Here is, in simple terms, how the system works. The ship uses a gas turbine engine for propulsion. A gas turbine is basically a jet engine that is inside a box. Gas turbines create great amounts of hot air under pressure. This air is used for several things, cooling the engine itself, high pressure bleed air, Prairie/Masker air, and finally for driving the power turbine. I can run down the operation of a gas turbine at little later. So the turbine makes the air and it is piped away for its different uses.
The Prairie/Masker is used in two different locations. First off the Prairie air is used in the propeller, called a screw in the Navy. Simple enough to remember, both start with a P. When the screw cuts through the water is can make a hole or cut. The resulting cut will then slam back full of water causing a loud noise, this is called cavitation. So to stop this from happening air is fed down the center of the propeller shaft to the propeller hub. From there it is fed through small holes in the edges of the propeller blades and fills the hole. So instead of the water crashing back together it slowly pushes the air out of the pocket and makes less noise.
Masker are is used to reduce the amount of machinery from being transmitted through the hull. The air is fed into a series of belts that run under the ship. This are creates a blanket of bubbles around the middle section of the ship. Sounds from the machinery has a harder time traveling through the air, into the ships hull, into air, and then into the water.
When these two systems are running in conjunction with one another it makes the ship sound like a rain storm. Since the submarine wouldn't know the weather top side they might ignore the rain storm and look for a mechanical sound. This can give the destroyer time to hear the sub and send it helo to make a kill. Or give them time to fire off one of there own torpedoes against the sub.
On the picture you can see a difference in the wake of the two ships. The Spruance class destroyer has the lighter colored wake. The supply ship does not. The destroyer is running its Prairie/Masker system. Like I said this is the bare bones simple explanation of these systems, but I hope it helped out some.
