Abatement of a Multi-Rotor Tonal Noise Component with Phase Control Technology
Ofek Hertzman, Shir Fligelman, Oksana Stalnov
Abstract
The annoying sound produced by a multi-rotor vehicle can bring a severe acoustic nuisance to an urban soundscape. Hence, it is essential to find a solution to attenuate the acoustic signature of a multi-rotor platform for further development and acceptance of these vehicles by the public. In this paper, an attempt is made to apply a phase control technique to attenuate acoustic signature, and the feasibility and potential are explored through an experiment. Phase control is a noise reduction technique that leverages destructive interference of the coherent acoustic source field between a system of propellers. The measurements were conducted in the newly established anechoic chamber at the Faculty of Aerospace Engineering at the Technion - Israel Institute of Technology. An array of 15 microphones were placed 1.5 m away from the center of a four-rotor rig on a spherical arc. The aerodynamic forces and moments were measured with load cells. Carefully selecting the relative azimuths blade angle (phase), the overall directivity and level of the tonal component at the blade passage frequency can be attenuated. It was found that with 90° phase between neighboring rotors, a reduction of 8 dB and at 3000 RPM and by 5 dB at 4100 RPM was observed at θ = 45° , in comparison to uncontrolled configuration.