Type 7828B)
were used to synthesize the binaural signals.
The experiment is composed of two parts. The first part investigates the air-borne sound localization
performance, whereas the second one investigates the borne-conducted sound localization performance. Afterthe first part of the experiment, the subject detached the headphone for air-borne reproduction, and then put
on the borne-conducted device. The sound incident from random directions was presented to the subject by
the bone conduction device of J1 and J4, and the subject answered the incident direction of the sound, whereas
the air-borne sound localization performance was measured by reproducing the same sound data as that of
the bone conduction reproduction. In order to reproduce the air-borne sound, the over-ear headphone (AudioTechnica, ATH-W1000Z) was used instead of the inner-ear type headphone, because the subject had to detachthe headphone only once between the first and second experiments.
It should be noted that the bone conduction sound was reproduced at the output volume so that the
subjective loudness of the white noise including the frequency components from 500 Hz to 8 kHz bands
reproduced by the bone conduction device is equivalent to that by the abovementioned over-ear headphone.
The result of the case in which the sounds were reproduced by the over-ear headphone is shown in
Fig. 7. Some erroneous recognitions concerning misjudgment between the front and back directions
were often seen as indicated by blue lines in the figure. This is caused by the use of a single HRTF
obtained by the head and torso simulator for all subjects. Next, the results of the cases in which thesounds were reproduced by the bone conduction device of J1 and J4 is shown in Fig. 8 (a) and (b),respectively. The results of the sound reproduction by the device of J1 for the contact positions of P2
and P3 are shown in Fig. 8 (a) whereas those of J4 for the same contact positions are shown in Fig. 8(b). Firstly, the sound localization performance of the case using the J4 device, especially in case ofsound incidence of 90 and 270 deg., increased compared to that using J1 device. It is because theformer device excites larger amount of bone conduction vibration especially in the higher frequency
range, as shown in the vibration characteristics of Fig. 4. It should be noted that the erroneous
judgement concerning the front and back direction indicated by the blue lines is also seen in theseresults with bone conduction sound reproduction. Secondly, there can be seen a slight difference inthe sound localization performances of the cases of the contact position P2 and P3, and the former
contact position indicates a little higher localization performance than the latter one, especially incase of sound incidence of 90 and 270 deg.

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Fig. 6 Overview of experiment.

Fig. 7 Result of reproducing air-borne sound.

Fig. 8 Results of reproducing bone conduction sound, (a)Device J1, (b)Device J4.

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4.2 Effect of panned binaural presentation with bone conduction reproduction
Further study on the sound image localization experiment using a bone conduction soundreproduction method, in which the intended direction of the reproduced sound images are quickly
panned to left and right in order to enhance the perception of the sound direction, was carried out. The
subject was surrounded by twelve discrete virtual sound sources arranged in every 30 degree. The
overview of sound reproduction method is shown Fig. 9. The subject had to answer which source wasradiating sound by the number of incident angle. As the participants of this experiment, five males intheir twenties were adopted. In this study, angles of 10, 20 and 30 degrees were used. The reproduction
sound was generated by convolution of the HRTF and the blink sound signal, which was adopted inthis study aiming at future application for the human guidance. In this experiment, the HRTF measured
with a KEMAR dummy head microphone provided by Massachusetts Institute of Technology [12] wasused. The procedure of generating the panned sounds was performed as follows. Firstly, as indicatedin Fig. 9(a), two virtual sound sources of Position A and B were additionally considered in addition
to the original presentation point of Position O. The presentation of the sound source is quickly panned
to Position A and B as shown in the time history of Fig. 9(b). Then, as can be seen in the figure, thesound incident from the Position O was made about 3 dB larger than that from positions A and B. The
sound was created with a switching time of 0.5 s between each sound. The presentation conditions of
the bone conduction element were P2 and J4 based on the examination results of the section 4.1. The
results of sound localization experiments conducted under the above conditions are shown in Fig. 10.
It should be noted that the red lines indicate correct answers, and the blue ones indicate the erroneous
judgement concerning the front and back direction. The 20 and 30 degrees of panned angles tended todecrease erroneous judgement concerning the front and back than the 10 degree of panned angle. From
this result, it turned out that it is appropriate to reproduce the panned angle at 20 degree. It alsosuggests that too large panned angle affects the sound image localization accuracy.

(a)Position AθPosition OΦ(b)Ex) main angle 30 degree(Position O)
panned angle 10 degree(Position A,B)
0.5 s
3 dB larger

Position B
Φ: main angle
θ : panning angle

25°

30°

35°

30°

25°

Fig. 9 Overview of sound reproduction method,(a) Procedure of generating the panned sounds, (b)Example of reproduction sound.

Fig. 10 Results of sound localization experiment.

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5. CONCLUSION

The sound localization performance of binaural signals reproduced by the bone conduction device has been
investigated. Detailed relationship between the sound localization performance and the contact conditions of
the bone conduction reproduction devices has been discussed. It has been indicated that differences of thecontact pressures between the head and the bone conduction device, and the contact positions of the bone
conduction devices on the subject’s head surface have large influence on the sound localization. Furthermore,as a results of a sound image localization experiment by using a bone conduction sound reproduction method,in which the intended direction of the reproduced sound images are quickly panned to left and right in orderto enhance the perception of the sound direction, it was found that the condition of the twenty-degree panning
was the best angle to clearly perceive the direction of incident sound.

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