Sound of Aerial Drones Underwater

Drones (formally called UAVs, Uncrewed Aerial Vehicles) are increasingly used as a tool for scientific research. Compared to vessels as platforms for observation, drones are relatively unobtrusive and allow scientists to approach and follow animals at close proximity. Drones are useful for collecting aerial images to assess body condition of an animal, for collecting samples of the exhaled breath (blow) of cetaceans (for DNA, microbiome, hormone samples, etc.), for attaching archival tags, or for conducting behavioral observations.

Drones produce a buzzing sound that can propagate below the water surface. Drones produce sound mostly at low frequencies (approximately 100–1000 Hz) but also in the mid frequency range (1–17 kHz). Some drones (e.g., the DJI Inspire 1) additionally produce sound at higher frequencies (approximately 28–36 kHz), a range not audible to humans but potentially to animals including toothed whales.

It is possible that the underwater sound produced by drones may affect the animals. The amplitude (loudness) of the sound of drones is greatly reduced underwater but it can remain audible (depending on the hearing capabilities of the animal) if the drone is in close enough proximity. The amplitude of the drone sound underwater varies greatly among different types of drones, with larger drones generally producing louder sounds. While potentially audible for some animals, the sound of drones is very quiet compared to motor vessel sound and therefore remains a valuable platform for observation.

To reduce the amplitude of the drone sound and therefore the potential impact on the observed animals, drone pilots usually increase the drone’s distance from the animal. This is effective, since larger distances to the receiver result in quieter sound. A drone pilot may increase the drone’s altitude, but interestingly, the drone sound decreases even more quickly if the horizontal distance to the receiver is increased, i.e., if the pilot flies the drone to the side of the animal, rather than higher. This is due to the way the sound is transmitted from air to water. Increasing horizontal distance to the animal under observation is therefore a great way to reduce noise impacts and decrease the risk of potential disturbance and behavioral changes.

Additional Links on DOSITS

• Hot Topic: Using Drones to Deploy Digital Acoustic Tags https://dosits.org/using-drones-to-deploy-digital-acoustic-tags/
• Science > How does sound propagate from air into water? https://dosits.org/science/movement/how-does-sound-propagate-from-air-into-water/ 

References

• Laute, A., Glarou, M., Dodds, F., Gomez Røsand, S. C., Grove, T. J., Stoller, A., … & Fournet, M. E. (2023). Underwater sound of three unoccupied aerial vehicles at varying altitudes and horizontal distances. The Journal of the Acoustical Society of America, 153(6), 3419-3419.
• Christiansen, F., Rojano-Doñate, L., Madsen, P. T., & 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, 277.
• Erbe, C., Parsons, M., Duncan, A., Osterrieder, S. K., & Allen, K. (2017). Aerial and underwater sound of unmanned aerial vehicles (UAV). Journal of Unmanned Vehicle Systems, 5(3), 92-101.