Phase Synchronization in Noise Control Methodology and Future Prospects

This presentation provides an overview of noise-mitigation potentials of propeller phase-locking in axial and edgewise flight. The investigation was conducted experimentally in the aeroacoustic facilities at the University of Bristol. Acoustic results were captured both in-plane and out-of-plane with respect to the rotor’s plane of rotation and revealed substantial noise reduction concentrated at the blade passing frequency, which is attributed to the destructive interference between acoustic wavefronts generated by each rotor. Noise emissions acquired in edgewise flight demonstrated different characteristic compared to that in axial flow. In axial flow, a relative phase angle of Δψ = 90° yielded most significant noise reduction regardless of the propeller advance ratios. While the in-plane observers in edgewise configuration follows this trend, for the out-of-plane observers in edgewise flight, the phase angle of maximum noise reduction is shifted to Δψ = 60°. This observation is potentially due to the unsteady loading intrinsic to propellers in non-axial flight. Increasing the advance ratio lead to an increase in noise reduction efficiency in axial flight, while the same effect is less observed in edgewise flight. Noise reduction in the blade passing frequency could be accompanied by amplifications in certain other blade passing harmonics. In axial flight, the second blade passing harmonic at Δψ = 90° is 15 dB greater than that at Δψ = 0°. In edgewise flight, the third and the fifth harmonics where amplified by 10 dB and 15 dB respectively when changing the relative phase from Δψ = 0° to Δψ = 90°. The overall sound pressure level of axial and edgewise configurations recorded a reduction of up to 6 dB and 10 dB respectively, despite these higher harmonic amplifications.