[Control of sound pressure in audible spot using parametric speakers]
Takumi Hakamata(1) , Hiroyoshi Yamashita(2) , Keisuke Watanabe(3) , Kotaro Hoshiba(4) , Takenobu Tsuchiya(5) , Nobuyuki
Endoh(6)(1) Kanagawa University, Japan, r201870089qz@jindai.jp(2) Kanagawa University, Japan, r201870092uf@jindai.jp(3) Kanagawa University, Japan, r201503845cn@jindai.jp(4) Kanagawa University, Japan, hoshiba@kanagawa-u.ac.jp(5) Kanagawa University, Japan, kenshin@kanagawa-u.ac.jp(6) Kanagawa University, Japan, endoh@kanagawa-u.ac.jpAbstract
To give the information to only the target person by sound, we have been researching about forming audible spotwith two parametric speakers. By transmitting modulated ultrasounds from each speakers respectively, audible
spot is formed only at the cross point of them. However, because of frequency characteristics of parametricspeakers, the area of audible spot is changed when the frequency of played sound is changed. To deal this issue,
we propose control method of sound pressure for forming stable audible spot in this paper. In proposed method,sound pressure of transmitted sound is controlled as same level at every frequency. By keeping constant soundlevel at every frequency, it is also expected to keep constant area of audible spot at every frequency. Proposed
method was evaluated by indoor environment. As the result, area change was suppressed from 70 % to 10 % byintroducing control method, and the availability of proposed control method was conﬁrmed.
Keywords: Parametric speaker, Audible spot, Single side band modulation

1INTRODUCTION

1.1 Background
Audio assist is often used to give the information at museums, stations, and so on. In case a normal loudspeakeris used for audio assist, because the sound diffuses spherically from the loudspeaker, giving the sound to only
the target person is difﬁcult. To solve this problem, a parametric speaker is sometimes used. A parametricspeaker consists arrayed ultrasonic speakers, and it has high directivity[1, 2, 3]. Transmitted ultrasound withmodulation is demodulated to audible sound by acoustic non-linearity of the air. By high directivity of aparametric speaker, the information can be given to the particular direction. Some systems using a parametricspeaker are already used for practical[4]. However, there is the problem caused by high directivity. Because
a parametric speaker emits the sound in one direction with high power, the sound propagates straightly withlow attenuation. In this case, reﬂected sounds from a wall, ﬂoor, ceiling reach to no-target person. Then, thetechnique to form audible spot instead of audible direction is required.
1.2 Related research
There are some studies to form audible spot. For example, Fukasawa et al. developed the system to form audible spot using 128 speakers placed around the area of 5m square[5]. They could form four audible spots in thearea with such a large scale system. To develop more smaller scale system, Matsui et al. proposed audible spot
forming system using two parametric speakers[6]. They achieved forming audio spot by transmitting modulated
signals by a kind of amplitude modulation from each speaker. However, there is the problem that the area ofaudible spot is changed when the frequency of played sound is changed. To form stable audible spot, additional
method is needed.

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1.3 Approach
To deal above issue, we propose control method of sound pressure for forming stable audible spot with twoparametric speakers in this paper. In proposed method, sound pressure of transmitted sound is controlled assame level at every frequency. By keeping constant sound level at every frequency, it is also expected to keep
constant area of audible spot at every frequency. Audible spot is formed in indoor environment using proposed
method, and sound ﬁeld is measured for evaluation and discussion.

2METHOD

For playing audible sound with a parametric speaker, the modulation considering acoustic non-linearity of theair is used. A career signal transmitted from a parametric speaker vc (t) and required audible sound vs (t) aredeﬁned as follows:vc (t) = Ac sin 2π fct,

(1)

vs (t) = As sin 2π fst.

