Active Noise Control of Small Axial Cooling Fans

1
Active Noise Controlof 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

2
Overview

• Introduction and Background
• Experimental Apparatus
• Theoretical Modeling
• Experimental Results
• Conclusions

3
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

4
Fan Noise Spectrum

• Harmonically related tones superimposed on a
  broadband spectrum (tones are dominant)

• Fundamental blade passage frequency (BPF)

5
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

8
On-axis Comparison

• 80 mm vs. 60 mm broadband (on axis,
  non-calibrated, d 1 m)

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Theoretical Modeling
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Theoretical Modeling
Optimum minimization of radiated power

• Nelson, Curtis, Elliott, Bullmore

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Global Measurements
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80 mm Fan Results
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60 mm Fan Results
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Global Comparison
Mean-square pressure reduction (MPR)
80 mm 80 mm 60 mm 60 mm Ideal Ideal
kd MPR kd MPR kd MPR
BPF 0.4 10.1 dB 0.5 13.6 dB 0.5 30 dB
2 x BPF 0.8 16.1 dB 1.0 17.6 dB 1.0 22 dB
3 x BPF 1.2 12.8 dB 1.5 10.5 dB 1.5 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

16
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

17
Acknowledgements

• Kent Gee, PSU
• Benjamin Faber, BYU
• Lance Locey, BYU
• BYU Acoustics Research Group