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

 Demodulation distance control based on analytic model between
film gas-lens depth and demodulation distance for parametric array
loudspeaker
Toshihiro FUJII1; Masato NAKAYAMA2, 3; Takanobu NISHIURA4
1

Graduate School of Information Science and Engineering, Ritsumeikan University, Japan
2
3

College of Design Technology, Osaka Sangyo University, Japan

Research Organization of Science and Technology, Ritsumeikan University, Japan
4

College of Information Science and Engineering, Ritsumeikan University, Japan

ABSTRACTParametric array loudspeaker (PAL) can transmit an audible sound to only a particular area by utilizing
ultrasound. We have previously proposed a demodulation distance control using a film gas-lens with the gas
different from the air to control a maximum demodulation distance (MDD), which is a distance withmaximum demodulation of the audible sound. The method selects the gas and adjusts the film gas-lens depth
to make the MDD equal to the desired MDD (D-MDD). In the method, the MDD has a large numerical error
because it employs a simple linear approximation with the MDD and the film gas-lens depth. In this paper,
we therefore propose a new MDD control based on an analytic model between film gas-lens depth and the
MDD. To design the analytic model, we investigate the MDD corresponding to the film gas-lens depth set at
equal intervals and design a regression curve. Next, to calculate the appropriate film gas-lens depth
corresponding to the D-MDD, we design the analytic model as an inverse function of the regression curve.
The analytic model can accurately adjust the film gas-lens depth to control the MDD. Finally, we confirmed
the effectiveness of the proposed method through the evaluation experiments.
Keywords: Parametric array loudspeaker, Demodulation distance, Film gas-lens

1. INTRODUCTION
Acoustic sounds are usually emitted by an electrodynamic loudspeaker. It is suitable fortransmitting the audible sound to many listeners because of its wider directivity. However, it isunsuitable for transmitting the audible sound in particular areas. Then, the attention has recently been
focused on a parametric array loudspeaker (PAL) [1, 2]. PAL can achieve a higher directivity by
utilizing ultrasound and transmit the audible sound to a particular area [3, 4]. The PAL utilizes theamplitude modulated (AM) wave which is designed by modulating an amplitude of ultrasound withan original audible sound. The AM wave is strongly emitted from the PAL, and it is graduallydemodulated into the original audible sound by the nonlinear interaction in the air. It is known thatthe sound pressure level (SPL) of the demodulated audible sound depends on the distance between the
PAL and the observed position [5, 6]. This is attributed that the demodulation insufficiently occurs at
close to the PAL. Thus, it is difficult to transmit the demodulated audible sound with the maximum
SPL to the listener at any positions. In this paper, we refer to the maximum demodulation distance(MDD) that the distance which the demodulated audible sound has the maximum SPL.
It is known that the MDD is changed by the density of the medium in which the emitted AM wave
is transmitted. Based on it, we have previously proposed a demodulation distance control using a film
gas-lens with the gas different from the air to control the MDD [7]. This method arranged the film
gas-lens in front of the PAL as the container to enclose the gas. Then, this method selects t he gas andadjusts the depth of film gas-lens to make the MDD equal to the desired MDD (D-MDD). However,
1
2, 3
4

is0278ex@ed.ritsumei.ac.jp
nakayama@ise.osaka-sandai.ac.jp
nishiura@is.ritsumei.ac.jp1061

this method has a large numerical error on controlling the MDD because it employs a simple linear
approximation with the MDD and the film gas-lens depth. It is difficult to accurately control the MDD
to any listener’s positions.
In this paper, we therefore propose a new MDD control based on an analytic model between thefilm gas-lens depth and the observed MDD (O-MDD). First, to design the analytic model, weinvestigate the O-MDD corresponding to the film gas-lens depth set at equal intervals and design a
regression curve in each gas through the preliminary experiments. Next, to calculate the appropriate
film gas-lens depth corresponding to the D-MDD, we design the analytic model as an inverse functionof the regression curve. The analytic model can accurately adjust the film gas-lens depth to control
the MDD. Thus, we can achieve the accurate MDD control for PAL by utilizing the analytic model.
We evaluated carbon dioxide (CO2 ), sulfur hexafluoride (SF 6 ), and helium (He) as the gas which
has different density compared with the air in order to confirm the validity of the proposed method.
As the results of the evaluation experiment, we confirmed the effectiveness of the proposed method.

