Pile driving of large steel monopiles in offshore waters has increased rapidly in recent years due to the expanding development of offshore wind energy. In particular, Phocoena phocoena (harbor porpoise) has been the focus of attention with respect to a possible negative impact. Impact pile driving, where a large steel monopile is driven 20-30 m into the seabed, is capable of generating very loud sound pressures, exceeding 230 dB re 1 μPa peak-peak in source levels and detectable at distances of tens of kilometers (Bailey et al. 2010). Such high sound pressures, coupled with the repetitive emission of sounds (1–2 strokes/s) at a high duty cycle (10%) gives the potential for exposing nearby animals to very high and potentially damaging sound exposure levels (Gordon et al. 2009). Besides the potential to inflict acute injury, the pile-driving noise has the potential to affect behavior of marine mammals over an even larger area.