Wikipedia
Prairie-Masker is a radiated noise reduction system fitted to some US Navy warships, including the Spruance, Oliver H. Perry, Arleigh Burke class destroyers, and the Ticonderoga class cruisers of the US Navy. The system was also installed during the 1960s on a limited number of post WWII Guppy III modified, and later diesel submarines.
The Masker and Prairie systems are designed to prevent the classification or identification of a warship's acoustic signature by another vessel, i.e. by a hostile submarine. Instead of hearing machinery, the ship sounds similar to rain on passive sonar. The Masker portion of the system is installed onto the hull of a vessel, usually near its machinery spaces. The Prairie portion of the system is designed to silence the vessels propellers. Originally classified, these systems are now used by several countries as part of their anti-submarine warfare solutions.