Abstract
Information about underwater noise from wave energy converters (WECs) can contextualize their potential contribution to soundscapes, as well as guide design refinement. Here, we compare underwater noise measurements around two versions of CPower’s SeaRAY®: an early, scale-model tested in Puget Sound in 2011 and kilowatt-scale prototype tested at the U.S. Navy’s Wave Energy Test Site (WETS) in 2024. In the intervening decade, multiple engineering changes were made to the WEC design that incorporated lessons learned from field testing, wave basin experiments, and numerical simulations. Over that time, the acoustic measurement system also evolved from a wave measurement buoy retrofitted with a hydrophone to the Drifting Acoustic Instrumentation System (DAISY) – a sensor suite engineered to collect high-quality acoustic data in energetic waves and currents.
Acoustic measurements around the scale-model SeaRAY in Puget Sound determined that it produced noise detectable to a range of 1500 m, with broadband sound pressure levels exceeding 130 dB at 300 m range. These noises were primarily frequency-modulated tones attributable to the power take-off (PTO) and intermittent, broadband impulses caused by end stop contact. The WETS SeaRAY had a similar overall architecture (a central nacelle connected to a pair of floats) but was roughly three times larger and heavier. Despite this, radiated noise, while still attributable to the power take-off, only exceeded ambient noise to a range of 150 m and, even at close range, was discernable for a narrower set of frequencies than for the Puget Sound scale-model. This reduced acoustic footprint is particularly notable because of the lower ambient noise levels at WETS attributable to limited vessel traffic. By comparing acoustic measurements to operational characteristics at WETS, we were able to identify a correlation between radiated noise and power take-off rotation rate. This could inform future design decisions and control strategies that minimize radiated noise. Finally, relative to the drifting hydrophones used in Puget Sound, the DAISYs deployed at WETS were able to characterize radiated noise over a wider frequency range and localize sounds attributable to the WEC using time-delay-of-arrival processing.
These acoustic snapshots demonstrate that changes to WEC design can substantially reduce their acoustic footprint, striking a balance between signaling WEC presence and altering animal behavior. Similarly, advances to environmental sensors can enable delivery of rich, timely information relevant to both technology developers and regulators.