Noise from Unconventional Aircraft: A Review of Current Measurement Techniques, Psychoacoustics, Metrics and Regulation

Purpose of Review

This review aims to evaluate and discuss the current advances in the measurement and assessment of the noise generated by unconventional aircraft, such as unmanned aircraft systems (UAS) and urban air mobility (UAM) vehicles. Building upon the findings of this review, research gaps are identified, and further work is proposed to enhance existing and emerging methods for the appropriate noise management of these advanced air mobility (AAM) technologies.

Recent Findings

Noise has been highlighted as one of the key concerns for the wider deployment of UAS and UAM operations. This is suggested to be due to having acoustic signatures with sound characteristics commonly associated with noise annoyance, such as ‘sharpness’ (the perceived proportion of high-pitched sonic energy) and ‘tonality’ (the perceptual prominence of concentrated sonic energy at discrete frequencies). These types of ‘psychoacoustic features’ are thought to be connected with observations of increased noise annoyance for AAM, compared with conventional aircraft and road vehicles, at the same level of sound exposure.

Summary

In the last few years, there has been a growing body of research on UAS and UAM noise. Research has focused on a comprehensive understanding of the sound sources of these unconventional aircraft under a wide range of operating and operational conditions. Based on gathered evidence, measurement protocols for both laboratory and field studies are very advanced for the acoustic characterisation of UAS in terms of sound level, frequency and directivity. Looking at the human response to UAS and UAM noise, loudness has been consistently reported as the main contributor to noise annoyance, with second-order contributions from other psychoacoustic features, such as sharpness, tonality and ‘amplitude modulation’ (fluctuations in loudness over time), varying among studies. Noise targets for UAS certification have been derived from existing regulations for conventional aircraft and rotorcraft, but might not account for the usually reported annoyance offset between UAS/UAM and conventional vehicles. Key research gaps identified include the lack of studies focusing on multiple events, and deeper understanding of the influence that personal or contextual factors may have on responses, which will be important for the development of robust methods for the assessment and minimisation of community noise annoyance due to the operation of these unconventional aircraft.

Graphical Abstract