On-Board Outdoor Acoustic Measurement of Multirotor Unmanned Air Vehicles: Effect of Flight Conditions on Emission and Perception

Urban Air Mobility development depends on overcoming several challenges. Noise emission is one of the most critical, making Unmanned Air Vehicles’ acoustic characterisation a scientific and regulatory priority. Through an on-board outdoor measurement campaign on three commercial multirotor drones (DJI Mavic 3M, Mavic 3 Classic, and DJI Matrice 4T) during hovering, this study analyses and shows the effects that the most important flight variables: altitude, propeller design and payload, have on the overall noise emission, its directivity and the psychoacoustic annoyance, more specifically through the descriptor developed by Boucher. The results showed that different altitudes (10, 15 and 20 m) do not affect noise emissions. Moreover, overall sound pressure levels may fail to discriminate drones’ noise annoyance. In fact, despite the lower sound pressure levels, for the specific platform and propeller configuration tested (DJI Mavic 3M with manufacturer low-noise blades), the low-noise-propeller configuration showed higher psychoacoustic annoyance than the standard-propeller one, driven primarily by higher roughness (and, to a lesser extent, tonality). Additional payloads (50–150 g) did not increase psychoacoustic annoyance monotonically. The heaviest Matrice 4T showed the most stable psychoacoustic profile, with psychoacoustic annoyance remaining below its unloaded baseline at all payload levels, while the lightest Mavic 3 Classic showed the largest swing. Moreover, considering the drone’s structural symmetry, the angular asymmetries revealed by the directivity analysis suggest hovering-stabilisation dynamics attributions. Overall sound pressure levels alone proved an unreliable predictor of drones’ noise annoyance across all tested flight conditions.