Especially during the installation of offshore wind turbines, the piling noise of hydraulic impact hammers induce considerable underwater sound emissions. Peak sound pressure levels have been measured high above 190 dB re 1 µPa at a distance of 750m from the pile driver. This construction noise of offshore wind turbines is potentially harmful to marine life, in particular to marine mammals and it induces flee reactions in a large area, as water is a very efficient conductor of sound.
Different zones of underwater noise immissions can be defined in the surrounding of a source of acoustic noise (Elmer et. al. 2007). Within the zone of audibility with moderate exposure levels, marine animals like harbour porpoises, harbour seals, grey seals and also fish will show some kind of reaction or change in their behaviour . At higher exposure levels, within the closer zone, underwater noise can induce temporary (TTS) or permanent (PTS) threshold shift. Important acoustic information might be masked caused by reductions in hearing sensitivity of an animal. Close to a very loud source of noise like pile driving, extreme intensity levels of underwater noise can cause physical trauma or death.
Even the lowest level of damage, which is a temporary threshold shift (TTS), must be avoided. Due to larger piles requiring higher driving energies, even higher underwater noise levels are expected in future offshore projects and this is also accompanied by an increasing number of erected offshore wind turbines.
Hence, effective noise reducing methods are in great demand to keep the limiting level of 160 dB (SEL) at 750m distance from offshore pile driving of the German Authority BSH. The mitigation of underwater noise is necessary, getting sound levels below recommended acoustic emission thresholds that are no longer harmful and disturbing to marine mammals and other protected animals.