Blade Number Effects in sUAS Rotor Noise Assessed Using Psychoacoustic Metrics and an Online Listening Test

quietdrones2026 Tracking Number 37

Blade number is a key design variable in sUAS and Urban Air Mobility (UAM) rotors in both aerodynamics and acoustics, yet the blade-number-induced changes in the spectral and temporal acoustic characteristics may not be captured by overall sound pressure level (OASPL) alone. In this study, we compare psychoacoustic metrics with subjective annoyance obtained from an online listening test for sUAS rotor noise with varying blade number. We obtained the noise data from the low Reynolds number rotor measurements conducted in the ISAE-SUPAERO anechoic chamber. Each rotor has a NACA 0012 airfoil, 0.25 m diameter, and a fixed pitch angle of 10° without any twist, while the number of blades varies from 2 to 4. The noise was recorded at 1.62 m using a 13-microphone semicircular array spanning −60° to 60°. The operating points were selected close to the theoretically required RPM to approximate an equal-thrust condition. The psychoacoustic metrics (loudness, sharpness, roughness, fluctuation strength, and tonality) and psychoacoustic annoyance (PA) based on More (2010) were computed using the MATLAB-based open-source code, Sound Quality Analysis Toolbox (SQAT). At −60° direction, OASPL and N_5 changed little with blade number, whereas 〖PA〗_5 increased as blade number increased. This trend was consistent with the increased sharpness for higher blade numbers, partially offset by reduced tonality. In an online listening test, 2-blade and 4-blade sounds at −60° direction were level-scaled to nominal levels of 60-80 dB in 5 dB steps (10 stimuli). Based on 41 valid responses, annoyance ratings did not differ significantly between blade numbers at 60-70 dB, while the 4-blade condition was rated significantly more annoying at 75-80 dB. In addition, the main annoyance factor selections changed from tonality/roughness for 2-blade to sharpness for 4-blade, which are qualitatively consistent with the objective metrics. Based on these results, we conclude that psychoacoustic metrics can complement sound level metrics in trade studies of blade numbers for sUAS/UAM rotor noise.