A noise reduction system called "Prairie Masker" was developed by the US Navy for several classes of its warships (including the Spruance, Perry, Arleigh Burke, and Ticonderoga) to reduce or mask their self-noise. The Prairie system is fitted to the ship's propellers, while Masker is fitted to the external hull in the vicinity of the propulsion plant. One method of masking the acoustic signature of a vessel that has been used in the past is known as the Masker System. In warships, air bubbles can be employed to mask potential targets or to provide alternate targets. Such systems are known as Masker. This system is in use by many NATO nations and works by injecting air around the hull of the vessel.
Masker also installed on a limited number of diesel submarines (post WWII - Guppy III) of the US Navy. The Masker-Prairie air compressor replaced No. 2 main diesel engine. A Guppy was far more than a minimum submarine conversion.
Prairie-Masker is used during both active and passive undersea warfare operations. Gas turbine ships routinely operate systems in port and at sea, to avoid marine growth from plugging holes in blade tips and masker belts. During ASW operations, there is no instantaneous way of determining if the airflow rates are accurate at any given time. Improper Prairie/Masker airflow rates are an ASW mission degrade. MACHALT Proposals Under Development will replace Prairie/Masker air system portable flow meters with an electronic airflow monitoring system.
Air bubbles can be employed to mask potential targets or to provide alternate targets. The large difference in characteristic impedance (c) between the air bubbles and the surrounding water make them very efficient as reflectors of acoustic energy. Very little sound will penetrate a curtain of air bubbles, making them very efficient as masking for noise sources.
Masker air forms an air bubble screen around the hull of the ship, reducing transmission of machinery noise to the surrounding waters. Masker creates acoustic impedance mismatch between hull and water, by way of the masker belts located around the hull, putting a blanket of air bubbles between the hull's machinery noise and the water. Masker air disguises low frequency machinery noise that radiates through the hull and cools bleed air for use in engine starting and motoring.
A basic method of preventing the transmission of sound in a medium requires the introduction of a significant density discontinuity in the medium. For example, sound attenuation in a low density medium requires the introduction of a high density material such as a slab of steel to create a high density discontinuity in the low density medium. Similarly, in a high density medium, sound attenuation can be achieved by introducing a low density material such as air as a discontinuity in the high density medium. Thus, a water/air interface would serve as an effective sound attenuator in water.
The large difference in characteristic impedance between the air bubbles and the surrounding water make them very effective as reflectors of acoustic energy. Very little sound will penetrate a curtain of air bubbles, making them veryefficient as masking for noise sources. Masker is used during both activeand passive undersea warfare operations.
Masker air forms an air bubble screen around the hull of the ship, reducing transmission of machinery noise to the surrounding waters. Masker creates acoustic impedance mismatch between hull and water, by way of the maskerbelts located around the hull, putting a blanket of air bubbles between the hull'smachinery noise and the water. Masker air disguises low frequency machinery noise that radiates through the hull and cools bleed air for use in engine startingand motoring.
The Masker radiated noise suppression system is designed to create an impedance mismatch around the parts of the hull which are inherently noisy. Through this mismatch the sound path from the hull or propeller to the surrounding water is blocked or distorted, resulting in a very much reduced level of radiated noise or a noise which is very difficult to detect and identify because of its distorted nature.
In sensitive areas such as the region surrounding the machinery compartment, the Masker system is used to disguise LF noise emanating from machinery that radiates through the hull by blowing air through a series of small nozzles mounted in the hull at about 12 psi. The air supply is bled off from the ship's high-pressure gas turbine (if fitted) or supplied by dedicated compressors. The bubbles so created remain trapped along these sensitive regions of the hull and mask the noise that would otherwise be directly radiated into the ocean from the vibrating hull plating.
The Masker Air System uses air from the ship's bleed air system via the bleed air cooler for discharge through emitter belts located around the underwater girth of the ship. The masker regulator valve reduces masker air pressure from 75 to 28 psig. After leaving the reducing valve, the air supply divides into two branches supplying air to the forward and aft emitter belts. On the FFG-7 the Emitter Belts are located at frames 177 and 253. Each belt is divided into port and starboard halves. Each belt has a separate air connection. Each emitter belt uses a solenoid operated valve to control air flow. The ACC controls these solenoid valves. Masker air discharges through each connection at a rate of 425 squared cubic feet per minute (SCFM) at approximately 12 psig. Perforations in the emitters allow discharge of Masker air from the keel to the water line. An orifice plate in the port side emitter belt balances air flow.
Bubbles have been used to acoustically decoy or mask a submersible vessel, however, this method is only partially effective as the bubbles create a reflective air column that draws attention or tends to inadequately clothe a submersible from acoustic probing. Unfortunately, the Masker System only operates within a narrow band of vessel speeds and is ineffective at speeds above 15 knots or at very low speeds where the air bubbles rise straight to the surface instead of streaming along the hull. Certain sea conditions also adversely effect the Masker System.
Furthermore, the Masker System increases the vessel's wake and this can be disadvantageous if the vessel is being attacked by wake-homing torpedoes. The Masker System also requires a supply of compressed air which itself can become a source of noise. Finally, air from the Masker System can be sucked into the cooling water intakes of the vessel, which can create additional problems as the cooling systems become inefficient.
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