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.
The mine may freely drift, be moored at a certain location and depth, or lie at the bottom of the ocean floor.
The first acoustic mines were deployed by the German navy in the Thames Estuary in October 1940. These mines could be set to broad spectrum or narrow spectrum sound signature. The broad setting would be triggered by the sound of any passing ship's propeller; the narrow would be set to only explode when the propeller of a large ship was detected. [1]
The US Navy deployed acoustic mines as an anti-submarine device from 1948. [2]
In naval warfare, an acoustic mine is defined as one being equipped with a hydrophone that functions as a listening device. This hydrophone listens for particular noises made by any ship or vessel's machinery (propellers). It can also detects structural sounds from the hull of a passing ship. An acoustic mine is mainly operated by a sensitive diaphragm located inside the hydrophone. This diaphragm is actuated when a certain noise reaches a predetermined intensity and duration. This initiates the closing of a switch causing an electrical circuit to flow and the mine to detonate.
When first encountering this type of mine in the 1940s the British Royal Navy responded with a series of devices that produced loud noises underwater to trigger the mines . Experiments were conducted with jackhammers, sirens, handgrenades, and bundles of loose metal pipes. They eventually settled on the jackhammer (known as breakers in the UK). This was initially set up to hammer against a bulkhead of a minesweeper but this resulted in the mine exploding dangerously close to the vessel. The hammer was then located in a metal box that was extended over the bows of the ship so that its sound preceded the ship itself. Towing the hammer in a floating vessel off to the side of the ship was also used. An off the shelf electrically powered breaker called a Kango was used and the approach became known as the "Kango sweep". [1]
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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, any vessel. Naval mines can be used offensively, to hamper enemy shipping movements or lock vessels into a harbour; or defensively, to protect friendly vessels and create "safe" zones. Mines allow the minelaying force commander to concentrate warships or defensive assets in mine-free areas giving the adversary three choices: undertake an expensive and time-consuming minesweeping effort, accept the casualties of challenging the minefield, or use the unmined waters where the greatest concentration of enemy firepower will be encountered.
Sonar is a technique that uses sound propagation to navigate, communicate with or detect objects on or under the surface of the water, such as other vessels. Two types of technology share the name "sonar": passive sonar is essentially listening for the sound made by vessels; active sonar is emitting pulses of sounds and listening for echoes. Sonar may be used as a means of acoustic location and of measurement of the echo characteristics of "targets" in the water. Acoustic location in air was used before the introduction of radar. Sonar may also be used for robot navigation, and SODAR is used for atmospheric investigations. The term sonar is also used for the equipment used to generate and receive the sound. The acoustic frequencies used in sonar systems vary from very low (infrasonic) to extremely high (ultrasonic). The study of underwater sound is known as underwater acoustics or hydroacoustics.
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 an electric potential when subjected to a pressure change, such as a sound wave. Some piezoelectric transducers can also serve as a sound projector, but not all have this capability, and some may be destroyed if used in such a manner.
A proximity fuze is a fuze that detonates an explosive device automatically when the distance to the target becomes smaller than a predetermined value. Proximity fuzes are designed for targets such as planes, missiles, ships at sea, and ground forces. They provide a more sophisticated trigger mechanism than the common contact fuze or timed fuze. It is estimated that it increases the lethality by 5 to 10 times, compared to these other fuzes.
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 sonobuoy is a relatively small buoy - typically 13 cm (5 in) diameter and 91 cm (3 ft) long - expendable sonar system that is dropped/ejected from aircraft or ships conducting anti-submarine warfare or underwater acoustic research.
The Aggressive-class minesweepers are a class of US-built minesweepers. They are designated as MSO, distinguishing them from the smaller coastal MSCs and inshore MSIs. Besides the US Navy, this class of vessels has also been used by the Belgian Navy and the Norwegian Navy, among others.
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.
The G7es (T5) "Zaunkönig" ("wren") was a passive acoustic torpedo employed by German U-boats during World War II. It was called the GNAT by the British.
Foxer was the code name for a British built acoustic decoy used to confuse German acoustic homing torpedoes like the G7 torpedo during the Second World War. A US version codenamed FXR was deployed at the end of September 1943 on all transatlantic escort vessels. A Canadian version was also built called the CAAT device. It was replaced in US service by the Fanfare noisemaker.
The Mark 24 mine is an air-dropped anti-submarine warfare weapon (ASW) incorporating passive acoustic homing system and torpedo integration. It was used by the United States, the British and Canadian forces during the Second World War and entered service in March 1943 and remained in use with the US Navy until 1948. Approximately 4,000 torpedoes were produced, with 340 ultimately being deployed during the war. 204 torpedoes were launched against submarine targets, with 37 Axis submarines being sunk and a further 18 damaged. The deceptive name of "Mark 24 Mine" was deliberately chosen for security purposes, to conceal the true nature of the weapon.
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 kilometres 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.
Anti-submarine warfare is a branch of underwater warfare that uses surface warships, aircraft, submarines, or other platforms, to find, track, and deter, damage, and/or destroy enemy submarines. Such operations are typically carried out to protect friendly shipping and coastal facilities from submarine attacks and to overcome blockades.
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.
The AN/UQQ-2 Surveillance Towed Array Sensor System (SURTASS), colloquially referred to as the ship's "Tail", is a towed array sonar system of the United States Navy.
RV Sir Horace Lamb was a Navy owned former mine warfare vessel assigned to the Columbia University, Geophysical Field Station research facility in Bermuda for acoustic research operating from 1959 to 1976. The ship was the former USS Redpoll (AMS-57/YMS-294), a YMS-1-class minesweeper of the YMS-135 subclass built and commissioned as YMS-294 in 1943.
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.
The University of California Division of War Research (UCDWR) was created at Scripps Institution of Oceanography at the University of California, San Diego campus during World War II to aid the U.S. Navy in war-related research.
Radio acoustic ranging, occasionally written as "radio-acoustic ranging" and sometimes abbreviated RAR, was a method for determining a ship's precise location at sea by detonating an explosive charge underwater near the ship, detecting the arrival of the underwater sound waves at remote locations, and radioing the time of arrival of the sound waves at the remote stations to the ship, allowing the ship's crew to use true range multilateration to determine the ship's position. Developed by the United States Coast and Geodetic Survey in 1923 and 1924 for use in accurately fixing the position of survey ships during hydrographic survey operations, it was the first navigation technique in human history other than dead reckoning that did not require visual observation of a landmark, marker, light, or celestial body, and the first non-visual means to provide precise positions. First employed operationally in 1924, radio acoustic ranging remained in use until 1944, when new radio navigation techniques developed during World War II rendered it obsolete.
Two closely related terms, Low Frequency Analyzer and Recorder and Low Frequency Analysis and Recording bearing the acronym LOFAR, deal with the equipment and process respectively for presenting a visual spectrum representation of low frequency sounds in a time–frequency analysis. The process was originally applied to fixed surveillance passive antisubmarine sonar systems and later to sonobuoy and other systems. Originally the analysis was electromechanical and the display was produced on electrostatic recording paper, a Lofargram, with stronger frequencies presented as lines against background noise. The analysis migrated to digital and both analysis and display were digital after a major system consolidation into centralized processing centers during the 1990s.