Harvesting power from the ocean is an important initiative for meeting global energy demands, ensuring energy security, providing energy equity, and reducing emissions. However, the spectrum of demonstrated capabilities for converting marine energy to electricity falls significantly short of the potential natural energy resources available in open ocean and tidal areas. For instance, an estimated 57% of the United States (U.S.) energy needs from 2019 could be met with marine energy sources, yet typically only 10% of marine energy resources are assumed technically available using the current power conversion technologies1. To close this technology gap, rapid, iterative, deployment and testing of the full range of readiness levels of marine energy converters (MECs) in a variety of high energy coastal and open water settings is critical. Yet, the introduction of novel renewable energy technologies and infrastructure has raised environmental concerns for some coastal area stakeholders. Uncertainties around the potential environmental effects of MECs often result in a conservative regulatory approach. This can lead to significant delays and higher project costs that act as barriers to device deployments and negatively impact marine energy project success and subsequent industry growth.

Concerns about the potential environmental effects of underwater noise produced by MECs have been consistently raised by community stakeholders and regulators in the consenting process2. Since many marine mammals3,4, fishes5,6,7, and invertebrates8 use sound for a variety of important life functions, the acoustic conditions of ocean habitats are critical for healthy marine ecosystems4,9. Thus far, there is no evidence that sounds from single or small numbers of operational MECs could cause mortality or acute auditory injury to marine animals. Rather, concerns are centered around arrays of devices and the potential for sound emissions to mask important biological or behavioral acoustic cues10 or disrupt vital behaviors11,12, impacting the long-term health and success of marine animals in these important habitats. Furthermore, device sound emissions may also contribute to coastal soundscapes that are already largely influenced by anthropogenic sources13,14,15. The lack of information resulting from a dearth of consistent, standardized measurements of underwater noise from MECs is a challenge for regulators responsible for managing these coastal areas16. There is particular uncertainty surrounding the acoustic emissions from wave energy converters (WECs), a class of MECs that transform kinetic and potential energy from ocean waves to mechanical or electrical energy, because there are a broad range of WEC device types and operating principles that affect the sound generated by a device. As a result, regulators often consider noise effects from WECs as having high uncertainty and high risk in the permitting process17.

In 2019, the International Electrotechnical Commission (IEC) published technical specification 62600-40, “Marine Energy – Wave, tidal and other water current converters – Part 40: Acoustic characterization of marine energy converters”, hereto referred to as the −40 TS18. The −40 TS details uniform methodologies to characterize sound produced by marine energy converters including specifications for sensors and sample rates, recording durations, hydrophone deployment methods and spatial configurations or sampling areas, and relevant co-temporal meteorological and oceanographic data. Two levels of acoustic characterization are described: Level A, which has higher spatial and temporal detail; and Level B, which has reduced spatial and temporal detail, but requires less effort and cost. The measurements performed in the use case provided in this paper are most closely aligned with the specifications for a Level A characterization of a WEC, which calls for measurements from three fixed hydrophones over the course of 6 months or sea states corresponding to 50% of the WEC’s annual energy production (AEP). A Level A characterization for WECs provides the added value of documenting sound emissions through a variety of environmental and power production states which is lacking in Level B characterization.

While the −40 TS provides detailed guidance as an engineering specification for acoustic characterization of WECs, including how data should be collected and analyzed, it does not discuss how these measurements should be interpreted in the context of effects on animals, a critical component for environmental regulatory decision making and project compliance monitoring. Yet, acoustic regulatory criteria for marine animals vary internationally, making standardized conversion of the −40 TS engineering characterization of sound emissions to animal effects thresholds for regulatory compliance challenging. In this paper, we use acoustic recordings from a WEC to connect the −40 TS characterization with regulatory criteria from the U.S. National Marine Fisheries Service (NMFS) regulatory guidance for understanding the impacts of measured anthropogenic sound on marine mammals19; furthermore, we discuss the results in the context of the European Union (EU) Marine Strategy Framework Directive (MSFD, 2008/56/EC)20. This directive requires Member States to achieve Good Environmental Status (GES) and has driven coordinated European research to develop underwater noise monitoring programs and to define thresholds relevant to GES for underwater noise21,22.

