Modal Technique for Active Control of Tones Radiated from a Ducted Fan

Abstract

This paper describes research in the development of a modal technique considered appropriate for the active noise control (ANC) of fan tones. Two essential issues were addressed to establish this technique. The e rst was to develop a waveform generation system, which can produce the arbitrary spinning modes necessary for tone cancellation. The second was to develop a modal detection system for the spinning modes. An examination of two systems showed that the desired spinning modes in the circumferential direction of a duct could be produced, and that the contents of the acoustic spinning modes in the duct could be determined with a high accuracy. A technological assessment of the modal technique composed of the two systems was challenged using a ducted fan. At a fan speed of 4125 rpm, the noise reduction in the blade-passage frequency could be achieved ranging from 2 to 10 dB without spillover caused by a secondary sound source. Although these experimental results are limited, they suggest that the modal technique for ANC of acoustic spinning modes in fan tones may be feasible. Additional technical issues such as the simultaneous noise reduction of higher harmonics and the sound power needed for the secondary sound sources should be resolved for further noise reduction.