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Active Noise Control of Small Axial Cooling Fans
Brian B. Monson, Scott D. Sommerfeldt, Connor Duke Department of Physics and Astronomy Brigham Young University 147th Meeting of the Acoustical Society of America New York, New York May 25th, 2004
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Overview Introduction and Background Experimental Apparatus
Theoretical Modeling Experimental Results Conclusions
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Active Noise Control Wave Superposition (destructive interference)
pT(p) = p1(p) + p2(p) ≈ 0 Source Coupling (modified radiation impedance) Z1 = Z11 + Z12 Z2 = Z22 + Z21
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Fan Noise Spectrum Harmonically related tones superimposed on a broadband spectrum (tones are dominant) Fundamental: blade passage frequency (BPF)
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Cooling Fan Control System
Developed by Gee, Sommerfeldt
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Statement of Problem Due to industry constraints, the previous fan ANC system developed is not optimized for a real world setting. Need a compact system to comply with increasing efforts to decrease the size of electronic office equipment (e.g. desktop computers).
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Cooling Fan Control System
Previous System 80 mm fan 32 mm loudspeakers 370 Hz BPF Modifications 60 mm fan 20 mm loudspeakers 600 Hz BPF Increase fan speed
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On-axis Comparison 80 mm vs. 60 mm broadband (on axis, non-calibrated, d = 1 m)
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Theoretical Modeling Control loudspeakers Fan
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Optimum minimization of radiated power
Theoretical Modeling Optimum minimization of radiated power Nelson, Curtis, Elliott, Bullmore
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Global Measurements 15° 18°
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80 mm Fan Results
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60 mm Fan Results
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Mean-square pressure reduction (MPR)
Global Comparison Mean-square pressure reduction (MPR) 80 mm 60 mm Ideal kd MPR BPF 0.4 10.1 dB 0.5 13.6 dB ~30 dB 2 x BPF 0.8 16.1 dB 1.0 17.6 dB 22 dB 3 x BPF 1.2 12.8 dB 1.5 10.5 dB 14 dB
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Conclusions Noise reduction does not yet achieve theoretical ideal predicted by Nelson et al., particularly with BPF Compact system, while remaining less obtrusive, is able to achieve as good of global noise control performance as previous system on first three harmonics Replacement of 80 mm fan with 60 mm fan and ANC system feasible
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Future Work Optimize loudspeaker enclosures for better low frequency response (including THD analysis) Examine and compare airflow output of fan with and without control Analyze control performance with increased 60 mm fan speed
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Acknowledgements Kent Gee, PSU Benjamin Faber, BYU Lance Locey, BYU
BYU Acoustics Research Group
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