2. CONVENTIONAL MAXIMUM DEMODULATION DISTANCE CONTROL BASEDON THE LINEAR APPROXIMATION WITH FILM GAS-LENS
We have proposed the MDD control based on the linear approximation with the film gas-lens as
the conventional method. This method arranged the film gas-lens in front of the PAL as the container
to enclose the gas. Figure 1 shows the overview of the conventional MDD control with film gas-lensᇱ
for PAL. In Fig. 1, ݈ represents the distance between the PAL and the listener’s head (D-MDD), ‫ݔ‬୫ୟ୶
represents the controlled MDD, ‫ ݓ‬represents the film gas-lens depth, ‫ݔ‬୥ୟୱ represents the MDD whenthe AM wave transmits through only the gas layer whose density is different from the air, ‫ݔ‬ୟ୧୰
represents the one when the AM wave transmits through only the air, and ߛ represents the ratio of ‫ݓ‬ᇱ
and the ratio ߛ of ‫ ݓ‬to ݈ are derived as following
to ݈. Conventionally, the controlled MDD ‫ݔ‬୫ୟ୶
linear approximation [7].
ᇱ‫ݔ‬୫ୟ୶ൌ ߛ‫ݔ‬୥ୟୱ ൅ ሺͳ െ ߛሻ‫ݔ‬ୟ୧୰ ǡ(7)‫ݓ‬ǡ
݈

(8)‫ݔ‬୥ୟୱ ൌଷߩ୥ୟୱ ܿ୥ୟୱ
ǡ
ߚ߱‫݌‬଴

(9)‫ݔ‬ୟ୧୰ ൌଷߩୟ୧୰ ܿୟ୧୰
ǡ
ߚ߱‫݌‬଴

(10)ߛൌ

where ߩ୥ୟୱ represents the density of the gas whose density is different from the air, ߩୟ୧୰ represents
the density of the air, ܿ୥ୟୱ represents the sound speed in the gas whose density is different from theair, and ܿୟ୧୰ represents the sound speed in the air. The method selects the gas (the density of the gasᇱequal to the D-MDD ݈. However,ߩ୥ୟୱ ) and adjusts the film gas-lens depth ‫ ݓ‬to make the MDD ‫ݔ‬୫ୟ୶g the MDD because it employs
p y a simple linear
this method has a large numerical error on controlling

Figure 1 – Overview of the conventional MDD control based on the linear approximation with thefilm gas-lens for PAL1062

approximation with the MDD and the film gas-lens depth as Eqs. (7) to (10). Therefore, it is difficult
to accurately control the MDD to any listener’s positions.

3. PROPOSAL OF MAXIMUM DEMODULATION CONTROL BASED ON
ANALYTIC MODEL BETWEEN FILM GAS-LENS DEPTH AND DEMODULATIONDISTANCE
3.1 Overview of the Proposed Method
We propose a new MDD control based on an analytic model between the film gas-lens depth andthe observed MDD (O-MDD). Figure 2 shows the overview of the proposed MDD control based on
the analytic model between the film gas-lens depth and the O-MDD. The method selects the gas (theᇱdensity of the gas ߩ୥ୟୱ ) and adjusts the film gas-lens depth ‫ ݓ‬to make the MDD ‫ݔ‬୫ୟ୶equal to thedesired MDD (D-MDD) ݈ as the same in the conventional method. First, to design the analytic model,ᇱcorresponding to the film gas-lens depth ‫ ݓ‬set at equal intervals
we investigate the O-MDD ‫ݔ‬୫ୟ୶and design a regression curve in each gas through the preliminary experiments. Details about
calculating regression curve are described in the next subsection. Next, to calculate the appropriate
film gas-lens depth ‫ ݓ‬corresponding to the D-MDD ݈, we design the analytic model as an inverse
function of the regression curve. The analytic model can accurately adjust the film gas-lens depth ‫ݓ‬ᇱ. Thus, we can achieve the accurate MDD control for PAL by utilizing theto control the MDD ‫ݔ‬୫ୟ୶
analytic model. As the results of the preliminary experiment, the regression curve of the O-MDDᇱ‫ݔ‬୫ୟ୶
corresponding to the film gas-lens depth ‫ ݓ‬set at equal intervals is derived as follows using an
exponential function.
ᇱ‫ݔ‬୫ୟ୶ൌ ‫ݔ‬ୟ୧୰ ሺߙ‫ݓ‬ሻǡ
(11)
where ߙ represents the fitting parameters of the regression curve in each gas, and it is determined
analytically from preliminary experimental results. Next, to calculate the appropriate film gas-lensdepth ‫ ݓ‬corresponding to the D-MDD ݈, we design the analytic model as an inverse function of theᇱis replaced with D-MDD ݈, the analytic model as anregression curve. From Eq. (11), O-MDD ‫ݔ‬୫ୟ୶
inverse function of the regression curve is derived as follows:ഀ݈‫ ݓ‬ൌ  ඨǤ‫ݔ‬ୟ୧୰

