Phase Synchronisation for Tonal Noise Reduction in a Multi-Rotor UAV

This study aims to investigate the effects of phase synchronisation on tonal noise reduction in a multi-rotor UAV using an electronic phase-locking system. Experiments at the University of Bristol explored the impact of relative phase angle, propeller spacing, and blade geometry on acoustic performance, including psychoacoustic annoyance. Results show that increasing the phase angle consistently reduces the sound pressure level (SPL) due to destructive interference. For the two-bladed configuration, the highest noise reduction occurred at relative phase angle Δψ=90∘, with a 19 dB decrease at the first blade-passing frequency (BPF). Propeller spacing had minimal impact when phase synchronisation was applied. The pitch-to-diameter (P/D) ratio also influenced results: for P/D=0.55, reductions ranged from 13–18 dB; and for P/D=1.0, reductions ranged from 10–20 dB. Maximum psychoacoustic annoyance was observed when propellers were in phase (Δψ=0∘), while annoyance decreased with increasing phase angle, confirming the effectiveness of phase control for noise mitigation. For the five-bladed configuration, the highest reduction of 15 dB occurred at Δψ=36∘, with annoyance levels also decreasing with phase offset.