Acoustic characterization of wave energy converters

Abstract

Wave energy converters produce sound as a consequence of their operation, but the specifics are not well-understood. Here, we present observations of two point-absorbing wave energy converters deployed at the US Navy’s Wave Energy Test Site in Kaneohe, HI. Measurements are obtained by free-drifting instrumentation packages which acoustical isolate the hydrophone from the surface expression to minimize masking by flow-noise (i.e., pseudo-sound generated by relative motion between hydrophone and water) and self-noise (i.e., propagating sound generated by the instrument package). Observations suggest that wave energy converters of different designs, even within the general class of point absorbers, produce different stereotypical sounds. Further, during normal operation, sound signatures can change substantially with sea state as new sound generation mechanisms come into play. One example of this is wave breaking around a shallow-hulled point absorber when waves exceed a critical steepness. The sound from bubble collapse is detectable up to several hundred kHz, whereas, below this critical steepness, wave converter sound is only detectable up to ten kHz. Finally, wave energy converter sounds are contextualized relative to other natural and anthropogenic sound sources to qualitatively explore their potential effects on marine animals.