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

Demodulation Characteristics in Propagation Process of
Amplitude-modulated Bone-conducted Ultrasound Presented to theNeck, Trunk and Arms
Koichiro Doi1; Riki Ogino1; Sho Otsuka1; Seiji Nakagawa1
1

Chiba University, Japan

ABSTRACT
Bone-conducted ultrasound (BCU) can be heard clearly and can transmit speech information using
amplitude-modulation (AM). Additionally, BCU is perceived even when presented to body parts distant fromthe head, like the neck, trunk, and arm. It is expected that demodulated low-frequency components appear asa result of the nonlinearity existing in the human body when amplitude-modulated BCU (AM-BCU) ispresented to such distal locations. First, to elucidate demodulation characteristics in the propagation processof such distantly-presented BCU, vibration at the external auditory meatus was measured when AM-BCU
was presented to the neck, shoulder, upper limb, breastbone and backbone. The results showed spectrum
peaks corresponding to the carrier frequency (30 kHz) and the modulation frequency. The level of the peak of
the carrier tended to decrease depending on the distance between stimulation and measurement points,
however, the peak of the modulator did not change with distance. Second, vibrations around the cartilage ofthe auricle, tragus and articulations, which have strong nonlinearity, were measured. The demodulation
components were larger for the auricle and tragus than for the peripheral articulations. These results suggestthat demodulation mainly occurs in the vicinity of the ear even when BCU is presented to distal body parts.
Keywords: Bone-conduction, Ultrasound, Amplitude-modulation, Demodulation

1. INTRODUCTION
High-frequency sound at least up to 100 kHz presented via bone conduction can be perceived evenby the sensorineural deaf (1). This “audible” ultrasound through bone conduction is referred to as
bone-conducted ultrasound (BCU). Additionally, BCU can transmit speech information using
amplitude-modulation (AM). Utilizing this information, we have developed a new hearing aid for theprofoundly deaf, called the bone-conducted ultrasound hearing aid (BCUHA) (2-4).
In BCUHA, both high pitch tone due to the ultrasonic carrier and the envelope of the modulated
signal are perceived (5). Profoundly deaf people perceive the envelope as temporal information. On
the other hand, normal hearing people may perceive it as a demodulated low frequency component
generated by the nonlinearity of the human body (6).
With bone-conducted devices, sound is usually presented onto a part of the head, such as mastoid
process of the temporal bone, using a vibrator. However, the devices have several disadvantages, suchas the discomfort of wearing a vibrator and difficulty of fixing it to the head. On the other hand, BCU
is perceived even when presented to the body parts distant from the head, like the neck, arm, and trunk(7). If it is possible to present BCU sound to distal body parts and obtain significant perception (8),these problems identified with bone-conducted devices can be solved. Furthermore, distantlypresented BCU can be applied to develop a new device that can present sound information selectivelyto the users.
When BCU is presented to a distal body part, vibration propagates through various tissues in thebody. Therefore, demodulation may occur in the propagation process through tissue with strongnonlinearity, such as articular cartilage. Demodulated sound may contribute to improving BCU soundquality.
In this study, to elucidate demodulation characteristics in the propagation process of such
distantly-presented BCU, vibration in the outer ear canal was measured when AM-BCU was presented
to a distal body part. Further, to examine the body parts in which more demodulation occurs, vibrations
around the cartilage that has strong nonlinearity were measured.

16604

www.cfme.chiba-u.jp/~nakagawa/index.html

2. EXPT. 1: MEASUREMENT OF VIBRATIONS IN THE EAR CANAL
2.1 Method
Seven subjects with normal hearing (one female and six males, 21-24 years) participated in theexperiment. Vibrations were measured using a small accelerometer (Ono sokki, NP-3211), which
was wrapped in a sponge tube and inserted into the ear canal (9). A vibrator (Murata Manufacturing
MA40E7S) was placed against the following body parts: ⅰ. Mastoid process of the temporal bone, ⅱ.
Sternocleidomastoid muscle (muscle of the neck), ⅲ. Clavicle, ⅳ. Acromial process, ⅴ. Brachialis
muscle (muscle of the upper arm), ⅵ. Brachioradial muscle (muscle of the lower arm), ⅶ. Sternum,ⅷ. The 10-12th thoracic vertebrae. To the mastoid, the vibrator was fixed using a headband, and tothe other parts, using an elastic support band (Fig. 1). The carrier frequency of BCU stimuli was 30
kHz and the modulation frequencies were varied at 0, 100, 300, 600, 800, 1000, 3200 and 6400 Hz.
2.2 Result
The spectrum peak corresponding to the carrier frequency (30 kHz) and the modulation frequency
were confirmed at all body parts and these differences ranged from 40 to 70 dB. According to theprevious study (10), the current results suggest that the demodulation component reached theperceptual level.
The level of the carrier component at each body part relative to the mastoid tended to decreasedepending on the distance between stimulation and measurement points, and the presentation bodyparts showed a significant effect on the attenuation amount of the vibration ( p<2.2e-16). However, thelevel of the modulation frequency component at each part relative to the mastoid did not change with
distance, and the presentation body parts showed no effect on the relative value (p=0.99) (Fig. 2).