Prior studies that have characterized the sound produced by WECs have used a wide variety of data collection methods and have reported different metrics, making comparison between studies challenging16. For instance, several studies have employed free drifting hydrophone systems, with the earliest documented study conducted around a one-seventh scale heave and surge point absorber WEC in Puget Sound, Washington, USA23. Acoustic characterization data was collected at ranges of 10 m – 1,500 m and acoustic frequencies from 20 Hz-20 kHz. Similarly, an attenuator WEC was characterized at the European Marine Energy Center using hydrophones cabled to a free-drifting vessel with the propulsion system turned off to avoid contamination24. Power spectral density calculations of received levels showed similar trends (although lower in amplitude) to bottom-mounted, fixed hydrophones located closer to the WEC. At the U.S. Navy and University of Hawaii Wave Energy Test Site (WETS), a point absorber WEC was also characterized with drifting hydrophones, and sound emissions from the power generator were observed between 50 Hz – 300 Hz25. The highest amplitude acoustic signals, detected in frequencies up to 5 kHz, were associated with mooring system components. Most recently, in 2024, advanced drifting hydrophone systems26 were used to characterize the acoustic emissions from the same WEC that was tested in Puget Sound in 2011, but at a larger scale and rated power capacity for open water testing in the open ocean at WETS. Using an array of drifting hydrophones, signals could be localized and attributed to specific components of the WEC at a limited range (up to 150 m) and remained below 120 dB re 1 \(\mu {Pa}\) during the operational conditions and frequency band where WEC sound was detected (60–900 Hz)27. Notably, the larger scale WEC was quieter due to engineering improvements informed by the previous study in 2011.

In addition to free drifting hydrophones, sound emission characterizations of operational WECs have also used fixed hydrophones on the seafloor or suspended in the water column. In an early study, a hydraulic point absorber was characterized using a fixed hydrophone moored 25 m from the WEC off the Danish North Sea coast28. The majority of WEC noise, described with median sound pressure levels, occurred in the frequency range 125–250 Hz with received sound pressure levels (SPL) 1–2 dB above ambient conditions. A study conducted off the coast of Portugal collected fixed hydrophone acoustic measurements around a semi-operational, seabed-anchored oscillating surge WEC29. Broadband WEC noise was observed from 50 Hz-20 kHz, with the peak in received levels occurring at a frequency of 125 Hz. Again, mooring system components noise was a significant sound source and a confounding signal for characterizing WEC generated sounds in the ambient field. The same WEC was further characterized at a test site in the United Kingdom30 with a hydrophone mounted on the seafloor at a distance of 200 m. A source level of 155 dB re \(\mu {Pa}\) at 1 m was estimated from those recordings with tonal peaks occurring at 30 Hz and 60 Hz attributed to the WECs power generator. Moreover, in the Mediterranean Sea, a study reported that full-scale WEC operations generated continuous low-frequency noise, predominantly below 4 kHz, with dominant spectral components under 100 Hz. Third-octave band sound pressure levels at 63 Hz reached approximately 126 dB re 1 \(\mu {Pa}\) at a distance of 40 m from the WEC31. Lastly, a bottom mounted acoustic vector sensor and hydrophone platform collected data at ranges of 100 m and 200 m from a WEC deployed off the southern California coast USA during low energy environmental conditions32. WEC-attributed sound emissions were observed in frequencies 200 Hz – 2.7 kHz with an estimated sound exposure level (SEL) of 139 dB re 1 \(\mu \textrm{Pa}^2\,\textrm{s}\).

Measurements of operational WEC sound emissions have shown the acoustic energy is primarily observed below 1 kHz and is consistently lower amplitude than other anthropogenic disturbances of regulatory concern (e.g. seismic exploration, pile driving, vessel noise). Nevertheless, the diversity in hardware, methodologies, analysis, and reporting of WEC characterizations has made cross-comparisons and device specific evaluations of acoustic emissions difficult. Adoption of the −40 TS by researchers in future acoustic characterizations of WECs and other marine energy converters has the potential to alleviate inconsistencies in measurement and analysis, providing a standardized approach. Furthermore, connecting −40 TS characterization to environmental regulatory threshold criteria and reporting metrics that conservation managers and decision makers are familiar with is a critical next step for permitting and licensing of marine energy deployments.

In this study we present acoustic data collected from three seafloor hydrophones deployed near a WEC tested off the coast of La Jolla, California. Data collection and analysis follow the Level A −40 TS to characterize WEC sound emissions through different power production states and environmental conditions. We then interpret these results in the context of two sets of regulatory criteria: the U.S. National Marine Fisheries Service (NMFS) Technical Guidance for Assessing the Effects of Anthropogenic Sound on Marine Mammal Hearing19 and the E.U. Marine Strategy Framework Directive (MSFD) descriptors 11.1 and 11.2. While the NMFS guidance focuses on auditory impacts to marine mammals, the MSFD descriptors provide a framework to evaluate the impact of underwater noise on indicator species and ecosystems33. The results not only provide valuable environmental effects information for a growing marine energy industry, but also demonstrate the utility and value of the −40 TS.