Active noise control on a large‐scale ducted fan with inlet wave‐number sensing Free

Active noise‐control (ANC) methods for reduction of turbofan inlet noise have been previously demonstrated on a business‐sized turbofan engine. The potential of ANC is investigated here on a large 4‐ft‐diameter ducted fan which has a geometry and tone frequencies which are more characteristic of current and future large production engines. The control inputs are implemented with a single circumferential array of acoustic drivers constructed with rare‐earth magnets. Both far‐field pressure transducers and an inlet wave‐number sensing technique are investigated as error‐sensing strategies to reduce the 2BPF tone of the ducted fan (approximately 1000 Hz.) The wave‐number sensing strategy involves filtering and combination of inlet‐mounted microphone signals to result in the observation of a particular wave number propagating axially out of the inlet. The ducted fan is equipped with a rotating microphone array in the inlet, which allows for modal analysis of the inlet acoustic field before and after control. The in‐duct mode measurements provide insight into the control mechanisms. The test facility is also equipped for making far‐field pressure measurements. Experimental results for a single circumferential control source array in conjunction with far‐field pressure and inlet wave‐number sensing techniques will be presented and discussed. [Work supported by NASA Langley Research Center.]