Control of tonal noise from subsonic axial fan. Part 2: active control simulations and experiments in free field

This paper deals with the global control of engine cooling fan noise in free field at the Blade Passing Frequency (BPF) and its first harmonic. The aim of this paper is to investigate the feasibility of using a single loudspeaker in front of the fan to cancel the tonal noise. A simplified model of fan noise, which only takes into account the most radiating circumferential mode of the forces acting by the fan on the fluid, is first combined with an unbaffled loudspeaker model to predict the residual sound field for various sensor configurations. Metrics for global control such as sound directivity or sound power attenuation reveal that the control is effective with this simplified model in the whole space at low frequency, depending on the number and location of the error sensors. However, for non-homogeneous flow, other circumferential modes may contribute to the sound radiation and then, the inverse model described in the companion paper is used to provide a more accurate extrapolated sound field from the reconstructed unsteady aerodynamic forces acting by the fan on the fluid. Simulation results demonstrate the global control in the downstream half space of the blade passing frequency and its first harmonic using a single error microphone and a single control source. A single-input-single-output (SISO) adaptive feedforward controller is implemented experimentally to drive the control loudspeaker. The tones at the BPF and at its first harmonic are attenuated by up to 28 and 18 dB, respectively at the far field error microphone.