1. 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

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

6. 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).

7. 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)

9. Theoretical Modeling
Control loudspeakers
Fan

10. Optimum minimization of radiated power
Theoretical Modeling
Optimum minimization of radiated power
Nelson, Curtis, Elliott, Bullmore

11. Global Measurements
15°
18°

12. 80 mm Fan Results

13. 60 mm Fan Results

14. 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

15. 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