(12)ᇱ
In addition, we can confirm the controlled MDD ‫ݔ‬୫ୟ୶is equal to D-MDD ݈ by substituting the ‫ݓ‬ᇱof Eq. (12) into Eq. (11). The controlled MDD ‫ݔ‬୫ୟ୶ is derived as follows:ഀ݈ᇱ‫ݔ‬୫ୟ୶ൌ ‫ݔ‬ୟ୧୰ ሺߙ‫ݓ‬ሻ ൌ ‫ݔ‬ୟ୧୰  ቌߙ  ඨቍ ൌ ݈Ǥ‫ݔ‬ୟ୧୰

(13)

Figure 2 – Overview of the proposed MDD control based on the analytic model between the film gas-lensdepth and the O-MDD.

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From above, the proposed method can accurately adjust the film gas-lens depth ‫ ݓ‬to control the MDDᇱ
to be equal to the D-MDD ݈ . Thus, we can achieve the accurate MDD control for PAL by‫ݔ‬୫ୟ୶
utilizing the analytic model.
3.2

Preliminary Experiment to Calculate the Regression Curve

3.2.1 Preliminary Experimental Conditions
Table 1 shows the experimental conditions in the preliminary experiment. In this experiment, we
utilized CO2 , SF6 , and He as the gas which has different density compared with the air. Table 2 showsthe experimental equipment and Table 3 shows the physical parameters of gases. We measured the
SPL of the demodulated audible sound in the front direction of the PAL at each distance and calculatedᇱ. Figure 3 shows the experimental arrangement of the PAL and film gas-lens. In thisthe O-MDD ‫ݔ‬୫ୟ୶preliminary experiment, we arranged the film gas-lens to enclose the gas in front of the PAL as shownin Fig. 3. The film gas-lens is constructed by the thin film about 2 ߤm and metallic frames. Besides,
the film gas-lens depth ‫ ݓ‬can expand and contract flexibly. We confirmed that the SPL of thedemodulated audible sound transmitted through the film gas-lens attenuated about 3 dB.
Table 1 – Experimental conditions.
Ambient noise level‫ܮ‬୅ ൌ ʹ͹Ǥ͸ dB

Sampling frequency

192 kHz

Quantization bit rate

32 bits

Sound source

TSP signal (ʹଵଽ points)

Evaluation frequency band

0~10 kHz

Evaluation distance

0, 0.30, and 0.50~3.50 m (0.25 m spacing)

Environment

Office room (ܶ଺଴ ൌ ͶͲͲ)Film gas-lens depth ‫ݓ‬

0.05~0.30 m (0.05 m spacing)

O-MDD when transmits throughonly the air ‫ݔ‬ୟ୧୰
Table 2 – Experimental equipment.
Parametric array loudspeaker

MITSUBISHI, MSP-50E

1.00 mTable 3 – Physical parameters of gases.
Gas

DensitySound speed
340 m/s

Microphone

SONY, ECM-88B

Air

1.29 kg/m3

Microphone amplifier

THINKNET, MA-2016C

CO2

1.98 kg/m3

260 m/s
134 m/s
970 m/s

Power amplifier

VICTOR, PS-A2002

SF6

6.16 kg/m3

A/D, D/A converter

RME, FIREFACE UFX

He

0.18 kg/m3

Figure 3 – Experimental arrangement of the PAL and the film gas-lens.

