Active control of axial-flow fan noise

Discrete-frequency axial-flow fan noise reduction using active noise control is described. The unique aspect of the current research is the use of the fan itself as the antinoise source in the active noise control scheme. This is achieved by driving the entire fan unit axially with an electrodynamic shaker which mechanically couples the solid surfaces of the fan to the acoustic medium. The fan unit is thus transformed into a crude loudspeaker. A near-field microphone serves as an error sensor, where transfer function measurements between the electrical input to the shaker and the electrical output of the microphone are found to be reasonably free of phase distortions and linear. A feedforward algorithm utilizing the output of a tachometer as a reference signal is used. The experimental apparatus is composed of a baffled fan unit in a free field. A small cylindrical flow obstruction is placed on the inlet side of the fan to enhance noise emissions at the blade-pass frequency and harmonics. The experiment successfully demonstrates the concept of active control of tonal fan noise using a shaken fan as the cancellation source. For the fan operating in a planar baffle, the fundamental blade-passage frequency sound-pressure level at the location of the error sensor is reduced by 20 dB, while the second and third harmonic levels are reduced by 15 and 8 dB, respectively. Placing a cabinet enclosure over the baffled fan did not affect these results significantly, and free-field sound power measurements indicate similar level reductions with the active control in operation. (C) 1997 Acoustical Society of America.