Currently all offshore wind farms reside outside the US, and the vast majority of underwater noise studies of operational wind farms were made in Europe. Two studies that stand out due to their thoroughness are a 2003 report on the Utgrunden wind farm off the east coast of Sweden [EU1], and a 2006 report on the Horns Rev wind farm off the west coast of Denmark [EU2]. The Utgrunden wind farm consists of only 7 turbines, and its location, local bathymetry, etc., are well described in the report. The Horns Rev wind farm possesses 80 turbines. The two reports show similar low-frequency noise spectra from individual turbines, which are characterized by a tonal structure associated with gear noise. The fundamental tone is typically between 150 and 200 Hz and has a sound pressure level less than 125 dB (re 1 μPa) at 100 m, with a few higher harmonics at lower sound pressure levels. There is no indication of significant (or even measurable at 100 m) wind-turbine noise above 1 kHz. Several authors have summarized noise measurements made on a number of other wind farms [e.g., EU3], and the noise spectra look much the same.
One very recent (2013) US report is of particular significance, titled "Acoustic Noise and Electromagnetic Study in Support of the Rhode Island Ocean SAMP" (Special Area Management Plan), because it focuses on "the environmental impact of an offshore wind farm consisting of 8 turbines in an area south of Block Island, Rhode Island" [US4]. In particular, the report discusses "the underwater acoustic noise generated by the various phases of the life cycle of a wind farm from site surveys, construction, operation, and decommissioning." Background noise levels were measured on site, and anticipated underwater noise from operational wind turbines was simulated based on measurements made at the Utgrunden offshore wind farm in Sweden [EU1]. While the impact on acoustical systems was not considered, "The modeling suggests that the 8 turbine wind farm would have little impact on marine life." At a distance 10 km south of the proposed site, and assuming "that the [eight] turbines are operating at the highest possible power setting for the wind conditions", it was determined that "additional noise from the wind turbines is significantly less than noise from shipping, wind and rain for the period covered by these measurements (5 weeks in October and November, 2008)."
Measurements of underwater noise due to pile driving are relevant to the future construction, even if not operation, of wind turbines in US coastal waters. A typical recent study reports measurements of pile driving in water of depth 25 ft performed during the Washington State Ferries 2006 Test Pile project, with and without three different noise abatement systems implemented: bubble screen, foam-walled steel pile, and double-walled steel pile [US3]. However, other than the similarities to wind turbines in terms of shallow water and noise abatement, the frequencies and sound pressure levels are much higher than those generated underwater by operational wind farms. The present report focuses only on operating wind farms, not their construction.
A determination of whether the underwater noise from wind farms impacts acoustical systems ultimately depends on signal-to-noise ratios and therefore ambient noise levels. To this day, the largest compilation of ambient ocean noise measurements reported in the open literature was published in 1984 [US1]. A 2003 National Research Council report on ocean noise and marine mammals calls attention to this paucity of ambient noise data [US2]. The report notes one particularly large collection of noise data compiled by the Naval Oceanographic Office (NAVOCEANO) for the US Navy, but points out that "Access to the databases listed is restricted, making it difficult to review them and use them for scientific purposes."
US.1
"Ambient Noise in the Sea"
R. J. Urick
The Catholic University of America
Published by Undersea Warfare Technology Office
Naval Sea Systems Command
Department of the Navy
Washington, D.C.
1984
US.2
"Ocean Noise and Marine Mammals"
Committee on Potential Impacts of Ambient Noise in the Ocean on Marine Mammals
Ocean Studies Board
Division on Earth and Life Studies
National Research Council of the National Academy of Sciences
Washington, D.C.
2003
US.3
"Underwater Acoustic Measurements from Washington State Ferries 2006 Mukilteo Ferry Terminal Test Pile Project"
A. MacGillivray
JASCO Research, 2101 - 4464 Markham Street, Victoria, British Columbia, Canada V8Z 7X8
Prepared for Washington State Ferries & Washington State Department of Transportation
Version 2
March 6, 2007
US.4
"Acoustic Noise and Electromagnetic Study in Support of the Rhode Island Ocean SAMP"
J. H. Miller, G. R. Potty, K. Vigness-Raposa, D. Casagrande, L. A. Miller, J. Nystuen, and P. M. Scheifele
Ocean Special Area Management Plan (SAMP) Volume 2
April 26, 2013
EU.1
"Utgrunden offshore wind farm-measurements of underwater noise"
Ingemansson Technology AB, Göteborg, Sweden
Report 11-00329-03012700
June 17, 2003
EU.2
"Measurement of underwater noise emitted by an offshore wind turbine at Horns Rev"
K. Betke
Institut für technische und angewandte Physik GmbHOldenburg, Germany
February 13, 2006
EU.3
"Underwater noise from three types of offshore wind turbines: Estimation of impact zones for harbor porpoises and harbor seals"
J. Tougaard, O. D. Henriksen, and L. A. Miller
J. Acoust. Soc. Am. vol. 125, 3766-3773
June 2009