Feasibility of a Length-limited Parametric Source for Active Noise Control Applications. Bhan Lam....

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Feasibility of a length-limited parametricsource for active noise control applications

Conference Paper · July 2014

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3 authors:

Bhan Lam

Nanyang Technological University

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Woon-Seng Gan


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Chuang Shi


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Available from: Chuang ShiRetrieved on: 26 May 2016

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ICSV21, Beijing, China, 13-17 July 2014 1

The 21 st International Congress on Sound and Vibration

13-17 July, 2014, Beijing/China

FEASIBILITY OF A LENGTH-LIMITED PARAMETRICSOURCE FOR ACTIVE NOISE CONTROL APPLICA-TIONSBhan Lam, Wong-Seng Gan, Chuang Shi

Digital Signal Processing Laboratory, School of Electrical & Electronic Engineering, Nan- yang Technological University

e-mail: [email protected]

In a previous work presented by the authors in ICSV20, we proposed a theoretical frameworkto generate a length-limited parametric source for active noise control (ANC). The length-limited parametric source consists of two parametric loudspeakers on the same axis. Twoseparate ultrasonic carrier frequencies are applied to the different parametric loudspeakers,resulting in different absorption lengths. By controlling the input phases and amplitudes ofthe two parametric loudspeakers, their secondary sound waves can suppress each other in theoverlapped area but retain in the difference area. An advantage of the length-limited paramet-ric source in active noise control is its ability to target a specific control region while mini-mizing the spillover effect of the conventional parametric source. However, the theoreticalframework from the previous study has not been experimentally validated, and this paperaims to do the latter. A concentrically-nested parametric loudspeaker, which is driven by twoultrasonic frequencies with user-controllable settings, was built to conduct this experiment.The experimental setup consists of National Instruments’ data acquisition modules and Brüel& Kjærs’ microphones. The sound field along the length of the two absorption distances wasmeasured to evaluate the suppression achieved in the overlapped region and retention in thedifference region. Two types of signals were tested in the length-limited parametric sourcesetup: (1) single tone and (2) linear chirp. In a non-adaptive ANC setup, a length-limited par-ametric beam was employed as the ‘anti- noise’ source to locally control the noise source pro-duced by a conventional loudspeaker. From the experimental observations, further investiga-tions can be conducted to improve the length-limited effect for narrowband signals.

1. Length-limited parametric secondary sources in ANC

One limitation of active noise control (ANC) systems, which utilises dynamic speakers as an-ti-noise sources, is the physical difficulty in collocating the noise source and anti-noise source toachieve an ideally wide ‘quiet -zone’ .1 Thus, ANC systems usually produce localised ‘quiet -zones’resulting in an undesired increase in sound pressur e level (SPL) at areas outside the ‘quiet -zones’,also known as the ‘spill -over’ effect. Moreover, even if the aim of the ANC system is to produce alocalised ‘quiet -zone’ , the ‘spill -over’ effect cannot be avoided due to the Omni-directional natureof dynamic speakers. On the contrary, a highly-directive ‘anti -noise’ source generated by a para-metric loudspeaker could potentially reduce the ‘spill -over’ area.


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21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014


ParametricSources

Second Beam

First Beam

Length-limited Beam

Figure 1: Illustration of proposed length-limited parametric source in ANC .2

Parametric speakers generate highly-directive audible sound beams based on the self-demodulation effect in the nonlinearity of air .3-5 Since Brooks et al. , numerous researchers havetested ANC systems with parametric ‘anti-noise ’ sources .2, 5, 6 Notably, Kidner et al. have reported areduction in the ‘spill -over’ area when a parametric anti-noise source was simulated based on thenumerical solution of the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation .7, 8 However, the‘spill -over’ effect still exists in t he area of the sound beam between the parametric source and the‘quiet -zone’ location. To further reduce the ‘spill -over’ effect in ANC systems using parametricanti-noise sources, an ANC structure using a length-limited parametric anti-noise source was pro-