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Table 4 – Fitting parameters ߙ of the regression curve in each gas.
Gas

Fitting parameter ߙ

CO2-1.138SF6-4.345He

1.176(b) SF6(c) He(a) CO 2ᇱ
Figure 4 – O-MDD ‫ݔ‬୫ୟ୶
corresponding to the film gas-lens depth ‫ ݓ‬set at equal intervals.
3.2.2 Preliminary Experimental Results
ᇱAs the results of the preliminary experiment, Fig. 4 shows the O-MDD ‫ݔ‬୫ୟ୶
corresponding to thefilm gas-lens depth ‫ ݓ‬set at equal intervals. In Fig. 4, the horizontal axis shows the film gas-lensᇱ. In addition, Table 4 shows the fitting
depth ‫ ݓ‬and the vertical axis shows the O-MDD ‫ݔ‬୫ୟ୶parameters ߙ of the regression curve in each gas. As shown in Table 1 and Fig. 4, since the O-MDDᇱ
is 1.00 m in the case of thewhen transmits through only the air ‫ݔ‬ୟ୧୰ was 1.00 m, the O-MDD ‫ݔ‬୫ୟ୶
film gas-lens depth ‫ ݓ‬ൌ Ͳ. As shown in Fig. 4 (a), (b), in the case of enclosing CO 2 or SF6 , gasesᇱdecreases with the increase of the film gas-lens
whose density is higher than air, the O-MDD ‫ݔ‬୫ୟ୶ᇱchanges more sharply in the case of
depth ‫ݓ‬. In addition, we can confirm that the O-MDD ‫ݔ‬୫ୟ୶
enclosing SF6 than that in the case of enclosing CO2. This is because SF6 has a larger density differencewith air than CO2. On the other hand, in the case of enclosing He, the gas whose density is lower thanᇱ
increases with the increase of the film gas-lens depth ‫ ݓ‬as shown in Fig. 4 (c).
air, the O-MDD ‫ݔ‬୫ୟ୶

4. EVALUATION EXPERIMENT ON CONTROL OF MAXIMUM DEMODULATIONDISTANCE
4.1 Experimental Conditions
In order to confirm the effectiveness of the proposed method , we conducted an evaluation
experiment on control of the MDD. Table 5 shows the evaluation distance and Fig. 5 shows theexperimental environment for the measuring the sound pressure distributions . We measured the SPLof the demodulated audible sound at the 94 microphone positions as shown in Fig. 5. In thisexperiment, we evaluated at 0.125 m spacing in the range of 0.5~1.5 m in order to confirm the detailed
MDD change as shown in Table 5 and Fig. 5. In addition, in order to confirm the reproducibility, thenumber of trials was set to 5. Other experimental conditions and equipment are the same as in Tables
1, 2. Table 6 shows the gas and film gas-lens depth ‫ ݓ‬corresponding the D-MDD ݈ calculated from
Eq. (12). We enclosed the gas and adjusted the film gas-lens depth ‫ ݓ‬in each condition in Table 6.
ᇱin eachWe calculate the average distance error (ADE) between the D-MDD ݈ and the O-MDD ‫ݔ‬୫ୟ୶ᇱ
condition in Table 6 and evaluated whether the O-MDD ‫ݔ‬୫ୟ୶ could be controlled to the D-MDD ݈.
ADE is derived as follows:்ͳᇱ ൌ ෍ห‫ݔ‬୫ୟ୶ǡ௜
െ ݈หǡ
ܶ

(14)

௜ୀଵ

ᇱ
where ܶ represents the number of trials, ‫ݔ‬୫ୟ୶ǡ௜
represents the ݅ th O-MDD. In addition, we
calculated the radiation angle from observed sound pressure distribution in each condition andevaluated the change of radiation characteristics. The radiation angle is defined as the angle to the1065Table 5 – Evaluation distance in the evaluation experiment on control of the MDD.
0, 0.30, 0.50~1.50 m (0.125 m spacing),

Evaluation distanceand 1.50~3.50 m (0.25 m spacing)

