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 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 on or ...

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 detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...

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 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, ca ...

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

propellers A propeller (colloquially 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 ...

. *

Hydrodynamic In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) ...

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 ...

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

Targeting

Hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...

s 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 on 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 device placed in water to damage or destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, an ...

s and

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

es such as the

CAPTOR mine The Mark 60 CAPTOR (Encapsulated Torpedo) is the United States' only deep-water anti-submarine naval mine. It uses a Mark 46 torpedo contained in an aluminum shell that is anchored to the ocean floor. The mine can be placed by either aircraft, s ...

can be programmed to distinguish the acoustic signatures of different vessels, leaving friendly vessels unmolested and attacking

high-value target In United States military terminology, a high-value target (HVT) is the term given to a person or resource that an enemy commander requires to complete a mission. The term has been widely used in the news media for Osama Bin Laden and high-ranki ...

s 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 facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a ...

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 section Radar cross-section (RCS), also called radar signature, is a measure of how detectable an object is by radar. A larger RCS indicates that an object is more easily detected. An object reflects a limited amount of radar energy back to the source. ...

s 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 (colloquially 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 ...

s to reduce cavitation, this led to the development of large slow turning propellers, today there is a preference now for

pump-jet 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 ( axial-flow pump), a centrifugal pump, or a mixed flow pump which is a combination of bot ...

propulsor {{short description, Mechanical device to propel a vessel 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. Th ...

s over propellers. *the fitting of

anechoic tile __NOTOC__ 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: *To absorb the sound waves ...

s 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 the

Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against Fr ...

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 recreatio ...

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

s 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 The Research Vessel ''Triton'' is a trimaran vessel owned by Gardline Marine Sciences Limited and a former prototype British warship demonstrator for the UK's Defence Evaluation and Research Agency (later QinetiQ). She was built as a techno ...

to test the principle of a large scale trimaran design.

Machinery monitoring and diagnosis

Tracking and study of animals