(this item first appeared in AEI’s lay summaries of new research)

Tougaard, Henriksen, Miller. Underwater noise from three types of offshore wind turbines: Estimation of impact zones for harbor porpoises and harbor seals. J. Acoust. Soc. Am. 125 (6), June 2009. 3766-3773.

Underwater noise was recorded from three different types of wind turbines in Denmark and Sweden. The authors note that virtually all airborne noise from the turbine blades is reflected off the surface of the water, while vibrations from the machinery are transmitted through the tower and into the foundation, from where it radiates out into the water column and seabed. In general, turbine noise was only measurable above ambient noise at frequencies below 500Hz, with total SPLs of 109-127db re 1uPa rms, measured at 14-20m from the turbines’ foundations. By comparing measured sound levels with audiograms of harbor seals and harbor porpoises, the researchers determined that the sounds were only slightly audible for the porpoises at ranges of 20-70m, whereas harbor seals may hear the sounds at ranges of 100m to several kilometers. As a bottom line, researchers suggest that behavioral changes are very unlikely in harbor porpoises except at very close ranges, while seals may have some behavioral reaction out to a few hundred meters. For both species, masking is predicted here to be low to non-existent (due to differences between vocalization frequency patterns and the predominantly low-frequency turbine noise), and the sound is not loud enough to cause physical injury, no matter how close the animals are.