Towards Abatement of Quadrotor’s Acoustic Signature with Phase Control Technology

The annoying sound produced by a multi-rotor vehicle can bring a severe acoustic nuisance to an urban soundscape. Hence, it is paramount to find a solution to attenuate the acoustic signature of a multi-rotor platform for further public development and acceptance of these vehicles. In the scope of the MSc thesis, a phase control technique was applied to attenuate acoustic signature, and the feasibility and potential are explored through an experiment. The noise reduction technique leverages destructive interference of the coherent acoustic source field between a system of propellers by controlling the relative phase between the blades in a multi-rotor configuration. In the first stage, a low-noise and aerodynamically efficient single-rotor configuration based on APC 14×5.5 propeller was designed. The design was based on modelling the static aerodynamic loads on the blade with Blade Element Momentum Theory. The assumption is that aerodynamic efficiency will result in the reduction of the sound propagation of the blade. In the second stage, single and multi-rotor configuration experiments were studied experimentally.

The single and the multi-rotor configurations were part of a measurement campaign 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 meters away from the center of the rotors’ rig on a spherical arc. In the multi-rotor configuration, by carefully selecting the relative azimuths blade angle (phase), the overall sound pressure directivity and level of the tonal component at the blade passage frequency can be attenuated. It was found that with destructive interference, a 90 degrees of phase angle between neighbouring rotors, a reduction of 4 dB at 3000 RPM and by 7 dB at 3800 RPM was observed, compared to uncontrolled configuration. Measured sound pressure levels were compared to the analytical acoustic model, and similar amplitudes and trends of the sound pressure levels were found between the model and the measurements.