posed .2 The length-limited parametric source, shown in Figure 1, is based on the principle of ANC,

where two beams on the same axis (with one at the same amplitude but out-of-phase with the other)nullifying each other in the overlapped region .9, 10 . Through simulations using the numerical solu-tions of the KZK equation, it has been shown that a parametric beam with a higher carrier frequency(shorter absorp tion distance) was able to reduce the ‘spill -over’ effect caused by the beam with alower carrier frequency .2 Hence, this paper aims to experimentally verify the existence of a length-limited parametric source and to deploy the length-limited beam (as the anti-noise source) in a sim-

ple non-adaptive ANC system.

2. Physical implementation of the length-limited source

With reference to simulations conducted in the previous study by the authors, the generationof the length-limited source is formulated based on the same source equations (Eq .(1) and Eq .(2)) employing the double-side band modulation (DSBAM) technique .2

)sin()]sin(1[ 11101 t t m p p d (1)

)sin()]sin(1[ 22202 t t m p p d (2)

However, the center frequencies 1 and 2 are set to 25 kHz and 40 kHz respectively, based

on the ultrasonic emitters (25 kHz and 40 kHz) that will be used in the experiment. The initialsound pressure 0 p of the source equations is set to 124 dB on the surface of both type of emitters.Based on the environmental parameters of the test environment (temperature at 23°C and hu-

midity at 50%), the absorption distances are approximately, 13.7 m and 6.5 m for 25 kHz and 40kHz parametric beams respectively. Since the absorption distance of the 40 kHz beam is shorter, its

parameters will be varied such that the optimum reduction is observed in the region where both beams overlap. Thus, the phases, 1 is fixed at 01 and 2 varies from 02 to 330° at inter-vals of 30°; and modulation indices, 1m is fixed at 11 m and 2m varies from 5.02 m to 0.9 at in-tervals of 0.2.

As both beams have to be located on the same axis and generated from the same source loca-

tion, a concentrically-nested parametric loudspeaker was proposed and built. The parametric speak-er is made up of an inner hexagonal array of 36 (40 kHz) emitters and an outer annular ring of 36(25 kHz) emitters, as shown in Figure 2. The arrays are driven separately by two analog outputs.


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36 X 40 kHzEmitters (Silver)

36 X 25 kHzEmitters (Yellow)

Class DAmp

NI BNC2110

B&K 4961

PC(Labview)

25 kHz Emitters

Class DAmp

NI PCI-6733

NI 9234

Acoustic D omain

Analog Domain

Double-side Open-ended Duct

40 kHz Emitters

Figure 2: Concentrically-nested parametric loudspeaker (Left). Block diagram of experimental setup.(Right)

3. Experimental validation of the length-limited source

The long absorption distances of the 40 kHz and 25 kHz emitters (~7 m & 14 m respectively)coupled with the short Rayleigh distances (~1.5 m for both and beam loses directivity in the far-field), required a test environment that has sufficient length and is not affected by the reflectionscaused by the lost in directivity. Thus, the experiment was confined in a 9.5 m long PVC pipe withan internal diameter of 0.16 m.

The generation and real-time parameter control of the DSBAM signals and data acquisition ofacoustic measurements was developed in a system design software (NI Labview) and implementedin a mix of hardware setup, as shown in Figure 2. In the setup, the parametric array is placed at one

end of the 9.5 m pipe and driven by two separately amplified DSBAM signals from the NI PCI-6733 output module. Acoustic measurements are made by lowering the B&K 4961 multi-field mi-crophone into the centre of the pipe at predefined distances from the parametric array (0.2 m to 4 mat intervals of 0.2 m, 4 m to 9.2 m at intervals of 0.4 m). Acoustic data from the microphone is ac-quired via the NI 9234 analog input module for post-processing. At each distance point, acousticdata is acquired for every combination of the varying 2m and 2 values (i.e., 36123 data points

per distance point).In line with the previous study, the experiment was conducted with the same modulating fre-

quency, 5.12 d kHz .2 In addition to the 1.5 kHz single tone modulating frequency, the exper-

iment was also conducted for a linear chirp from 0.5 kHz to 3 kHz with duration of 1 s.