Table 6 – Gas and film gas-lens depth ‫ ݓ‬corresponding the D-MDD ݈.
D-MDD ݈Gas

Film gas-lens depth ‫ݓ‬0.500 mSF60.16 m

0.625 mSF6

0.11 m0.750 mCO20.25 m0.875 mCO20.12 m1.125 mHe

0.10 m1.250 mHe

0.19 m1.375 mHe

0.27 m

1.500 mHe

0.34 mFigure 5 – Experimental environment for measuring the sound pressure distributions.
point at which the SPL of the demodulated audible sound is attenuated by 6 dB in the lateral direction
based on a point 1.00 m from the front of the PAL.
4.2 Experimental Results
As the results of this experiment, Fig. 6 shows the ADE between the D-MDD ݈ and the O-MDDᇱ‫ݔ‬୫ୟ୶
in each condition. In Fig. 6, the horizontal axis shows the error between the D-MDD ݈ and theᇱand the vertical axis shows the D-MDD ݈. From Fig. 6, we confirmed that the proposed
O-MDD ‫ݔ‬୫ୟ୶
method can control the MDD with an error within 0.047 m on average. We also confirmed that thevariance was 0 and it is reproducible. Figure 7 shows the estimated MDD of the conventional andproposed methods. In Fig. 7, the horizontal axis shows the MDD and the vertical axis shows the Dᇱ
MDD ݈ . The average error between estimated MDD and O-MDD ‫ݔ‬୫ୟ୶
was 0.432 m in theconventional method and 0.047 m in the proposed method. From these results, we confirmed that the
proposed method can control the MDD more accurately than the conventional method.
Figure 8 shows the sound pressure distributions in each condition. As shown in Fig . 8, we
confirmed that the high SPL was obtained near the D-MDD ݈ in each condition. Table 7 shows theradiation angles in each condition. As shown in Table 7, in the case of enclosing CO2 or SF6 , gases
whose density is higher than air, we confirmed that the radiation angle spread compared to the one in
the case of the AM wave transmits through only the air. Besides, we confirmed that the radiation angle
in the case of enclosing SF6 spread larger than that in the case of enclosing CO2 . This is because SF6
has a larger density difference with air than CO2. However, this spread of the radiation angle barely
affects the performance of the PAL in practical use. Furthermore, we confirmed that the radiation
angle spread with the increase of the film gas-lens depth ‫ ݓ‬in the case of enclosing CO2 and SF6 .
Similarly, the radiation angle narrowed down with the increase of the film gas-lens depth ‫ ݓ‬in thecase of enclosing He. From these results, we confirmed that the influence of the gas increased with
the increase of the film gas-lens depth ‫ݓ‬. Thus, we confirmed that the proposed method can control
the MDD to be equal to any listener’s positions.

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ᇱ
in each condition.
Figure 6 – Experimental results for ADE between the D-MDD ݈ and the O-MDD ‫ݔ‬୫ୟ୶

Figure 7 – Estimated MDD of the conventional and proposed methods.

(a) Air(‫ݔ‬ୟ୧୰ =1.000 m)(b) SF6(d) CO2(c) SF6(e) CO2(D-MDD ݈=0.750 m,(D-MDD ݈=0.500 m,(D-MDD ݈=0.625 m,ᇱᇱᇱ
=0.750 m,
O-MDD ‫ݔ‬୫ୟ୶
=0.500 m, O-MDD ‫ݔ‬୫ୟ୶
=0.500 m, O-MDD ‫ݔ‬୫ୟ୶and ‫=ݓ‬0.25 m)
and ‫=ݓ‬0.16 m)
and ‫=ݓ‬0.11 m)(f) He(D-MDD ݈=1.125 m,
ᇱO-MDD ‫ݔ‬୫ୟ୶=1.125 m,
and ‫=ݓ‬0.10 m)(g) He(h) He(D-MDD ݈=0.875 m,
ᇱO-MDD ‫ݔ‬୫ୟ୶
=0.875 m,
and ‫=ݓ‬0.12 m)(i) He(D-MDD ݈=1.250 m,(D-MDD ݈=1.375 m,(D-MDD ݈=1500 m,ᇱᇱᇱ
O-MDD ‫ݔ‬୫ୟ୶=1.250 m, O-MDD ‫ݔ‬୫ୟ୶=1.250 m, O-MDD ‫ݔ‬୫ୟ୶
=1.500 m,
and ‫=ݓ‬0.19 m)
and ‫=ݓ‬0.27 m)
and ‫=ݓ‬0.34 m)

Figure 8 – Experimental results for sound pressure distributions in each condition.

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Table 7 – Experimental results for radiation angles in each condition.
Gas

Radiation angle

Air (‫ݔ‬ୟ୧୰ =1.000 m)

3.26 deg.

ᇱSF6 (D-MDD ݈=0.500 m, O-MDD ‫ݔ‬୫ୟ୶
=0.500 m, and ‫=ݓ‬0.16 m)

9.92 deg.

