Directional acoustic measurements around a small-scale tidal turbine

Characterizing the underwater sound produced by tidal turbines over a range of operating conditions is helpful to contextualize their impact, if any, on natural soundscapes. Traditionally, acoustic characterization around marine energy installations has involved omnidirectional hydrophone-based systems that cannot directionally distinguish sounds from a marine energy installation from other sounds in the environment such as boats and marine mammals. Here, results are presented from a deployment of two stationary directional acoustic systems and an array of drifting hydrophones in the vicinity of a small tidal turbine (i.e., <2 kW rated power) in Sequim Bay, WA. 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 sounds over periods of days to weeks, and the drifting hydrophones measured turbine sound during peak flow speeds without contamination from flow noise.