(2)

Ac and As are amplitudes of a career signal and audible sound, fc and fs are frequencies of them, t is the time.
As the modulation method, double side band (DSB) modulation which is a kind of amplitude modulation isoften used[7]. Modulated signal by DSB based on Eq. 2 for playing Eq. 1 is given as follows:
vdsb (t) =Ac sin 2π fct +As
Assin 2π ( fc + fs ) + sin 2π ( fc − fs )t.
22(3)The ﬁrst term is a career signal, the second one is upper side band (USB) signal and the third one is lower
side band (LSB) signal. By transmitting vdsb (t) from one parametric speaker, vdsb (t) is demodulated due toacoustic non-linearity of the air, and difference tone between a career signal and USB signal or LSB signal at
the frequency of fs . Single side band (SSB) modulation which is simpliﬁed DBS is also used[8]. Modulated
signal by SSB is given as follows:
vssb (t) = Ac sin 2π fct +Assin 2π ( fc − fs )t.
2(4)In SSB, USB is omitted from DSB. Compared to DSB, audible sound vs (t) played by SSB modulated signal
has half the amplitude but harmonic distortion is suppressed.
A career signal and a side band signal are transmitted from one parametric speaker. Then, audible area spreads
in the direction of transmission. In this research, for forming audible spot, a career signal and a side bandsignal are transmitted from two parametric speakers, respectively. By separating them, audible area is formed
only at the cross point of them. For high audio quality, SSB modulation is used in this system. A signaltransmitted from the ﬁrst speaker vssb1 (t) and that from the second speaker vssb2 (t) are given from Eq. 4 asfollows:
vssb1 (t) = Ac sin 2π fct,As
vssb2 (t) =sin 2π ( fc − fs )t.
2(5)
(6)

However, because of frequency characteristics of parametric speakers, amplitudes of a career signal and a sideband signal, Ac and As /2, change with the frequencies of them. In case that, the area of audible spot alsochanges. To keep constant area, Ac and As /2 are controlled at every frequency as follows:
Ac =As.
2(7)By introducing this control method, it is expected to form stable audible spot at every frequency.

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Figure 2. Frequency - sound pressure characteristics of speakers when input voltage is
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Figure 1. Experimental conﬁguration.
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3

EVALUATION EXPERIMENTTo evaluate the performance of proposed method, the experiment was performed.
3.1 Experimental conﬁguration
Figure 1 shows experimental conﬁguration. Two parametric speaker, No. 1 were No. 2, are located at thedistance of 500 mm from audio spot forming point. Directions of transmission from each speaker were crossedat the angle of 90 degree. vssb1 and vssb2 were transmitted from the speaker No. 1 and No. 2, respectively.
The cross point of transmitted directions deﬁned as {x, y} = {0, 0}, and sound ﬁeld was measured by obtaining
acoustic signals using the microphone at some points. Measurement points were set as every 9 mm in thex range from -90 mm to 90 mm and the y range from -90 mm to 90 mm. This is the experiment for abasic study. Then, the side band signal which has single frequency was transmitted from the speaker No. 2,and audible sound which has single frequency was played. In this experiment, to play audible sound at thefrequency fs =0.1 to 2 kHz, the frequency of the career signal ( fc in Eq. 5) was set as 40 kHz, and that of the2692

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Figure 4. Measured sound ﬁelds when (a) fs = 0.1 kHz, (b) fs = 1 kHz, (c) fs = 2 kHz. (i) Results withoutcontrol, (ii) with control.

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Figure 5. The area of audible spot.

