This study investigates the differences in sound emissions of multi-rotor unmanned aerial systems (UAS) between hover and cruise. We therefore conducted onboard acoustic measurements on two different UAS types, each equipped with multiple propeller configurations. The recorded data allowed for a detailed comparison of noise emissions at fixed emission angles, focusing on both traditional acoustic metrics and psychoacoustic parameters. The results reveal notable differences between hover and cruise, with certain configurations exhibiting a sudden increase in sound emission levels of up to 4 dB above specific cruise speeds. While some UAS/propeller combinations showed a linear relationship between cruise speed and sound emission levels, others experienced abrupt level increases beyond a critical velocity of 2–4 m/s, along with changes in loudness and roughness. These variations were not directly correlated with rotor speed changes, indicating that aerodynamic transitions, such as shifts in blade-wake/blade-vortex interactions, strongly influence sound generation. Spectral analyses support the occurrence of an aerodynamic regime change between hover and cruise, with the effect saturating at higher cruise speeds. Consequently, noise and auralization models, but also evaluations on low-noise propellers that are based on measurements at hover state only may yield substantial inaccuracies when extrapolated to cruise state.
Meister et al. (Sun,) studied this question.