acoustic weapons

In World War II, underwater weapons fitted with a sensor which would detect the sound of a ship's propellers. The sensor would steer a torpedo towards the ship or, if fitted to a mine, detonate it when it was close enough to the ship's hull.

Acoustic torpedoes were introduced by the German navy in 1943 but were never very successful: some even homed in on the submarine that launched them. The Allies sought to counter such weapons by towing a device behind ships which made more noise than the ship's machinery, and so either attracted the torpedo to itself rather than to the ship or else triggered the mine at a nonlethal distance.