Prairie - Masker Prairie / Masker is a radiated noise reduction system fitted to some warships of western origin , including the DD - 963 and DDG - 51 - class destroyers and the CG - 47 - class cruisers . The Masker portion consists of two bands fitted to the outside of the hull adjacent the vessel ' s engine rooms , compressed air is then forced into the bands and escapes through machined perforations to create a barrier of air bubbles in the sea about the hull , thus trapping machinery noise . Prairie works via the same principles but is fitted either near to or on the ships propellors . The systems are designed to prevent classification / identification by radiated noise of the warship in question by acoustical analysis , i . e . by a hostile submarine . Instead of hearing machinery , the ship sounds similar to rain on passive sonar . Originally classified , these systems are now used by several countries as part of their anti - submarine warfare solutions . The masker portion of the system is based upon creating a speed of sound ( acoustic impedance ) mismatch between the bubble curtain and the seawater . Acoustic waves encountering material with a radically different speed of sound do not penetrate , but bounce back . The speed of sound is dependent upon the square root of stiffness of a material divided by its density . In a bubble cloud the density is mostly unchanged - that of the water - but the stiffness is that of air . The result is that the speed of sound in a cloud of bubbles in water is a factor of almost ten slower than in water itself and a factor of three slower than it is in air . Thus , sounds within the ship ( or submarine ) hull which would otherwise go out into the water and propagate for a long distance are reflected back into the hull and eventually dissipated therein . You can experience one manifestation of this physics by filling a glass with water and with carbonated water and tapping the side with a spoon . The water - filled vessel will ring . The vessel with bubbles in it will ' thud . ' ( Thanks to Alan Ellinthorpe , a navy acoustician , for this experiment . ) The noise quieting of the Prairie screw actually works on a different principle . The principal noise problem with a screw is cavitation . The pressure behind the moving blade may become so low that it is less than the vapor pressure of water at that depth . This results in a vapor bubble forming . When the bubble gets out of the low pressure area , it collapses right back into water and makes a loud noise in doing so . If , however , the trailing edge of the screw emits a small amount of air , then the cavitation bubbles have a bit of air within them . The collapse of the water vapor does not completely close the bubble , and thus there is little noise generated thereby . The Prairie Masker was also installed on a limited number of diesel submarines ( post WWII - Guppy III and later diesel boats ) of the US Navy . In submarine use the air for 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 . When used by a surface vessel ( anti - submarine destroyer ) , it is so effective in hiding its acoustic signature that often they are requested not to use the system when exercising with submarines . This is because of the possibility of a collision with the submarine which might come at or near the surface in close proximity to the destroyer without hearing it . As might be expected , keeping a set of small holes clear of fouling under the surface of a ship of submarine can present a problem . On submarines 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 can not survive in fresh water .