Psychoacoustic analysis of the perceptual influence of rotational speed fluctuations in an urban mobility vehicle with distributed ducted fans

Novel propulsion concepts are being developed for urban air mobility (UAM). This study analyses the noise emissions of a virtual UAM vehicle equipped with 26 distributed, ducted, low-speed fans. Synthetic flyover sounds are generated using an auralization framework, which involves noise predictions, sound propagation, and the binaural audio rendering at the observer position. Since noise emissions of distributed propulsion are characterized by interference effects, this paper discusses the influence of rotational speed fluctuations of the distributed fans on the sound perception of the auralized sounds. These rotational speed fluctuations are modelled as different ranges of random and constant deviations from the nominal speed, in contrast to the baseline case with the 26 fans operating synchronously. The results of a listening experiment performed to evaluate the perceptual differences between the different configurations seem to indicate that relatively small fluctuations in the rotational speed of the fans (e.g. $\pm1\%$ with respect to the nominal rpm) already notably improve the perceived noise annoyance. The observed differences in annoyance ratings are reasonably well explained by sound metrics that consider the tonal nature of sound, such as the effective perceived noise level (EPNL), tonality, and the psychoacoustic annoyance model by Di et al.