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Radiated noise reduction system fitted to some western warships

Prairie Masker is a radiated noise reduction system fitted to some western warships, including the Oliver Hazard Perry class frigates, Spruance and 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 vessel's propellers. Originally classified top secret, these systems are now used by several countries as part of their antisubmarine warfare systems.

Ship silencing is an important part of reducing unwanted noise, which can severely limit a naval vessel's active and passive undersea warfare capability and decrease the range or probability of detection by an unfriendly vessel. The US Navy maintains a Ship Silencing Program to address these problems. The goals of the Ship Silencing Program are a reduction of sonar self noise over the frequency range of passive capable sonar and the reduction in the ship's radiated noise to reduce detection by enemy submarines. Self noise is the noise generated by a ship that has an effect on its own sonar and sensors. Radiated noise is the noise generated by a ship that has an effect on other ship's sensors, especially submarines. The Masker Prairie countermeasure systems are key elements in this program to reduce radiated noise and self noise that are not reduced by controlling the sounds at their source or their transmission to the hull. The use of air bubbles and their effects upon acoustic wave propagation began to be studied systematically during the Second World War as part of a general effort to understand sound in submarine warfare. In the United States, this research effort was directed by the National Defense Research Committee (NDRC) and was carried out by various US Navy laboratories. The advances made in this research were published in The Physics of Sound in the Sea in 1946. Results of that study most pertinent to this discussion are found in Part IV of that volume Acoustic Properties of Wakes. The use of bubble curtains have found wide spread applicability in reducing noise or the damaging effect of shock waves in a variety of industry from: underwater blasting, to pile driving, to explosive metal forming.) on the other hand, is fitted either near to or on the ships propellers and compressed air is pumped through small holes in the edges of the propeller. As might be expected, keeping a set of small holes clear of fouling under the surface of a ship or submarine can present a problem. On submarine installations this was prevented by running a trickle of fresh water through the system when the ship was in port. Most of the creatures that foul an ocean going ship's hull cannot survive in fresh water. A similar system called Agouti was fitted to Royal Navy frigates and such vessels as .

In submarine use the large volume of air needed to operate the system needs to come from the surface, and therefore Prairie Masker can only be used when the diesel submarine is snorkeling. This is not a serious limitation since the major noise source that needs quieting are the diesel engines, which are used only when snorkeling.