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.

Military use

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 Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

tracking.

Sound characteristics

The acoustic signature is made up of a number of individual elements. These include: *Machinery noise: noise generated by a ship's engines, propeller shafts, fuel pumps, air conditioning systems, etc. *

Cavitation Cavitation in fluid mechanics and engineering normally is the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When sub ...

noise: noise generated by the creation of gas bubbles by the turning of a ship's

propellers A propeller (often called a screw if on a ship or an airscrew if on an aircraft) 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 flu ...

. *

Hydrodynamic In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in moti ...

noise: noise generated by the movement of water displaced by the hull of a moving vessel. These emissions depend on a hull's dimensions, the installed machinery and ship's displacement. Therefore, different

ship class A ship class is a group of ships of a similar design. This is distinct from a ship type, which might reflect a similarity of tonnage or intended use. For example, is a nuclear aircraft carrier (ship type) of the (ship class). In the course o ...

es will have different combinations of acoustic signals that together form a unique signature.

Targeting

Hydrophones and

sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances ( ranging), communicate with or detect objects o ...

operating in passive mode can detect acoustic signals radiated by otherwise invisible submarines, and use these signals to target attacks. Modern

naval mine A naval mine is a self-contained explosive weapon placed in water to damage or destroy surface ships or submarines. Similar to anti-personnel mine, anti-personnel and other land mines, and unlike purpose launched naval depth charges, they are ...

s and

torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, such ...

es 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 An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and hangar facilities for supporting, arming, deploying and recovering carrier-based aircraft, shipborne aircraft. Typically it is the ...

from its escorts.

Countermeasures and acoustic quieting

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 *fitting of machinery with the best possible mechanical tolerances and designed to produce a minimum of noise. *decoupling the machinery from the hull by mounting machinery on rubber mounting blocks. *designing

propeller A propeller (often called a screw if on a ship or an airscrew if on an aircraft) 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 flu ...

s to reduce cavitation, this led to the development of large slow turning propellers, today there is a preference now for pump-jet propulsors over propellers. *the fitting of anechoic tiles to the hull, however ill fitting and loose anechoic tiles can themselves be a source of noise. *hydrodynamic efficiency to minimise the perturbation of water. *care in minimising protrusions from the hull.

Trimaran warships

For a time Britain's

Royal Navy The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King of the United Kingdom, King. Although warships were used by Kingdom ...

toyed with the idea of the

trimaran A trimaran (or double-outrigger) is a multihull boat that comprises a main hull and two smaller outrigger hulls (or "floats") which are attached to the main hull with lateral beams. Most modern trimarans are sailing yachts designed for recrea ...

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 A superstructure is an upward extension of an existing structure above a baseline. This term is applied to various kinds of physical structures such as buildings, bridges, or ships. Aboard ships and large boats On water craft, the superstruct ...

to mechanically isolate them from the water. This project got as far as the construction of the research ship

RV Triton The research vessel ''Triton'' is a trimaran vessel owned by Gardline group, Gardline Marine Sciences Limited and a former prototype British warship Emergency vehicle, demonstrator for the UK's Defence Evaluation and Research Agency (later Qine ...

to test the principle of a large-scale trimaran design.

References