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

Overview Introduction and Background Experimental Apparatus Theoretical Modeling Experimental Results Conclusions

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

Fan Noise Spectrum Harmonically related tones superimposed on a broadband spectrum (tones are dominant) Fundamental: blade passage frequency (BPF)

Cooling Fan Control System Developed by Gee, Sommerfeldt

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).

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

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

Theoretical Modeling Control loudspeakers Fan

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

Global Measurements 15° 18°

80 mm Fan Results

60 mm Fan Results

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

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

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

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