Active Noise ControlActive Noise Controlof Small Axial Cooling Fansof Small Axial Cooling Fans

Brian B. Monson, Scott D. Sommerfeldt, Brian B. Monson, Scott D. Sommerfeldt, Connor DukeConnor Duke

Department of Physics and AstronomyDepartment of Physics and AstronomyBrigham Young UniversityBrigham Young University

147th Meeting of the Acoustical Society of 147th Meeting of the Acoustical Society of AmericaAmerica

New York, New YorkNew York, New YorkMay 25th, 2004May 25th, 2004

OverviewOverview Introduction and BackgroundIntroduction and Background

Experimental ApparatusExperimental Apparatus

Theoretical ModelingTheoretical Modeling

Experimental ResultsExperimental Results

ConclusionsConclusions

Active Noise ControlActive Noise Control Wave Superposition Wave Superposition

(destructive (destructive interference)interference)

ppTT(p) = p(p) = p11(p) + p(p) + p22(p) ≈ 0(p) ≈ 0

Source CouplingSource Coupling(modified radiation (modified radiation

impedance)impedance)

ZZ11 = Z = Z1111 + Z + Z1212

ZZ22 = Z = Z2222 + Z + Z2121

Fan Noise SpectrumFan Noise Spectrum

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

Fundamental: blade passage frequency (BPF)

Cooling Fan Control Cooling Fan Control SystemSystem

Developed by Gee, Sommerfeldt

Statement of ProblemStatement of Problem

Due to industry constraints, the Due to industry constraints, the previous fan ANC system developed previous fan ANC system developed is not optimized for a real world is not optimized for a real world setting.setting.

Need a compact system to comply Need a compact system to comply with increasing efforts to decrease with increasing efforts to decrease the size of electronic office the size of electronic office equipment (e.g. desktop computers).equipment (e.g. desktop computers).

Cooling Fan Control Cooling Fan Control SystemSystem

Previous SystemPrevious System 80 mm fan80 mm fan 32 mm loudspeakers32 mm loudspeakers 370 Hz BPF370 Hz BPF

ModificationsModifications 60 mm fan60 mm fan 20 mm loudspeakers20 mm loudspeakers 600 Hz BPF600 Hz BPF Increase fan speedIncrease fan speed

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

Theoretical ModelingTheoretical ModelingControl loudspeakers

Fan

Theoretical ModelingTheoretical Modeling

Nelson, Curtis, Elliott, BullmoreNelson, Curtis, Elliott, Bullmore

Optimum minimization of radiated power

Global MeasurementsGlobal Measurements

15°

18°

80 mm Fan Results80 mm Fan Results

60 mm Fan Results60 mm Fan Results

Global ComparisonGlobal Comparison

80 mm80 mm 60 mm60 mm IdealIdeal

kdkd MPRMPR kdkd MPRMPR kdkd MPRMPR

BPFBPF 0.40.4 10.1 10.1 dBdB

0.50.5 13.6 13.6 dBdB

0.50.5 ~30 ~30 dBdB

2 x 2 x BPFBPF

0.80.8 16.1 16.1 dBdB

1.01.0 17.6 17.6 dBdB

1.01.0 22 dB22 dB

3 x 3 x BPFBPF

1.21.2 12.8 12.8 dBdB

1.51.5 10.5 10.5 dBdB

1.51.5 14 dB14 dB

Mean-square pressure reduction (MPR)

ConclusionsConclusions Noise reduction does not yet achieve Noise reduction does not yet achieve

theoretical ideal predicted by Nelson theoretical ideal predicted by Nelson et et alal., particularly with BPF., particularly with BPF

Compact system, while remaining less Compact system, while remaining less obtrusive, is able to achieve as good of obtrusive, is able to achieve as good of global noise control performance as global noise control performance as previous system on first three harmonicsprevious system on first three harmonics

Replacement of 80 mm fan with 60 mm Replacement of 80 mm fan with 60 mm fan and ANC system feasiblefan and ANC system feasible

Future WorkFuture Work

Optimize loudspeaker enclosures for Optimize loudspeaker enclosures for better low frequency response better low frequency response (including THD analysis)(including THD analysis)

Examine and compare airflow output Examine and compare airflow output of fan with and without controlof fan with and without control

Analyze control performance with Analyze control performance with increased 60 mm fan speedincreased 60 mm fan speed

AcknowledgementsAcknowledgements

Kent Gee, PSUKent Gee, PSU Benjamin Faber, BYUBenjamin Faber, BYU Lance Locey, BYULance Locey, BYU BYU Acoustics Research GroupBYU Acoustics Research Group