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 co ntrol sound source consists of loudspeakers arranged in a ring around the fan duct, The error sensor location is in the fan duct. The purpos e of this experiment is to demonstrate that the in-duct error sensor r educes the mode spillover in the far field, thereby increasing the eff iciency of the control system. The control system is found to reduce t he blade passage frequency tone significantly in the acoustic far fiel d when the mode orders of the noise source and of the control source a re the same, when the dominant wave in the duct is a plane wave. The p resence 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 cont rol system is also found to be stable and convergent when the first ci rcumferential 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 i n the far field on an are around the fan inlet by as much as 20 dB wit h none of the sound amplification associated with mode spillover.