Active control of fan-generated tone noise

An experiment to control the noise radiated from the inlet of a ducted fan using a time domain active adaptive system is reported on. The control sound source consists of loudspeakers arranged in a ring around the fan duct, The error sensor location is in the fan duct. The purpose of this experiment is to demonstrate that the in-duct error sensor reduces the mode spillover in the far field, thereby increasing the efficiency of the control system. The control system is found to reduce the blade passage frequency tone significantly in the acoustic far field when the mode orders of the noise source and of the control source are the same, when the dominant wave in the duct is a plane wave. The presence of higher order modes in the duct reduces the noise reduction efficiency, particularly near the mode cut on where the standing wave component is strong, but the control system converges stably. The control system is also found to be stable and convergent when the first circumferential mode is generated in the duct both by the fan and by the control source, The control system is found to reduce the fan noise in the far field on an are around the fan inlet by as much as 20 dB with none of the sound amplification associated with mode spillover.