An acoustic mine is a type of naval mine which monitors audio activity in its vicinity. Depending on its design, it will either actively send out audio pulses, not unlike a sonar, listening to the speed at which the echo returns to it or passively listen to its environment, depending only on the noise that is made without its interference.
Further variants exist, as this mine may either freely drift, be moored at a certain location and depth, or lie at the bottom of the ocean floor.
In the broad discussion of naval warfare, an acoustic mine is defined as one being equipped with a hydrophone that functions as a main listening device for the mine itself. This hydrophone listens for particular noises made by any ship or vessel's machinery (propellers) and also detects normal clanks from the hull of any ship. An acoustic mine is mainly operated by its diaphragm located inside the hydrophone. This diaphragm is actuated when a certain noise reaches a predetermined intensity and time. Therefore, this initiates the closing of a switch causing an electrical circuit to then complete and for the mine to detonate. This then leads to the question as to whether or not these mines detonate under false pretenses one being normal oceanic activity? For the majority of the time the answer to this question is simply "no" due to one main reason. Unlike pressure mines that detonate under the circumstance of water displacement caused by the hull of a ship, acoustic mines have a set range that is determined before it is laid. This means that it is supposed to receive a signal to detonate only when sounds are within this range (normal range intensity usually set close to the noise produced by a vessel's propellers). However, there is one drawback to the invention of the acoustic mine that disturbs its effectiveness. Its ability to accumulate dirt and other sea organisms is quite intense meaning that at any time it may fail to work or detonate by accident.
Levie, Howard S. Mine Warfare at Sea. Norwell, MA. Kluwer Academic Publishers, 1992.
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