Active control of drag noise from a small axial flow fan

Abstract

Noise sources in an axial flow fan can be divided into fluctuating axial thrust forces and circumferential drag forces. For the popular design of a seven-blade rotor driven by a motor supported by four struts, drag noise dominates. This study aims to suppress the drag noise globally by active control schemes. Drag noise features a rotating dipole and it has to be cancelled by a secondary source of the same nature. This is achieved experimentally by a pair of loudspeakers positioned at right angles to each other on the fan rotational plane. An adaptive LMS feedforward scheme is used to produce the control signal for one loudspeaker and the time derivative of this signal is used to drive the other loudspeaker. The antisounds radiated by the two loudspeakers have a fixed phase relation of 90° forming a rotating dipole. An open-loop control scheme is also implemented for the purpose of comparison and easier implementation in real-life applications. The results show that the globally integrated sound power is reduced by about 13 dB for both closed-and open-loop schemes. A possible limiting factor for the cancellation performance is found to be the presence of higher order modes of drag noise.