3. EXPT. 2: MEASUREMENT OF VIBRATIONS AROUND CARTILAGE
3.1 Method
Six males with normal hearing (21-24years) participated in the experiment. Measurement points
were following parts: Ⅰ. Tragus, Ⅱ. Pinna, Ⅲ. Cartilago thyroidea, Ⅳ. Shoulder, Ⅴ. Thoracic
vertebrae, Ⅵ. Cartilago costalis, Ⅶ. Elbow. The vibrator and the accelerometer were placed so that
it was located at the center of the measurement points. For the tragus and pinna, the accelerometer
was inserted into the ear canal, and for the other parts, it was fixed on the skin using masking tapeand wax. The AM-BCU stimuli were identical with indicated in 2.1.
3.2 Result
The level of the carrier component relative to the modulation frequency component at the tragus
and pinna were larger than at distal parts, and the presentation body parts showed a significant effecton the relative value (p<2.2e-16). In distal parts, the thoracic vertebrae showed the largest relativevalue.

4. DISCUSSION
The level of the carrier component tended to decrease depending on the distance between stimulation andmeasurement points, however, the demodulation component did not change as a result of distance. Inaddition, more demodulation component was generated at the tragus and the pinna than in distal cartilage.
These results suggest that demodulation mainly occurs in the vicinity of the ear, even when BCU is presented
to distal body parts.

ACKNOWLEDGEMENTS

ⅰⅲ
ⅴⅶ
Fig. 1 – Examples of the placements of BCU
stimuli: i. Mastoid process, ⅲ. Clavicle, ⅴ.
Brachialis muscle, ⅶ. Sternum

REFERENCES

Acceleration relativeto the mastoid (dB)This work was supported by JSPS KAKENHI Grant Number 17H02079 and a Research Grant in 14
the Natural Sciences from the Mitsubishi Foundation for SN.
-16
-12
-8
-4
04

Modulation
frequency(Hz)

ⅱⅲ

ⅳ

ⅴ

ⅵ

ⅶ

ⅷ

100
300
600
800
1000
3200
6400

Fig. 2 – Spectrum peaks corresponding to thedemodulation component at each body part
relative to the mastoid process

6605

1.

M L Lenhardt, R Skellett, P Wang, A M Clarke. Human ultrasonic speech perception. Science. 1991;
253: p. 82–5.
2. S Nakagawa, Y Okamoto, Y Fujisaka. Development of Bone-conducted ultrasonic hearing-aid(BCUHA) for the sensorineural deaf. Trans. Jpn. Soc. Med. Bio. Eng. 2006; 44(1): p. 184-9.
3. S Nakagawa, C Fujiyuki, T Kagomiya. Development of Bone-Conducted Ultrasonic Hearing Aid for
the Profoundly Deaf: Assessments of the Modulation Type with Regard to Intelligibility and Sound
Quality. Jpn. J. Appli. Phys. 2012; 51:07GF22.
4. S Nakagawa, C Fujiyuki, T Kagomiya. Development of a Bone-Conducted Ultrasonic Hearing Aid for
the Profoundly Deaf: Evaluation of Sound Quality Using a Semantic Differential Method. Jpn. J.
Appli. Phys. 2013; 52:07HF06.
5. K Fujimoto and S Nakagawa. Nonlinear explanation for bone-conducted ultrasonic hearing. Hear. Res.
2005; 204: p. 210-5.
6. Ito K and Nakagawa S, Self-demodulation of amplitude-modulated signal components in
amplitude-modulated bone-conducted ultrasonic hearing. Jpn. J. Appli. Phys. 2015; 54:07HF07.
7. S Nakagawa, T Sakaguchi, M Yamaguchi, M Tonoike, S Imaizumi, H Hosoi, Y Watanabe.
Characteristics of auditory perception of bone-conducted ultrasound. Tech. Rep. IEICE WIT99-15.
1999; p. 89.
8. S Nakagawa, R Ogino, S Otsuka, Assessment of detection threshold and temporal resolution ofdistantly presented bone-conducted ultrasonic hearing, Jpn. J. Appli. Phys. 2018; 57:07LD22.
9. Ito K and Nakagawa S. Perception Mechanisms of Bone-Conducted Ultrasound Assessed by Acoustic
Characteristics in the External Auditory Meatus. Jpn. J. Appl. Phys. 2010; 49:07HF31.
10. Ito K and Nakagawa S, Perception of bone-conducted ultrasound assessed by its loudness
characteristics, Proc. ICA. 2007; 19(1-PPA-09-13): p.1-6.

6606