Directional acoustic measurements around a small-scale tidal turbine

Characterizing the underwater sound produced by tidal turbines is essential to understand their environmental impact. Two stationary acoustic vector sensor systems and an array of drifting hydrophones were deployed in the vicinity of a small tidal turbine (i.e., <2 kW rated power) in Sequim Bay, WA. The turbine itself was also equipped with hydrophones on its foundation. Both directional sensing modalities effectively localized and characterized sounds from the turbine. Turbine sound consisted of tones between 100 Hz and 4 kHz and their harmonics, intermittent impulses, and frequency-modulated sounds that varied with turbine operation. Directional processing facilitated the identification of low-intensity sounds from the turbine system that would likely have been overlooked in measurements from a single hydrophone. The vector sensors effectively characterized changes in turbine sound throughout the turbine deployment, and the drifting hydrophones measured turbine sound during peak flow speeds with minimal flow noise contamination. While the deployment of directional acoustic sensors for acoustic characterization of all marine energy converters is neither feasible nor recommended, they can provide uniquely valuable information and simplify the attribution of sounds to sources in an acoustically complex environment.