On-field noise measurements and acoustic characterisation of multi-rotor small unmanned aerial systems

The noise signature of small Unmanned Aerial Systems (sUAS) is highly influenced by specific operating and weather conditions. Accurate noise assessment of sUAS operations requires field noise measurements that capture all the complexity of sound emission and propagation during realistic flight situations. This paper presents a measurement and analysis framework for the acoustic characterisation of sUAS through the calculation of conventional noise metrics (LAmax and LAE), frequency and directivity features. Using a multi-channel measurement approach, and back-propagating the sound from ground microphone to source, the presented framework allows the calculation of acoustic hemispheres for a selection of acoustic metrics. Important findings are that (i) the framework is robust for a variety of multi-rotor sUAS varying in size and configuration; (ii) broadband noise and tonal noise are the dominant noise sources during flyover for larger sUAS and smaller sUAS respectively; (iii) the maximum noise radiation of the sUAS tested is found in the rear arc of polar directivity; and (iv) angles of maximum radiation of amplitude modulated noise have been found for most sUAS tested at Θ angles ±30∘. This work is intended to be relevant in establishing common protocols regarding sUAS acoustic certification between environmental policymakers, stakeholders, and industry.