To reduce greenhouse gas emissions, an increasing amount of energy is being generated from offshore wind farms. Their construction and operation might cause severe disturbance for the harbour porpoise, the only cetacean species breeding in the German Bight. Offshore wind turbines are hardly audible to harbour porpoises in their vicinity, but during the foundation process considerable noise emissions are produced which can affect the behaviour and – depending on the distance from the sound source – cause temporary hearing threshold shift, permanent hearing threshold shift or even death of harbour porpoises. In order to minimise these effects, noise mitigation systems during pile driving and acoustic harassment devices to drive harbour porpoises out of the endangered area before pile driving were developed. Until 2017, the seal scarer was mandatory as acoustic harassment device. However, seal scarers led to decreased porpoise detection rates in much larger distances than intended, when 1 km is usually rendered sufficient to avoid temporary and permanent hearing threshold shift. Therefore, the FaunaGuard Porpoise module is now prescribed and used as deterrent device. The development of the FaunaGuard on the one hand aimed at deterring all harbour porpoises from a radius of 1 km around offshore wind farm piling locations before the start of noise-intensive piling. On the other hand, it was intended that deterrence by a FaunaGuard should not lead to such large-scale disturbance as caused by a seal scarer (partly 7 km and more). Although the respective project-specific evaluations indicated that a FaunaGuard is highly effective, a cross-project analysis and a comparison with data from the previous procedure for piling with previous seal scarer operation were still pending. Thus, this study investigated the following research topic: How do harbour porpoises respond to the FaunaGuard and subsequent piling during the construction of offshore wind farms in the North Sea, in comparison to the seal scarer as acoustic harassment device? In four offshore wind farm projects, harbour porpoise detection rates were monitored acoustically: (1) Continuously every minute at different distances from the piling, and (2) every minute from a few hours before FaunaGuard operation until a few hours after the piling at 750 m and 1,500 m distance from the piling. Stationary and mobile Cetacean Porpoise Detectors (C-PODs) were used for this purpose. Assuming that the detection rates indicate the physical presence and absence of harbour porpoises, this study showed the following: (1) The detection rates of harbour porpoises were decreased by 48 % during FaunaGuard operation at smaller distances up to around 1.5 km, compared to a period of on average six hours before the operation of the device, without leading to long-term deterrence. (2) During the operation of the FaunaGuard, reduced detection rates were observed only up to a distance of around 2 to 2.5 km, so that in contrast to the seal scarer, obviously no large-scale disturbance occurred. (3) In spite of environmental differences among the offshore wind farms, detection rates decreased between 37 % and 75 % during FaunaGuard operation in the vicinity of piling locations. Furthermore, no far-reaching disturbance and long-term deterrence were to be expected in any of the wind farms positioned in different regions of the German Bight, North Sea. Therefore, the FaunaGuard appeared to be applicable in various areas in the North Sea for scaring harbour porpoises away from the danger zone around the pile-driving site. (4) After the first 20 to 25 minutes of FaunaGuard operation, harbour porpoise detection rates nearly declined to zero in the close range of up to 1.25 km distance. Longer operation times of the device seemed to lead only to a small increase in the maximum distance and intensity of deterrence. (5) At close range, no meaningful differences were found between the FaunaGuard and the seal scarer. However, in 5 to 10 km distance (mean around 8 km), the seal scarer had a much more far-reaching effect than the FaunaGuard. The detection rates during FaunaGuard operation decreased by only 12 % compared to the detection rates in the six hours before, but by 94 % when using a seal scarer. Due to the shorter effect range of the FaunaGuard as well as to improved noise mitigation systems, the response to the FaunaGuard and subsequent pile driving at larger distances seemed to be lower than shown by studies where pilings with a seal scarer as acoustic harassment device were investigated. The FaunaGuard was assessed as a highly effective acoustic harassment device to displace harbour porpoises from a small-scale area in the short term and thus to prevent a temporary or permanent hearing threshold shift. All results from the present study indicated that a FaunaGuard should be used instead of a seal scarer for future construction of offshore wind farms, assuming there is no habituation effect. Although this study only covers projects in the North Sea, we suppose that the FaunaGuard will also work in the Baltic Sea. The FaunaGuard is an important step forward to a less harmful piling procedure in the North and Baltic Seas.