Acoustic signature

For the article regarding audio files, seeAcoustic fingerprint.For signature reduction in general, seeStealth technology.

The termacoustic signatureis 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

Main articles:Acoustic homingandAcoustic torpedo

The analysis of acoustic signatures is an important adjunct topassive sonarused to track naval warships and weapons. Similar methods have been used to identify aircraft, especially before the development of sophisticatedradartracking.

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.
• Cavitationnoise: noise generated by the creation of gas bubbles by the turning of a ship'spropellers.
• Hydrodynamicnoise: 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, differentship classeswill have different combinations of acoustic signals that together form a unique signature.

Targeting

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

Modernnaval minesandtorpedoessuch as theCAPTOR minecan be programmed to distinguish the acoustic signatures of different vessels, leaving friendly vessels unmolested and attackinghigh-value targetswhen faced with multiple possible targets, e.g. distinguishing anaircraft carrierfrom its escorts.

Countermeasures and acoustic quieting

Main article:Acoustic quieting

Warship designers aim to reduce the acoustic signature of ships and submarines just as much as they aim to reduce theradar cross sectionsand 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 possiblemechanical tolerancesand designed to produce a minimum of noise.
• decoupling the machinery from the hull by mounting machinery on rubber mounting blocks.
• designingpropellersto reduce cavitation, this led to the development of large slow turning propellers, today there is a preference now forpump-jetpropulsorsover propellers.
• the fitting ofanechoic tilesto 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

TheRV Triton

For a time Britain'sRoyal Navytoyed with the idea of thetrimaranhulledFuture 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 usingpropulsorspowered by adiesel-electricpower plant, with the diesels being placed in thesuperstructureto mechanically isolate them from the water. This project got as far as the construction of the research shipRV Tritonto test the principle of a large-scale trimaran design.

See also

• Sonar
• Underwater acoustics
• Stealth ship
• Type 45 destroyer
• Anti-submarine warfare
• Submarine warfare
• Type 212 submarine
• Teardrop hull
• Spectrogram