Fundamentally, a high background noise level already exists in the ocean due to wind, waves, and precipitation: depending on the frequency, this is around 80-90 dB, comparable to a passing lorry or a hair dryer. The scale is logarithmic, meaning that a 3 dB increase physically represents a doubling of the noise level, even if human perception does not register it as such.
In order to protect the already heavily pressured marine environment, the Bundesumweltministerium established the "Schallschutzkonzept" in 2013 for the construction of offshore wind turbines. The threshold values were determined based on the research findings of a study that investigated the risk of (temporary) hearing loss in harbour porpoises (Phocoena phocoena). When these massive wind turbines are constructed on the high seas, foundations must be driven into the seabed using heavy machinery. This activity is akin to heavy stomping in the flat above: it reverberates loudly. Very loudly.
Consequently, a legal limit was established: within the immediate vicinity of the piling sites, a maximum noise level of 160 dB may be measured. To comply with these thresholds, several methods are employed to contain the piling noise, analogous to the foam-insulated walls of a recording studio: guide sleeves enclose the pile, and bubble curtains additionally shield the surrounding environment. In this way, a reduction of 20-25 dB can be achieved.
Air bubbles can therefore trap noise and thereby protect the animals outside the zone – not only harbour porpoises.
Sources:
BMU (2013): Konzept für den Schutz der Schweinswale vor Schallbelastungen bei der Errichtung von Offshore-Windparks in der deutschen Nordsee (Schallschutzkonzept)
Possenti (2024): The present and future contribution of ships to the underwater soundscape. https://doi.org/10.3389/fmars.2024.1252901
