Use of dipole resonator configurations for bi-directional attenuation of plane wave blade tone noise propagations

Flow-excited, tunable quarter-wavelength resonators can be integrated into the shrouds of ducted subsonic axial fans. This study explores their efficacy in reducing plane wave propagations of tonal noise generated through rotor stator interaction. The use of two tunable resonator chambers oriented axially on either side of the blade region enables a dipole-like secondary sound field to be passively generated. Sources are then tuned in terms of magnitude and phase to reduce bi-directional propagations of blade tone noise. Resonators are oriented such that they are driven directly by the existing periodic pressure field in the region of the rotor's blade-tips, exploiting the nearly 180 degree phase change of the pressure field incident on the fan's shroud across the rotor blade plane. Individual resonator response is governed by the opening's proximity to the rotor blade's leading edge and through adjustment of the impedance of the individual resonator tube's back wall. The dipole resonator magnitude is adjusted by the tuning the resonators nearer or further from the BPF, or by resonator axial spacing and resonator impedance. Phasing is controlled by modifying the relative circumferential position of the resonators relative to adjacent stator vanes. Dipole resonator source effectiveness is verified on a 260 mm diameter radiator cooling fan mounted in an anechoically terminated, ducted facility. Blade passage frequency (BPF) noise reductions of 12.9 dB and 11.6 dB were achieved simultaneously in the upstream and downstream directions, to levels within 5 dB of the broadband level.