ᇱSF6 (D-MDD ݈=0.625 m, O-MDD ‫ݔ‬୫ୟ୶
=0.500 m, and ‫=ݓ‬0.11 m)

9.67 deg.

ᇱCO2 (D-MDD ݈=0.750 m, O-MDD ‫ݔ‬୫ୟ୶
=0.750 m, and ‫=ݓ‬0.25 m)

6.39 deg.

ᇱCO2 (D-MDD ݈=0.875 m, O-MDD ‫ݔ‬୫ୟ୶
=0.875 m, and ‫=ݓ‬0.12 m)

5.88 deg.

ᇱHe (D-MDD ݈=1.125 m, O-MDD ‫ݔ‬୫ୟ୶=1.125 m, and ‫=ݓ‬0.10 m)

4.87 deg.

ᇱHe (D-MDD ݈=1.250 m, O-MDD ‫ݔ‬୫ୟ୶=1.250 m, and ‫=ݓ‬0.19 m)

3.99 deg.

ᇱHe (D-MDD ݈=1.375 m, O-MDD ‫ݔ‬୫ୟ୶=1.250 m, and ‫=ݓ‬0.27 m)

3.73 deg.

ᇱHe (D-MDD ݈=1500 m, O-MDD ‫ݔ‬୫ୟ୶
=1.500 m, and ‫=ݓ‬0.34 m)

3.62 deg.

5. CONCLUSIONSIn this paper, we proposed a new MDD control based on an analytic model between the film gaslens depth and the O-MDD for PAL. First, to design the analytic model, we investigate the O-MDD
corresponding to the film gas-lens depth set at equal intervals and design a regression curve in eachgas through the preliminary experiments. Next, to calculate the appropriate film gas-lens depth
corresponding to the D-MDD, we design the analytic model as an inverse function of the regression
curve. The analytic model can accurately adjust the film gas-lens depth to control the MDD. Thus, we
can achieve the accurate MDD control for PAL by utilizing the analytic model. It becomes able totransmit the audible sound with maximum SPL to the listener at any points. As the results of theevaluation experiment, we confirmed the effectiveness of the proposed method and the proposed
method can control the MDD more accurately than the conventional method. In future work, we willattempt to the MDD control by temperature or humidity.

ACKNOWLEDGEMENTS
This work was partly supported by JST COI, and JSPS KAKENHI Grant Numbers JP18K19829,
JP18K11365, and JP19H04142.

REFERENCES
1. P. J. Westervelt, “Parametric acoustic array,” Journal of the Acoustic Society of America, vol. 35, no. 4,
pp. 535-537, 1963.
2. K. Aoki, T. Kamakura, and Y. Kumamoto, “Parametric loudspeaker-characteristics of acoustic field andsuitable modulation of carrier ultrasound,” Elections and Communications in Japan, vol. 74, no. 9, pp.
76-82, 1991.
3. C. Shi, and W. S. Gan, “Development of parametric loudspeaker: a novel directional sound generation
technology,” Proceedings of IEEE Potentials, vol. 29, no. 6, pp. 20-24, 2010.
4. M. Yoneyama, J. Fujimoto, Y. Kawamoto, and S. Sasabe, “The audio spotlight: An application of
nonlinear interaction of sound waves to a new type of loudspeaker design,” Journal of the Acoustic Societyof America, vol. 73, no. 5, pp. 1532-1536, 1983.
5. D. Ikefuji, M. Nakayama, M. Morise, and T. Nishiura, “Weighted double sideband modulation toward
high quality audible sound on parametric loudspeaker,” IEEE International Conference on Acoustics,
Speech and Signal Processing 2013, pp. 843-847, 2013.
6. K. A. Naugol’nykh, S. I. Soluyan, and R. V. Khokhlov, “Nonlinear interaction of sound waves in anabsorbing medium,” Soviet Physics Acoustics, vol. 9, pp. 155-159, 1963.
7. K. Ariyoshi, T. Fukumori, M. Nakayama, and T. Nishiura, “Evaluation on reproducing sound field control
with film acoustic-lens for parametric loudspeaker,” in INTER-NOISE and NOISE-CON Congress andConference Proceedings. Institute of Noise Control Engineering, pp. 1881-1888, 2017.

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