Prairie-Masker is a noise reduction system that was first used on United States Navy warships in the 1970's, as of the late 1990's the technology is used in several navies including the Australian, British and Canadian Royal Navies as well as the Hellenic Navy. In the US Navy it is used on Arleigh Burke class guided missile destroyers and Ticonderoga class guided missile cruisers.
The system is designed to prevent the identification or classification of a warship by masking its acoustic signature. Instead of hearing the sound of the engines running and the propeller spinning it creates a sound on sonar similar to rain hitting the surface of the water. There are two different components to this system, the Masker portion and the Prairie portion.
The Masker portion is designed to mask the ship's engine noise on enemy sonar and to also increase the ship's own sonar effectiveness by reducing self-noise. The Masker portion typically 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 within the hull where it is dissipated.
The Prairie portion is actually built into the ship's propellers, on older ships it was built into the hull above them. Small holes are built into the edges of the propellers through which compressed air is pumped thereby disguising the sound of the propeller as bubbles in the water.