3.1 Single ToneFigure 3 shows the sound pressure level (SPL) plots of the 1.5 kHz single tone, for 2 from 0°

to 330° along the entire length of the pipe at the defined locations, and for 2m = 0.5 (Figure 3( a)),

2m = 0.7 (Figure 3( b)) and 2m = 0.9 (Figure 3( c)). From comparison of results plotted in Figure 3, atrend of reduction can be observed in the first 4 m when the phase of the 40 kHz source, 2 = 150°.By observation, the most significant reduction is achieved when 2m = 0.9 at 2 = 150°.

To determine the SPL reduction due to the shorter 40 kHz beam, a 1.5 kHz tone was generat-ed with only the 25 kHz source. Figure 4 shows the comparison between the SPLs of the 1.5 kHztone without the 40 kHz source and the length-limited beams with 2m = 0.5, 0.7 and 0.9 at 2 =150°. The average SPL reduction is determined in the first 6.4 m of the pipe, which corresponds to


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the absorption distance of 6.5 m for the 40 kHz source, and are: (1) 5.57 dB reduction for 2m = 0.5,(2) 7.59 dB reduction for 2m = 0.7, and (3) 9.82 dB reduction for 2m = 0.9.

0 1 2 3 4 5 6 7 8 90

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Distance From Emitter in Pipe (m)

Phase,

2

(Deg)

dB Plot for 1.5 kHz Sine at m2

= 0.5

dB/20 Pa

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Phase,

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(Deg)


= 0.9

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(c)

Figure 3: SPL values of the 1.5 kHz single tone along the length of the pipe for (a) 2m = 0.5, (b) 2m =

0.7 and (c) 2m = 0.9, for 2 from 0° to 330°.


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0 1 2 3 4 5 6 7 8 90

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180210

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(D

eg)


dB/20 Pa

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(c)

Figure 5: SPL values for the linear chirp signal along the length of the pipe for (a)2

m = 0.5, (b)2

m =

0.7 and (c) 2m = 0.9, for 2 from 0° to 330°.


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0 1 2 3 4 5 6 7 8 925

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75Comparision between 25 kHz Beam and Length-limited beams


d B / 2 0

P a

25 kHz Beam OnlyLength-limited Beam ( =180 , m=0.2)Length-limited Beam ( =180 , m=0.3)

Length-limited Beam ( =180 , m=0.4)Length-limited Beam ( =180 , m=0.5)Length-limited Beam ( =180 , m=0.7)Length-limited Beam ( =180 , m=0.9)

Length-limited AreaOverlapped Area of 2 Beams

Figure 6: Comparison of linear chirp dB values along the length of the pipe between the 25 kHz sourcelength-limited beams.

0 1 2 3 4 5 6 7 8 9 1035

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70ANC of 1.5kHz Tone at 8 m

d B / 2 0

P a

Distance From Emitter in Pipe(m)

ANC OffANC On

Local Control at 8m

Noise Source at 8 m

Figure 7: SPL values of local ANC at 8 m with a length-limited parametric source as the ‘anti -noise’ source.

4. ANC with a length-limited parametric ‘anti -noise’ source

Since the existence of a length-limited parametric source has been validated experimentallyfor the single tone case, the feasibility of deploying a length-limited parametric source for ANCapplications can be tested. Using the same experimental setup, a noise source (1.5 kHz single tone)generated by a conventional speaker (GENELEC 1029A) is introduced at the 8 m position in the

pipe. The distance of 8 m was chosen as it is greater than the absorption length of a typical 40 kHz parametric array (~6.5 m) and within the length-limited area (~6.5 m to 14 m). A length-limited beam generated with 2 = 150° and 2m = 0.9 will be used as the ‘anti -noise’ source to generate asingle tone (1.5 kHz) for local control of the conventional noise source at 8 m. In contrast to tradi-tional ANC, the phase and amplitude of the noise source at 8 m is adjusted such that there is opti-mum cancellation by the length-limited ‘anti -noise’ source.


