During the installation of offshore wind farm foundations and deep water foundations, the underwater piling noise of hydraulic impact hammers induces considerable hydro sound that is radiated from the piles into the surrounding water. These very high underwater noise emissions can reach dangerous levels and are potentially harmful to marine life in a large area. Both knowledge about the complex mechanisms and dynamic interactions of the hammer and pile system and knowledge about the acoustic behavior of the piles, the shallow water, and the ground are necessary to understand the generation and radiation of underwater piling noise and to develop effective noise mitigation systems. The new mitigation method of hydro sound dampers (HSD) uses curtains of robust air-filled elastic bladders showing high resonant effects, similar to air bubbles, and special dissipative PE-foam elements to reduce impact noise. These HSD elements are fixed to pile surrounding fishing nets. HSD-systems are independent of compressed air and not influenced by currents. The theoretical background, numerical simulations, laboratory tests, and offshore tests with noise mitigations between 17 dB to 35 dB (SEL) are described.