Many marine wildlife species, including mammals, fish, and invertebrates, use sound or acoustic signals in the ocean for a variety of life functions, such as communication, foraging, navigation, and reproduction. Underwater noise is a stressor of concern for regulators who permit the deployment of marine energy devices. Sound emissions from these devices may increase background noise levels, which can mask important sounds for aquatic life, causing stress and changes in their behavior. To better understand potential sound disturbances from marine energy, the Triton team studies many aspects of underwater noise in the marine environment, including the particle motion component of sound and how to reduce unwanted acoustic flow noise. 
 

Acoustic particle motion and marine energy  

The Acoustic Particle Motion project aims to understand how the particle motion component of acoustic energy from a marine energy device may impact the environment. All sources of sound underwater generate both pressure and particle motion components. Sound pressure is a scalar measurement, while particle motion is a vector quantity with directionality associated with how loud the sound is from the motion of the water molecules. Studies have shown that particle motion is likely detected by most invertebrates and all fishes, whose otoliths, or ear bones, act as small accelerometers. Meanwhile, sound pressure is only detectable by a subset of fishes who have specialized structures (e.g., cod), and likely not by any invertebrates. Yet only the pressure amplitude of sound waves measured by hydrophones is typically used for quantifying underwater noise effects on marine species. To date, there is limited information from particle motion measurements in the field which creates a significant data gap for this potential acoustic disturbance to fishes and invertebrates. Particle motion is gaining recognition from regulators as an essential element for understanding the possible environmental effects of marine energy projects. 

To help further progress the particle motion measurements for marine energy-related underwater noise made by the Integral Consulting Noisespotter™, the Triton team worked with industry partners to build and test a portable, battery-operated, particle motion sensor and hydrophone instrument package. This instrument consists of a three-axis accelerometer as well as a reference hydrophone to provide a comprehensive assessment of underwater noise at each deployment location.   

The team conducted the first of several field deployments in Sequim Bay, Washington, to test the particle motion sensor and hydrophone instrument package. Next, the Acoustic Particle Motion team will deploy the sensor package in a higher-energy environment around wave energy converters at the U.S. Navy Hawai'i Wave Energy Test Site. Through this testing and development work, the project aims to increase understanding of the possible effects of acoustic particle motion to better characterize underwater noise from marine energy devices and its effects on sensitive fish and invertebrate species. 

Lastly, the project has engaged with experts on particle motion and industry regulators to develop recommendations for acoustic particle motion field measurements and data collection at marine energy sites to identify research gaps for understanding effects on marine animals. These efforts aim to build on advancements in technology for measuring particle motion and contribute to a valuable knowledge base so regulators and marine energy industry stakeholders can evaluate whether mitigation strategies are necessary. 

To learn more about Triton's Acoustic Particle Motion Project, read Triton Explains: Acoustic Particle Motion and Marine Energy

Flow noise mitigation

The Triton team is also performing research that addresses how to reduce flow noise that may contaminate underwater sound measurements caused by water flowing by a hydrophone. Flow noise is pseudo-sound (non-acoustic pressure fluctuations from turbulence) caused by the flow of water past an acoustic sensor, like the sound you hear in your ear while riding a bicycle. As part of this research, Triton is developing and testing various flow shields to reduce the effects of flow noise from turbulence in the energetic flow around hydrophones in a range of environmental conditions. Initial flow shield tests took place in Sequim Bay where the team deployed the shield-donned hydrophones into the currents in the Sequim Bay tidal channel, just outside of the PNNL research campus in Sequim, in an area that has been sited for future tidal turbine testing, research, and development. Through these studies, the team aims to inform practical underwater noise mitigation strategies that can be used by both researchers as well as wave and tidal energy developers to improve underwater noise characterization at high-energy marine energy sites. 

The project team published a paper titled Performance of Three Hydrophone Flow Shields in a Tidal Channel presenting results from a study that evaluated the performance of three hydrophone flow shields in the Sequim Bay tidal channel at PNNL-Sequim. Read the paper here!