side band signal ( fc − fs in Eq. 6) was set as 39.9 to 38 kHz.
Figure 2 and Figure 3 show frequency - sound pressure characteristics when input voltage is 2 Vpp and input- sound pressure characteristics of speakers. The horizontal axes represent the frequency and input voltage, respectively, and the vertical axis represents sound pressure. Sound pressures under each situation were measured
at the distance of 500 mm. As shown in Figure 2 and Figure 3, these characteristics can be approximated aslinear equation. Using this characteristics, sound pressures of the career signal and the side band signals werecontrolled as Eq. 7. In particular, As /2 was adjusted to Ac at each frequency.
3.2 Experimental results
Figure 4 shows measured sound ﬁelds. Figure 4(a), (b) and (c) show results when fs ware 0.1, 1 and 2 kHz.
Figure 4(i) and (ii) show results without control and with control. The color indicates sound pressure level(SPL) of each fs . When fs was 1 kHz, it can be seen that high SPL spots were formed around {x, y} = {0, 0}
in both case without control and with control. Middle SPL part observed around high SPL spots seems to be
the effect of side lobes of each speaker. In results without control, decrement of the area of high SPL spot withincrement of fs can be seen. This is because that the amplitude of the side band signal declines with decrementof the frequency from 40 kHz as shown in Figure 2. On the other hand, comparing with with results withoutcontrol, the area of formed high SPL spot with control seems not to change.
3.3 Discussion
To evaluate about changes of the area of audible spot quantitatively, the area was measured. Audible area
was deﬁned as the spot that SPL is higher than 60 dB. Figure 5 shows the change of the area of audible
spot. The horizontal axis represent the frequency of audible sound fs , and the vertical axis represents thearea of audible spot. As shown in the result without control, the area decreased approximately 70 % with theincreasing frequency. In the result with control, area change was suppressed to approximately 10 %. As results,
the availability of proposed control method was conﬁrmed. However, audible sound which has single frequency
was played in this experiment. Therefore, we will study about forming audible spot which includes multiple
frequency with proposed method in future work.

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4CONCLUSION

In this paper, we proposed sound pressure control method for forming stable audible spot with two parametricspeakers. The area of formed audible spot without sound pressure control changes because of frequency characteristics of speakers. To keep constant area of audible spot, sound pressure of transmitted sound is controlled
as same level at every frequency in proposed method. As the result of evaluation experiment, the availabilityof proposed control method was conﬁrmed. In future work, we will study about forming audible spot which
includes multiple frequency with proposed method.

REFERENCES[1] Tomoo, K.; Masahide, Y.; Kazuo, I. Developments of Parametric Loudspeaker for Practical Use, Proceedingsof 10th International Symposium on Nonlinear Acoustics, Kobe, Japan, July 24-28, 1984, pp 147-150.
[2] Bennett, B. M.; Blackstock, T, D. Parametric array in air, The Journal of the Acoustical Society of America,
Vol 57 (3), 1974, pp 562-568.
[3] Yoneyama, M.; Fujimoto, J.; Kawamo, Y.; Sasabe, S. The audio spotlight: An application of nonlinear
interaction of Sound waves to a new type of loudspeaker design, The Journal of the Acoustical Society ofAmerica, Vol 73 (5), 1983, pp 1532-1536.
[4] Kortbek, J. K.; Grønbæk, K. Interactive Spatial Multimedia for Communication of Art in the Physical Museum Space, Proceedings of the 16th ACM international conference on Multimedia, Vancouver, Canada,
October 26-31, 2008, pp 609-618.
[5] Fukasawa, Y.; Shinagawa, K.; Horio, K.; Mitsuhashi, K.; Deguchi, A.; Kusunoki, F.; Inagaki, S.; Mizoguchi,H. Loudspeaker array system for exhibition support into museum Effectiveness veriﬁcation of sound spot
for science education, Proceedings of IEEE International Conference on Systems, Man and Cybernetics,Singapore, Singapore, October 12-15, 2008, pp 1175-1180.
[6] Matsui, T.; Ikefuji, D.; Nakayama, M.; Nishiura, T. A design of audio spot based on separating emission ofthe carrier and sideband waves, Proceedings of Meetings on Acoustics, Vol 19 (1), 2013, pp 1-9.
[7] Wang, Y.; Chen, M.; Li, H.; Zhou, Z. Deﬁning the Parameters of Truncated Square-rooting DSB forParametric Loudspeaker, Proceedings of IEEE International Conference on Mechatronics and Automation,
Harbin, China, August 5-8, 2007, pp 1689-1693.
[8] Chen, M.; Qin, X.; Xu, L.; Du, Y.; Xu, L. The Distortion Analysis of the Single Side Band Method
for Parametric Loudspeaker Based on Orthogonal Envelope Detection, Proceedings of the 2nd International
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