Acoustic signature
Encyclopedia
Acoustic signature is used to describe a combination of acoustic emissions of ships and submarines.
These emissions depend on a hull's dimensions, the installed machinery and ship's displacement. Therefore different ship class
es will have different combinations of acoustic signals that together form a unique signature.
s and Sonar
operating in passive mode can detect acoustic signals radiated by otherwise invisible submarines, and use these signals to target attacks.
Modern naval mine
s and torpedo
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 from its escorts.
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
For a time the 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 propulsor
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
to test the principle of a large scale trimaran design.
Contributing factors
The acoustic signature is made up of a number of individual elements. These include:- Machinery noise: noise generated by a ships engines, propeller shafts, fuel pumps, air conditioning systems, etc.
- CavitationCavitationCavitation is the formation and then immediate implosion of cavities in a liquidi.e. small liquid-free zones that are the consequence of forces acting upon the liquid...
noise: noise generated by the creation of gas bubbles by the turning of a ship's propellers. - Hydrodynamic 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
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, the is a nuclear aircraft carrier of the Nimitz class....
es will have different combinations of acoustic signals that together form a unique signature.
Targeting
HydrophoneHydrophone
A hydrophone 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 electricity when subjected to a pressure change...
s and Sonar
Sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...
operating in passive mode can detect acoustic signals radiated by otherwise invisible submarines, and use these signals to target attacks.
Modern naval mine
Naval mine
A naval mine is a self-contained explosive device placed in water to 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 enemy vessel...
s and torpedo
Torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...
es such as the CAPTOR mine
CAPTOR mine
The CAPTOR is the United States Navy's primary anti-submarine naval mine. This deep-water mine is laid by ship, aircraft or submarine, and is anchored to the ocean floor. When its sonar detects a hostile submarine, the CAPTOR launches a Mark 46 torpedo.The name CAPTOR is short for enCAPsulated...
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.
Countermeasures
Warship designers aim to reduce the acoustic signature of ships and submarines just as much as they aim to reduce the radar cross sectionRadar cross section
Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...
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 propellerPropellerA propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...
s to reduce cavitation, this led to the development of large slow turning propellers, today there is a preference now for pump-jetPump-jetA pump-jet, hydrojet, or water jet, is a marine system that creates a jet of water for propulsion. The mechanical arrangement may be a ducted propeller with nozzle, or a centrifugal pump and nozzle...
propulsorPropulsorA 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 and Pump-jet are examples....
s over propellers. - the fitting of anechoic tileAnechoic tileAnechoic 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...
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
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...
toyed with the idea of the trimaran
Trimaran
A trimaran is a multihulled boat consisting of a main hull and two smaller outrigger hulls , attached to the main hull with lateral struts...
hulled Future Surface Combatant
Future Surface Combatant
The Global Combat Ship is a ship design and construction programme currently underway by the United Kingdom's Ministry of Defence to replace the Royal Navy's 13 Type 23 frigates. The programme has been in development since 1998, initially under the designation "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 propulsor
Propulsor
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 and Pump-jet are examples....
s powered by a diesel-electric
Diesel-electric
Diesel-electric transmission or diesel-electric powertrain is used by a number of vehicle and ship types for providing locomotion.A diesel-electric transmission system includes a diesel engine connected to an electrical generator, creating electricity that powers electric traction motors...
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
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...
to test the principle of a large scale trimaran design.
See also
- Underwater acousticsUnderwater acousticsUnderwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water and its boundaries. The water may be in the ocean, a lake or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and...
- Stealth shipStealth shipA stealth ship is a ship which employs stealth technology construction techniques in an effort to ensure that it is harder to detect by one or more of radar, visual, sonar, and infrared methods...
- Type 45 destroyerType 45 destroyerThe United Kingdom's Type 45 destroyer is an air defence destroyer programme of the Royal Navy which will replace its Type 42 destroyers. The first ship in the class, HMS Daring, was launched on 1 February 2006 and commissioned on 23 July 2009. The ships are now built by BAE Systems Surface Ships...
- Anti-submarine warfareAnti-submarine warfareAnti-submarine warfare is a branch of naval warfare that uses surface warships, aircraft, or other submarines to find, track and deter, damage or destroy enemy submarines....
- Submarine warfareSubmarine warfareNaval warfare is divided into three operational areas: surface warfare, air warfare and underwater warfare. The latter may be subdivided into submarine warfare and anti-submarine warfare as well as mine warfare and mine countermeasures...
- Upholder/Victoria class submarineUpholder/Victoria class submarineThe Upholder/Victoria-class submarines, also known as the Type 2400 , are diesel-electric Fleet submarines designed in the UK in the late 1970s to supplement the Royal Navy's nuclear submarine force....
- Teardrop hullTeardrop hullA teardrop hull is a submarine hull design which emphasizes hydrodynamic flow above all other factors. Benefits over previous types include increased underwater speed and a smaller acoustic signature, making detection by sonar more difficult...
- SpectrogramSpectrogramA spectrogram is a time-varying spectral representation that shows how the spectral density of a signal varies with time. Also known as spectral waterfalls, sonograms, voiceprints, or voicegrams, spectrograms are used to identify phonetic sounds, to analyse the cries of animals; they were also...