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SPL values along the length of the pipe for local control at 8 m are plotted in Figure 7. Theconventional noise source at 8 m was reduced by 18.40 dB from 58.56 dB to 40.16 dB. The dis-crepancy in the waveform for the first 7.6 m of the length-limited beam is due to perturbationscaused by the downstream reflections of the conventional noise source at 8 m.

5. Conclusion

This paper investigated the feasibility of deploying a length-limited source in a local ANCsetup. Firstly, an experimental validation of the length-limited source proposed in a previous studywas conducted for three types of test frequencies .2 The experiment was confined in a 9.5 m longPVC pipe and SPL measurements were taken at predefined intervals along the pipe. A reduction of9.82 dB for single tone was achieved in the length of the pipe corresponding to the absorption dis-tance of the 40 kHz beam. However, there is no conclusive evidence of the length-limited beamformation for a linear chirp. Next, the local ANC of a noise source generated by a conventionalspeaker at the 8 m position of the pipe was demonstrated. An 18.40 dB reduction of a 1.5 kHz sin-gle tone noise at 8 m was achieved with the length-limited par ametric ‘anti -noise’ source. Based onthe results, local ANC was achieved with reduced ‘spill -over’ effect for a single tone. Although ithas been preliminarily shown that a length-limited source can be applied in ANC applications forsingle tones, it is worth noting the perturbations caused by the beating phenomenon between thecarriers of the two parametric sources. Further investigation is required to prevent the introductionof unwanted difference frequencies due to the beating effect.

REFERENCES1 S. M. Kuo and D. R. Morgan, Active noise control systems: algorithms and DSP

implementations : Wiley, 1996. 2 C. Shi and W. S. Gan, "Using length-limited parametric source in active noise control

applications," presented at the 20th International Congress on Sound and Vibration (ICSV20), Bangkok, Thailand, 2013.

3 M. Yoneyama, J. i. Fujimoto, Y. Kawamo, and S. Sasabe, "The audio spotlight: An application of nonlinear interaction of sound waves to a new type of loudspeaker design," The Journal of the Acoustical Society of America, vol. 73, p. 1532, 1983.

4 C. Shi and W.-S. Gan, "Development of parametric loudspeaker," Potentials, IEEE, vol. 29, pp. 20-24, 2010.

5 W.-S. Gan, J. Yang, and T. Kamakura, "A review of parametric acoustic array in air," Applied Acoustics, vol. 73, pp. 1211-1219, 2012.

6 L. A. Brooks, A. C. Zander, and C. H. Hansen, "Investigation into the feasibility of using a parametric array control source in an active noise control system," in Proceedings of ACOUSTICS , 2005, pp. 39-45.

7 Y.-S. Lee, "Numerical solution of the KZK equation for pulsed finite amplitude sound beams in thermoviscous fluids," 1993.

8 M. R. F. Kidner, C. Petersen, A. C. Zander, and C. H. Hansen, "Feasibility study of localized active noise control using an audio spotlight and virtual sensors," in Proc acoustics , 2006.

9 C. Hedberg, K. Haller, and T. Kamakura, "A self-silenced sound beam," Acoustical Physics, vol. 56, pp. 637-639, 2010.

10 H. Nomura, C. M. Hedberg, and T. Kamakura, "Numerical simulation of parametric sound

generation and its application to length-limited sound beam," Applied Acoustics, vol. 73, pp. 1231-1238, 2012.
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Feasibility of a Length-limited Parametric Source for Active Noise Control Applications. Bhan Lam....

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