Abstract
This paper addresses one of the recognized barriers to the unrestricted adoption of Unmanned Aircraft (UA) in mainstream urban use—noise—and reviews existing approaches for estimating and mitigating this problem. The aircraft noise problem is discussed upfront in general terms by introducing the sound emission, propagation, and psychoacoustic effects. The propagation of sound in the atmosphere, which is the focus of this paper, is then analysed in detail to isolate the environmental and operational factors that predominantly influence the perceived noise on the ground, especially looking at large-scale low-altitude UA operations, such as in the envisioned Urban Air Mobility (UAM) concepts. The physics of sound propagation are presented, considering all attenuation effects and the anomalies due to Doppler and atmospheric effects, such as wind, thermal inversion, and turbulence. The analysis allows to highlight the limitations of current mainstream aircraft noise modelling and certification approaches and, in particular, their inadequacy in addressing the noise of UA and, more generally, UAM vehicles. This finding is important considering that, although reducing noise at the source has remained a priority for manufacturers to enable the scaling up of UAM and drone delivery operations in the near future, the impact of poorly considered propagation and psychoacoustic effects on the actual perceived noise on the ground is equally important for the same objective. For instance, optimizing the flight paths as a function of local weather conditions can significantly contribute to minimizing the impact of noise on communities, thus paving the way for the introduction of full-scale UAM operations. A more reliable and accurate modelling of noise ground signatures for both manned and unmanned low-flying aircraft will aid in identifying the real-time data stream requirements from distributed sensors on the ground. New developments in surrogate sound propagation models, more pervasive real-time sensor data, and suitable computing resources are expected to both yield more reliable and effective estimates of noise reaching the ground listeners and support a dynamic planning of flight paths.
Highlights
Sound refers to the mechanical energy transmitted through air or any other medium by longitudinal wave motion
This paper addressed the aircraft noise problem, reviewing the current noise emission, propagation, and psychoacoustic factors as they apply to manned aircraft, and the current work being carried out to develop suitable approaches for the modelling and mitigation of Unmanned Aircraft (UA) noise
Considering the unique challenges that would emerge with the widespread adoption of Urban Air Mobility (UAM), as well as drone delivery operations, this paper highlights the advantages of surrogate noise propagation models that both consider a broader array of environmental and operational factors as compared to legacy aircraft noise models and could potentially inform future noise certification standards for low-flying aircraft in urban environments
Summary
Sound refers to the mechanical energy transmitted through air or any other medium by longitudinal wave motion. A large body of work has tackled the noise of commercial transport aircraft, which are typically passenger-carrying, due to their significant impact on the people in proximity of airports, and on the passengers who are exposed to such noise during their travel. In this regard, there is plenty of work detailing noise reduction strategies for jet and turbofan engine aircraft [1,2]. The measured noise levels are adjusted to the reference profile to calculate the EPNL, which is the time integral of the Perceived Noise Level (PNLT) over the noise event duration
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