An underwater noise study (including use of numerical modelling) has been undertaken by the National Physical Laboratory to estimate the likely underwater noise levels generated by the construction, operation and decommissioning of the Atlantic Array Offshore Wind Farm. The output of this study has informed the assessment of effects of subsea noise on sensitive marine receptors throughout the Environmental Statement (ES). A total of three foundation locations were modelled which were chosen based on bathymetry and location to bathymetric features to provide broad representation of propagation conditions across the Atlantic Array site. For the construction phase, the modelled source was based on the use of a jacket foundation with a pin-pile diameter of 3.5 m installed using a 2,100 kJ hammer, with a typical hammer blow energy at 1,400 kJ. The propagation model used was based on an energy flux approach which calculates the sound energy transmitted through the water column. Results are presented in terms of sound exposure level (SEL) and peak sound pressure level (peak SPL) received level output as a function of range away from each modelled pile location whilst accounting for seabed properties and varying bathymetry. The modelling indicates that there is considerable variation in noise propagation across the Atlantic Array site due to variations in bathymetry, with good sound propagation conditions on a westerly bearing from the site. It has been estimated that mortality of marine mammals or fish would be unlikely to occur except in very close proximity to the pile or in the case of prolonged noise exposure close to the pile. Whilst it is possible that fish larvae mortality would occur, there is indicative evidence that there will be no statistically significant effect on survival rates beyond 100 m from the pile assuming they drift with the tide. It was also estimated that instantaneous injury (auditory) of marine mammals would be unlikely to occur beyond 200 m from the pile during full piling and less than 50 m during the soft-start, although prolonged exposure to the noise (SEL dose) may increase the risk at larger ranges. Assuming that hammer blow energy of 1,400 kJ will typically be used across the Atlantic Array site with the potential to peak at 2,100 kJ, it has been estimated that pinnipeds may suffer temporary threshold shift (TTS) of hearing sensitivity and exhibit a fleeing response to the underwater noise from the foundation installation at ranges of 550 m to 850 km, respectively. For harbour porpoise around mid-water column (i.e. foraging etc), the possible avoidance area around the foundation was estimated to be 17.5 km to 29.0 km, for the maximum hammer blow energy of 2,100 kJ, depending location of the foundation within the site and the surrounding bathymetry.