Underwater noise is a severe annoyance and danger to marine life. The innovative HSD (hydro sound damper) leads to an effective general method to reduce underwater noise. HSD was developed and patented between 2007 and 2010 by the author to reduce general marine noise and especially offshore piling noise. The theory and the acoustical background of the new noise mitigation method will be presented. HSD systems use nets with gas filled elastic balloons and special PE-foam elements with high dissipative effects to reduce continuous and impact noise. The resonance frequency of the HSD-elements, the optimum damping rate for impact noise, the distribution and the effective frequency range can be fully controlled. Offshore tests and serial applications in offshore wind farms in Germany and Great Britain demonstrate this new effective way to reduce the very high offshore piling noise. HSD systems are very small systems and easy to handle. They are independent of external air supply, not influenced by tidal currents, not expensive and easy adaptable to different applications. Underwater noise reductions by HSD systems and bubble curtains are measured by an independent acoustics institute when using the biggest hammers in the world. Achieved results between 14 dB (decibels) (SEL—sound exposure level) and more than 27 dB (SEL) are presented and discussed.