Применение активного контроля воздушного шума вентилятора мультимедийного проектора
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Electronic Journal «Technical Acoustics» http: //www. ejta. org
Andrey Troshin*, Andrey Bykov
IT Magic Co. Ltd. 4rd WonWoo Bldg, 907−16 Deachi-dong, Gangnam-gu, Seoul Republic of Korea
Implementation the active noise control to fan airborne noise of multimedia projector
Received 23. 08. 2006, published 11. 09. 2006.
Implementation the active noise control technology using Digital Signal Processing (DSP) core is presented. The case of airborne noise for EPSON projector fan is studied. The noise source ranking based on separate running method has been implemented to reveal the most significant noise source among three fans incorporated into multimedia projector. A short duct system containing noise cancelling speaker, reference and error microphones and DSP board with ported adaptive feed forward algorithm to cancelling critical tonal components of the fan noise were applied. The suppression of 6… 10 dB for airborne noise of the fan in multimedia projector at tonal frequencies was obtained.
Active noise control (ANC) means that noise reduction happens due to generation of the anti sound using special technology based on adaptive algorithm. Theoretical basis of active noise control was developed by Nelson and Elliott and can be found in the reference . Further development of active noise control technology was proposed by Colin H. Hansen  and Scott D. Snyder . The core of Digital Signal Processing (DSP) algorithms for active noise control can be found in the book  written by Sen M. Kuo, Dennis R. Morgan.
Active noise control means not only suppression the acoustic field using electroacoustic equipment and DSP technology. Sometimes the noise reduction can be provided by means of active vibrational control if acoustic field strongly correlated to vibration field.
Noise reduction of multimedia equipment like computer fans and hard disks, fans of multimedia projector becomes an actual problem nowadays. The proper mechanical and aerodynamic design can substantially reduce both structure borne and airborne noise components. Active noise control could be ultimate solution.
There are some advantages for active noise control implementation in the case of the small-sized fan. First an acoustic field inside fan duct is one dimensional. This fact allows developing a very simple and therefore cost effective single channel active noise control system. Usually, for three dimensional sound fields the active noise control solution is hardly to be cost effective. The multi channel system with compensational loudspeakers separated by distance equal to one half of wavelength is required for the general case. The small sizes of the fan make possible to suppress rather high frequency component which could be higher
Corresponding author, e-mail: atroshin@yahoo. com
than 500 Hz. The latest value is well-known like a high frequency limitation for application of the active noise control.
The aim of the paper is to report the practical results about implementation the active noise control solution to the case of airborne noise for cooling fan incorporated into multimedia projector.
1. NOISE SOURCE OBJECTIVES AND STUDY
The most important step to the noise control is the noise source ranking. It means that we should find out which source contributes more power into acoustic field comparing to others. There are three fans incorporated into multimedia projector. The arrangement and denotation of the fans inside projector casing is shown in Fig. 1.
Fig. 1. Fans allocation and denotation in the multimedia projector
To determine which fan contributes more acoustic power into total acoustic field the separate running test was performed. Each fan was run while others were turned off.
Noise narrow band spectra for three fans are shown in Fig. 2. Measurement was performed with frequency resolution 1. 465 Hz, number of averaging — 50. The noise spectrum of the product contains the prominent tonal components and Fan N2 contributes much more power at tonal components into total acoustic field than others. Thus, it is expedient to implement the active noise control solution to Fan N2.
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Fig. 2. Narrow band noise spectrum of the fans, separate running test: black — Fan N1, red — Fan N2, blue- Fan N3
2. ACOUSTIC AND DSP HARDWARE CONFIGURATIONS
Hardware block diagram and basics of active noise cancellation are shown in Fig. 3.
Noise canceling speaker
Fig. 3. ANC hardware block diagram and basics of active noise control
The ANC system comprises two high sensitive microphones, anti noise canceling loudspeaker, power amplifier and DSP board with adaptive feed forward algorithm. The acoustic hardware configuration is depicted in Fig. 4. Error and reference signal are picked-up by reference and error electret-type microphones respectively and then sent to analog input section of DSP board. Both signals converted to appropriate digital format using audio codec. The adaptive algorithm was downloaded to DSP core and performed the generation of
compensational signal which inputs to power amplifier. After gaining the compensational signal drives noise canceling loudspeaker. The DSP electronic block and power amplifier are shown in Fig. 5.
Fig. 4. Configuration of acoustic hardware: 1 — short duct, 2 — noise cancelling loudspeaker, 3 — reference microphone, 4 — error microphone
3. TESTING RESULTS
Testing results are shown in Fig. 6. It can be seen that six tonal components are reduced by 6.. 10 dB and it is audible. The more high efficiency was not achieved due to very short delay in primary transfer path. The low group delay codec was implemented. This short delay in primary transfer path is due to space limitation which leads to implement a short duct system.
500 1000 1500 2000 2500 3000
Fig. 6. Fan noise spectra: black — before running ANC, red — after running ANC
It has been found that centrifugal cooling fan incorporated into multimedia projector generates significant amount of tonal noise. The feed forward active noise control system based on stand alone embedded system was implemented in situ. In order to compensate the total delay in the system to provide convergence of adaptive algorithm the off-line modeling and acoustic feedback compensation technology were implemented. Details about off-line modeling can be found in . It has been found that rather high critical frequencies can be compensated. The measured efficiency of ANC system of 6. 10 dB was obtained.
Authors express their gratitude to IT Magic Co. Ltd, Republic of Korea and Sony Corporation, Japan for financial support of this project. Special thank goes to Dr. Chung-mo Kang, CEO of IT Magic Co. Ltd for his kind help.
1. Nelson P. A., Elliott S. J. Active control of sound. Academic press, 1992.
2. Colin H. Hansen. Understanding active noise cancellation. Spon Press, London and New York, 2001.
3. Scott D. Snyder. Active Noise Control. Primer AIP Press Springer, University of Adelaide, 2000.
4. Sen M. Kuo, Dennis R. Morgan. Active noise control: systems algorithms and DSP implementations. John Wiley and Sons Inc, 1996.