Information technology (IT) noise is very prevalent in today’s society. Active noise control (ANC) has shown promise in minimizing the effect of fan-induced IT noise on users. Much of the previous research has concentrated on axial cooling fans, such as those found in desktops and servers. This approach was based on the concept of minimizing radiated acoustic power in a model of the fan radiation, and using those results to determine appropriate nearfield locations for the error sensor(s). This paper describes modifications to this previous method to develop a modeling approach to implement active noise control with a centrifugal blower, such as those found in fan trays and laptop computers. This model has been used to predict tonal noise inside and outside the duct, as well as how to best develop an ANC system for such an idealized setup. Differences between the axial fan model and the centrifugal blower model are discussed, as well as some limiting assumptions for each model.

Previous work by these authors in active control of axial fans suggests an approach that can be successful in applying active control to small centrifugal fans used in fan trays and laptop computers. The modeling and analysis strategies developed for axial fans were modified for use with centrifugal fans mounted in a rectangular exhaust duct. Experimental verification allowed for proper inclusion of damping in the model. By minimizing the sound power radiated from the duct, optimal error sensor placement was predicted. Experimental results verified the effectiveness of placing the error sensor at these locations. Using predicted control source and error sensor locations, the rectangular duct was replaced by a centrifugal fan and duct attached to a heat sink, with the total dimensions being the same as the previous rectangular duct. The experimental results indicate that significant global reduction of the radiated tonal fan noise can be achieved.

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Previous work on active control of exhaust noise from small centrifugal fans demonstrated significant reductions of the blade passage frequency (BPF) tone. A fan and heat-sink were placed within a mock-up notebook computer case, and control of the fan exhaust noise was measured. It was found that control of the BPF in the exhaust did not significantly affect noise radiated from the fan inlets into the notebook casing, suggesting that exhaust noise and inlet noise may be controlled separately without one adversely affecting the other. In the current work, a two dimensional half-space, source coupling model has been developed to calculate the field within the notebook casing caused by the inlet noise. As a first approximation, free-space boundary conditions were used. A two-dimensional space was constructed to test the model, and error sensor placement was predicted. Measurements of radiated sound power show significant reduction of the blade passage frequency tone. Factors influencing experimental agreement with the model are discussed, such as modal effects and primary source location.

Active noise control (ANC) has shown promise in minimizing the effect of fan noise on users. Recent research by the authors has developed a model which is used to implement ANC on the inlets of centrifugal cooling fans. This model is based on minimizing radiated acoustic power in a model of the fan radiation and using those results to determine appropriate nearfield locations for the error sensor(s). Though this approach has been experimentally verified in an idealized setting, it was not verified in a more realistic situation. This paper describes how this model was expanded from its idealized setting to a mock laptop enclosure. When necessary modifications to the model were made, tonal noise can be predicted in the nearfield of the fan inlets, which allows one to develop an effective compact, realistic ANC setup for use in the mock laptop enclosure. With this ANC setup, significant global reduction of the inlet tonal noise can be achieved.