Effects of constructing the DanTysk offshore wind farm (German Bight, 80 turbines, 6 m diameter foundations) were studied by passive acoustic monitoring of pile-driving noise and harbour porpoise Phocoena phocoena echolocation. An acoustic deterrence device (seal scarer) was used to protect porpoises from hearing loss and bubble curtains were used to attenuate the pile-driving noise. Porpoise occurrence, quantified by echolocation signals, decreased when the seal scarer was engaged, during pile driving and up to 5 h after pile driving stopped. This effect extended out to 12 km, less than the 18-25 km reported from other pile drivings performed without bubble curtains. The bubble curtains thus effectively reduced the temporary habitat loss and risk of hearing loss. The 2 bubble curtains each attenuated the noise by between 7 and 10 dB, when used separately, and 12 dB when used together. Attenuation was most pronounced above 1 kHz, where the pile-driving noise at larger distances was comparable to or lower than ambient noise. This suggests that noise regulation should be based on frequency-weighted sound levels in addition to broadband levels, to assure that mitigation measures are effective in reducing impact on animals and not only in fulfilling legal requirements. The strong reaction to the seal scarer raises concern that it may surpass the reactions to the pile-driving noise itself, when operating with bubble curtains, and calls for a re-evaluation of the specifications of seal scarer sounds.