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 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.