The first passive acoustic torpedo was the G7es T-5 Zaunkönig torpedo deployed in late World War II by the German U-Boat fleet. This weapon was developed to attack escort vessels and merchant ships in convoys. In July 1944, Russian commando frogmen discovered T-5 torpedoes aboard a sunken German submarine, U-250. Torpedoes were safely delivered to surface ships. Key components of the G7es T-5 Zaunkönig torpedo were later ordered by Joseph Stalin to by given to British naval specialists. The capture of U-505 marked the second time that Allied forces gained access to this technology.
Since its introduction, the acoustic torpedo has proven to be a effective against surface ships as well as serving as an anti-submarine weapon. Today, acoustic torpedoes are mostly used against submarines.
Before a torpedo is launched, the target must be 'boxed in'. A fire control system on the firing platform will set an initial search depth range which is passed to the weapon's microprocessor. The search parameters cover the expected depth of the target.
In general, acoustic torpedoes are equipped with a mid-frequency acoustic sensor designed either to detect the sound of churning water from a moving vessel, or actively to locate targets using SONAR. Acoustic torpedoes can be compared to modern fire-and-forget guided missiles. What this means is the enemy (most likely a submarine) will be detected by sonar in any direction it goes. The torpedo will start with passive sonar, simply trying to detect the submarine. Once the torpedo's passive sonar has detected something, it will switch over to an active sonar and will begin to track the target. At this point, the submarine has probably started evasive maneuvers and may have even deployed a noisemaker. The torpedo's logic circuitry, if not fooled by the noise maker, will home in on the noise signature of the target submarine.