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An acoustic mine is a type of sea mine which responds to pressure changes and acoustic information, detonating when a ship of a certain size passes it. Acoustic mines have historically been laid in contentious sea lanes during times of warfare; modern mines are actually quite sophisticated, and many are capable of custom programming so that they will not, for example, pose a threat to friendly traffic. Acoustic mines are among a large family of tools used to gain and hold naval territory during periods of warfare. Because an acoustic mine is a military tool, ownership of such devices is typically restricted to the military or research organizations.
Early acoustic mines, such as those used in the Second World War, simply used directional pickups which passively listened for the sounds of passing ships and submarines. Modern acoustic mines typically are capable of both passive and active modes, much like the sonar systems on ships. Passive listening modes in an acoustic mine can be sensitized to the sound of specific engines or other characteristic acoustic signatures, while active modes can send out acoustic pulses to seek out and identify targets.
In addition to responding to acoustic information, many acoustic mines are also programmed to respond to changes in pressure. As ships and submarines travel, they displace large amounts of water, causing changes in pressure which can be read by sensitized monitors in an acoustic mine. Many acoustic mines are capable of differentiating between large and small ships, allowing them to target larger military ships without damaging fishing boats and other smaller craft.
There are several ways to deploy an acoustic mine. Classically, militaries have used free-floating mines, which can cause problems when they drift outside the sea lanes they are deployed in. Others tether their mines with anchors so that they remain in a particular location and at a specific depth. It is also possible to scatter sea mines along the ocean floor; this technique is often used to exploit submarines, which tend to hug the ocean floor in enemy territory.
Along with other types of mines, acoustic mines have been a topic of some contention. Globally, mining causes thousands of injuries annually, because mines are unable to differentiate between innocent and potentially threatening traffic. Anti-mining organizations have tried to convince global militaries to abandon the practice of mining, or to use mines which are easier to disable; for example, some acoustic mines can be equipped with computerized systems which respond to a coded pulse which can be used to disable the mines.