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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 by passing ships.
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 of 1940. These mines could be set to broad spectrum or narrow spwectrum sound signature. The broad would be triggered by the sound of any passing ships propeller; the narrow would be set top only explode when the propellor of a large ship was detected. 
In naval warfare, an acoustic mine is defined as one being equipped with a hydrophone that functions as a 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. This initiates the closing of a switch causing an electrical circuit to then complete and for the mine to detonate.
- Moffatt, Iain (1st July 2005). "MINES AND MINESWEEPING TECHNIQUES OF WW2" (PDF). Edinburgh Model Boat Club. Check date values in:
Levie, Howard S. Mine Warfare at Sea. Norwell, MA. Kluwer Academic Publishers, 1992.
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