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The term acoustic signature is used to describe a combination of acoustic emissions of sound emitters, such as those of ships and submarines. In addition, aircraft, machinery, and living animals can be described as having their own characteristic acoustic signatures or sound attributes, which can be used to study their condition, behavior, and physical location. [1]
The analysis of acoustic signatures is an important adjunct to passive sonar used to track naval warships and weapons. Similar methods have been used to identify aircraft, especially before the development of sophisticated radar tracking.
The acoustic signature is made up of a number of individual elements. These include:
These emissions depend on a hull's dimensions, the installed machinery and ship's displacement. Therefore, different ship classes will have different combinations of acoustic signals that together form a unique signature.
Hydrophones and sonar operating in passive mode can detect acoustic signals radiated by otherwise invisible submarines, and use these signals to target attacks.
Modern naval mines and torpedoes such as the CAPTOR mine can be programmed to distinguish the acoustic signatures of different vessels, leaving friendly vessels unmolested and attacking high-value targets when faced with multiple possible targets, e.g. distinguishing an aircraft carrier from its escorts.
Warship designers aim to reduce the acoustic signature of ships and submarines just as much as they aim to reduce the radar cross sections and infra-red signals. For submarines, as a prime factor in how they can be detected the reduction of the acoustic signature is a primary goal.
The acoustic signature can be reduced by
For a time Britain's Royal Navy toyed with the idea of the trimaran hulled Future Surface Combatant. These would have had a very low acoustic signature. With three blade-like hulls, these ships would have cut through the water with a minimum of hydrodynamic noise. Radiated mechanical noise would also be minimised by using propulsors powered by a diesel-electric power plant, with the diesels being placed in the superstructure to mechanically isolate them from the water. This project got as far as the construction of the research ship RV Triton to test the principle of a large-scale trimaran design.
Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery. These shock waves are strong when they are very close to the imploded bubble, but rapidly weaken as they propagate away from the implosion. Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal, causing a type of wear also called "cavitation". The most common examples of this kind of wear are to pump impellers, and bends where a sudden change in the direction of liquid occurs. Cavitation is usually divided into two classes of behavior: inertial cavitation and non-inertial cavitation.
A propeller is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon a working fluid such as water or air. Propellers are used to pump fluid through a pipe or duct, or to create thrust to propel a boat through water or an aircraft through air. The blades are shaped so that their rotational motion through the fluid causes a pressure difference between the two surfaces of the blade by Bernoulli's principle which exerts force on the fluid. Most marine propellers are screw propellers with helical blades rotating on a propeller shaft with an approximately horizontal axis.
Sonar is a technique that uses sound propagation to navigate, measure distances (ranging), communicate with or detect objects on or under the surface of the water, such as other vessels.
A pump-jet, hydrojet, or water jet is a marine system that produces a jet of water for propulsion. The mechanical arrangement may be a ducted propeller, a centrifugal pump, or a mixed flow pump which is a combination of both centrifugal and axial designs. The design also incorporates an intake to provide water to the pump and a nozzle to direct the flow of water out of the pump.
A minehunter is a naval vessel that seeks, detects, and destroys individual naval mines. Minesweepers, on the other hand, clear mined areas as a whole, without prior detection of mines. A vessel that combines both of these roles is known as a mine countermeasures vessel (MCMV).
A propulsor is a mechanical device that gives propulsion. The word is commonly used in the marine vernacular, and implies a mechanical assembly that is more complicated than a propeller. The Kort nozzle, pump-jet and rim-driven thruster are examples.
The baffles is the area in the water directly behind a submarine or ship through which a hull-mounted sonar cannot hear. This blind spot is caused by the need to insulate the sonar array, commonly mounted near the bow, from the noise of the vessel's machinery.
Prairie-Masker is a radiated noise reduction system fitted to some western warships, including the Oliver Hazard Perry-class frigates, Spruance and Arleigh Burke-class destroyers, and the Ticonderoga-class cruisers of the US Navy. The system was also installed during the 1960s on a limited number of post WWII Guppy III modified, and later diesel submarines.
A stealth ship is a ship that employs stealth technology construction techniques in an effort to make it harder to detect by one or more of radar, visual, sonar, and infrared methods.
A double hull is a ship hull design and construction method where the bottom and sides of the ship have two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship, and a second inner hull which is some distance inboard, typically by a few feet, which forms a redundant barrier to seawater in case the outer hull is damaged and leaks.
Anechoic tiles are rubber or synthetic polymer tiles containing thousands of tiny voids, applied to the outer hulls of military ships and submarines, as well as anechoic chambers. Their function is twofold:
An acoustic torpedo is a torpedo that aims itself by listening for characteristic sounds of its target or by searching for it using sonar. Acoustic torpedoes are usually designed for medium-range use, and often fired from a submarine.
A towed array sonar is a system of hydrophones towed behind a submarine or a surface ship on a cable. Trailing the hydrophones behind the vessel, on a cable that can be kilometers long, keeps the array's sensors away from the ship's own noise sources, greatly improving its signal-to-noise ratio, and hence the effectiveness of detecting and tracking faint contacts, such as quiet, low noise-emitting submarine threats, or seismic signals.
The Gotland-class submarines of the Swedish Navy are modern diesel-electric submarines, which were designed and built by the Kockums shipyard in Sweden. They are the first submarines in the world to feature a Stirling engine air-independent propulsion (AIP) system, which extends their underwater endurance from a few days to weeks. This capability had previously only been available with nuclear-powered submarines.
A submarine hull has two major components, the light hull and the pressure hull. The light hull of a submarine is the outer non-watertight hull which provides a hydrodynamically efficient shape. The pressure hull is the inner hull of a submarine that maintains structural integrity with the difference between outside and inside pressure at depth.
An acoustic mine is a type of naval mine which monitors audio activity in its vicinity. Depending on its design, it will either passively listen to its environment, depending only on the noise that is made by passing ships or actively send out audio pulses, not unlike a sonar, listening to the lapse in time in which the echo returns to it.
Geophysical MASINT is a branch of Measurement and Signature Intelligence (MASINT) that involves phenomena transmitted through the earth and manmade structures including emitted or reflected sounds, pressure waves, vibrations, and magnetic field or ionosphere disturbances.
Acoustic quieting is the process of making machinery quieter by damping vibrations to prevent them from reaching the observer. Machinery vibrates, causing sound waves in air, hydroacoustic waves in water, and mechanical stresses in solid matter. Quieting is achieved by absorbing the vibrational energy or minimizing the source of the vibration. It may also be redirected away from the observer.
Noise and vibration on maritime vessels are not the same but they have the same origin and come in many forms. The methods to handle the related problems are similar, to a certain level, where most shipboard noise problems are reduced by controlling vibration.
Silent running is a stealth mode of operation for naval submarines. The aim is to evade discovery by passive sonar by eliminating superfluous noise: nonessential systems are shut down, the crew is urged to rest and refrain from making any unnecessary sound, and speed is greatly reduced to minimize propeller noise. The protocol has been in use since the latter part of World War I, when hydrophones were invented to detect U-boats.