Hot off the press: Noise Levels of Unmanned Aerial Vehicles with Implications for Impacts on Marine Mammals

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In collaboration with the Marine Bioacoustics Lab at Aarhus University, we are delighted to bring to your attention a recent paper published in Frontiers in Marine Science entitled:

Noise levels of multi-rotor unmanned aerial vehicles with implications for potential underwater impacts on marine mammals

Full Citation: Christiansen, F., Rojano-Doñate, L., Madsen P.T. and Bejder, L. 2016. Noise Levels of Multi-Rotor Unmanned Aerial Vehicles with Implications for Potential Underwater Impacts on Marine Mammals. Frontiers in Marine Science  3 doi: 0.3389/fmars.2016.00277

Abstract:
Despite the rapid increase in the use of unmanned aerial vehicles (UAVs) in marine mammal research, knowledge of the effects of UAVs on study animals is very limited. We recorded the in-air and in-water noise from two commonly used multi-rotor UAVs, the SwellPro Splashdrone and the DJI Inspire 1 Pro, to assess the potential for negative noise effects of UAV use (Figure 1). The Splashdrone and Inspire UAVs produced broad-band in-air source levels of 80 dB re 20 μPa and 81 dB re 20 μPa (rms), with fundamental frequencies centered at 60 Hz and 150 Hz. The noise of the UAVs coupled poorly into the water, and could only be quantified above background noise of the recording sites at 1 m depth when flying at altitudes of 5 and 10 m, resulting in broad-band received levels around 95 dB re μPa rms for the Splashdrone and around 101 dB re μPa rms for the Inspire (Figure 2). The third octave levels of the underwater UAV noise profiles are (i) close to ambient noise levels in many shallow water habitats, (ii) largely below the hearing thresholds at low frequencies of toothed whales, but (iii) likely above the hearing thresholds of baleen whales and pinnipeds (Figure 2, Figure 3). So while UAV noise may be heard by some marine mammals underwater, it is implied that the underwater noise effect is small, even for animals close to the water surface. Our findings will be valuable for wildlife managers and regulators when issuing permits and setting guidelines for UAV operations. Further, our experimental setup can be used by others to evaluate noise effects of larger sized UAVs on marine mammals.

Figure 1. (A) Experimental setup of the UAV noise exposure study. The UAVs were hovering at fixed altitudes above the acoustic array at the heights indicated in the figure. The same trial was also carried out above a microphone placed on land. (B) The SwellPro Splashdrone and (C) DJI Inspire 1 Pro used in the experiment. Note: figure is not drawn to scale.

Figure 1. (A) Experimental setup of the UAV noise exposure study. The UAVs were hovering at fixed altitudes above the acoustic array at the heights indicated in the figure. The same trial was also carried out above a microphone placed on land. (B) The SwellPro Splashdrone and (C) DJI Inspire 1 Pro used in the experiment. Note: figure is not drawn to scale.

Figure 2. Representation of the in-air recordings of the SwellPro Splashdrone and the DJI Inspire 1 Pro flying at 10 m altitude. (A,D) spectrograms of the received noise at water surface where specific harmonic and subharmonic frequency bands are visible. (B,E) relative power spectra of the received noise. (C,F) waveforms of the source level noise produced for each UAV. (G) Power spectral density of the received noise at 10 m for the SwellPro Splashdrone (red line) and the DJI Inspire 1 Pro (blue line). Self-noise of the recorder (black line) is shown for comparison.

Figure 2. Representation of the in-air recordings of the SwellPro Splashdrone and the DJI Inspire 1 Pro flying at 10 m altitude. (A,D) spectrograms of the received noise at water surface where specific harmonic and subharmonic frequency bands are visible. (B,E) relative power spectra of the received noise. (C,F) waveforms of the source level noise produced for each UAV. (G) Power spectral density of the received noise at 10m for the SwellPro Splashdrone (red) and the DJI Inspire 1 Pro (blue). Self-noise of the recorder (black line) is shown for comparison.

Figure 3. Audiograms of a harbor porpoise (Phocoena phocoena, Kastelein et al., 2002), a bottlenose dolphin (Tursiops truncatus, Johnson, 1967), a northern elephant seal (Mirounga angustirostris, Kastak and Schusterman, 1999) and the predicted audiogram of a fin whale calf (Balaenoptera physalus, Cranford and Krysl, 2015). Ambient third-octave sound pressure levels (TOLs) in dB re 1 μPa RMS in five different shallow-water habitats: North Sea (Willie and Geyer, 1984), Baltic (Willie and Geyer, 1984), Scotian shelf (Piggott, 1964), Exmouth (Hermannsen et al. unpublished) and Koombana bay (Jensen et al., 2009). SwellPro Splashdrone and DJI Inspire 1 Pro received TOLs in dB re 1 μPa RMS at 1 m depth when UAVs hovering at 5 m altitude.

Figure 3. Audiograms of a harbor porpoise (Phocoena phocoena, Kastelein et al., 2002), a bottlenose dolphin (Tursiops truncatus, Johnson, 1967), a northern elephant seal (Mirounga angustirostris, Kastak and Schusterman, 1999) and the predicted audiogram of a fin whale calf (Balaenoptera physalus, Cranford and Krysl, 2015). Ambient third-octave sound pressure levels (TOLs) in dB re 1 μPa RMS in five different shallow-water habitats: North Sea (Willie and Geyer, 1984), Baltic (Willie and Geyer, 1984), Scotian shelf (Piggott, 1964), Exmouth (Hermannsen et al. unpublished) and Koombana bay (Jensen et al., 2009). SwellPro Splashdrone and DJI Inspire 1 Pro received TOLs in dB re 1 μPa RMS at 1 m depth when UAVs hovering at 5 m altitude.

 

Please note: We want to emphasize that this study was carried out under strict permitting conditions and that the pilot (F Christiansen) was trained and licensed to use UAVs for scientific purposes. With the use of recreational UAVs increasing rapidly around the world, regulators need to take a precautionary approach when setting up guidelines and regulations for the public, to minimize potential negative impacts from inexperienced and irresponsible operators.

Download the paper: The article can be downloaded HERE, or alternatively, please email Fredrik Christiansen for a PDF at f.christiansen@murdoch.edu.au

Acknowledgements: We thank M. L. K. Nielsen, K. R. Sprogis, J. Totterdell (Marine Information and Research Group, Australia) and J. A. Tyne for assisting during the field trials. We thank J. N. Smith for technical assistance with the SoundTrap. We thank Global Unmanned Systems (http://www.gus-uav.com) and Victorian UAS Training (http://www.victorianuastraining.com.au) for UAV technical support and training. We thank Associate Editor R. Harcourt and two reviewers for their constructive comments which helped to improve this manuscript. The UAVs in this study were operated under a Remotely Piloted Aircraft System License (ARN: 837589) and two UAV Operator Certificates (CASA.UOC.0136 and CASA.UOC.1-YC6NP-03), in accordance with regulations by the Australian Civil Aviation Safety Authority (CASA).

Lars Bejder PhD
Lars Bejder PhD
Professor Lars Bejder PhD is the Research Leader of the Murdoch University Cetacean Research Unit.
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