For the sustainable development of the offshore renewable energy industry, it is necessary to reduce or avoid the damaging effects of noise (such as death or permanent hearing damage), from activities which produce high sound pressure levels, such as pile driving, on marine mammals. One way to achieve this is to ensure that marine mammals are not present in areas where loud noises are being produced, by deterring them by means of safe sounds produced by Acoustic Mitigation Devices (AMDs). Research is needed to determine whether AMDs can mitigate immediate vicinity impacts of loud sounds, which could give rise to temporary or permanent hearing damage, injury or death of marine mammals. In the North Sea, the two most abundant marine mammal species are the harbour porpoise (Phocoena phocoena) and the harbour seal (Phoca vitulina). COWRIE commissioned SEAMARCO to carry out four studies on the audibility of sounds produced by three selected AMDs and their effect on the behaviour of harbour porpoises and harbour seals (playback experiments). In addition to achieving these aims, SEAMARCO estimated the distances at which sounds from AMDs are audible to, and elicit behavioural responses in, harbour porpoises and harbour seals.