Acoustic torpedo

Lore & Background

The first passive acoustic torpedoes were developed nearly simultaneously by the United States Navy and Germany during World War II. Germany deployed the G7e/T4 Falke in March 1943 from U-boats U-603 and U-758, but it was quickly replaced by the G7es T5 Zaunkönig in August 1943. The T5 saw widespread use starting in September 1943 against North Atlantic escort vessels and merchant ships in convoys. On the Allied side, the US Navy developed the Mark 24 mine, an aircraft-launched anti-submarine passive acoustic homing torpedo. First production units were delivered in March 1943, and the type scored its first verified combat kills in May 1943. About 204 Mark 24s were launched against submarine targets, sinking 37 Axis submarines and damaging 18 more.

Reader's Guide

The acoustic torpedo proved to be an effective weapon against both surface ships and submarines. Its introduction in the Battle of the Atlantic gave German U-boats a 'fire and forget' weapon capable of homing in on merchant ships and attacking escorts at close range. This temporarily reversed Allied gains in mid-1943, as convoys faced renewed threats. The Allies countered with towed noisemakers such as the British Foxer and US FXR, which decoyed torpedoes away from ships. Of about 700 G7es torpedoes fired, only around 77 hit. The capture of German T5 torpedoes from U-505 (June 1944) and U-250 (September 1944) allowed Allied and Soviet forces to study the technology. Today, acoustic torpedoes are mostly used against submarines, with modern systems employing computer-controlled beam patterns and both passive and active sonar.

Did You Know?

• The German G7e/T4 Falke was first deployed in March 1943 by U-boats U-603 and U-758.
• The US Mark 24 'mine' was actually an aircraft-launched acoustic homing torpedo that sank 37 Axis submarines.
• The Foxer decoy was a British-built towed noisemaker that confused German acoustic torpedoes but also rendered the ship's own ASDIC ineffective.
• In September 1944, the Soviet Navy recovered T5 torpedoes from the sunken German submarine U-250 in shallow waters.

The Wartime Race to Hear the Hunt

During the Second World War, two naval powers independently raced to give their torpedoes the ability to listen for their prey. The United States Navy and Germany both produced the first passive acoustic homing weapons in close succession. On the German side, the G7e/T4 Falke entered service aboard U-603, U-758, and U-221 in March 1943, yet only a handful were actually fired before the type was retired in favor of its successor, the G7es T5 Zaunkönig, by August of that year. The T5 then saw broad deployment in September 1943, striking North Atlantic escort vessels and merchant convoys. Across the Atlantic, the U.S. Navy fielded the Mark 24, an aircraft-launched anti-submarine weapon designed to home on submerged targets. Production units reached the fleet in March 1943, and the first confirmed kills arrived in May. Over the course of the war, roughly 204 Mark 24s were launched at submarine targets, resulting in the sinking of 37 Axis boats and the damaging of another 18.

Listening in the Deep: How Acoustic Homing Works

At the heart of every acoustic torpedo sits an array of transducers mounted on the weapon's nose. By applying phase delays to the signals received by these sensors, the torpedo constructs a series of directional acoustic beams—essentially zones of varying sensitivity to incoming sound depending on the angle from which noise arrives. In early designs these beam patterns were fixed, but modern weapons allow an onboard computer to reshape them dynamically. The torpedo can operate in two modes: passive sonar, which simply listens for the target's own engine rumble, machinery vibration, or propeller cavitation, and active sonar, which emits pulses and listens for the echo bouncing off the hull. The engagement sequence typically begins with passive search; once a likely signature is detected, the weapon switches to active tracking. At that point the target may attempt evasive maneuvers or deploy a noisemaker decoy, and the torpedo's logic circuitry must distinguish the true acoustic signature from the false one. Before launch, a fire-control system on the firing platform boxes in the target and transmits an initial depth-search window to the torpedo's microprocessor.

Turning the Tide in the Atlantic

The arrival of acoustic homing torpedoes reshaped the Battle of the Atlantic almost overnight. German U-boats suddenly possessed a fire-and-forget weapon capable of closing to just three or four hundred yards and striking merchant ships or their escorts with lethal precision. By the summer of 1943, however, a wave of Allied countermeasures—Coastal Command raids in the Bay of Biscay, merchant aircraft carriers in convoys, hedgehog projectiles, upgraded radar, and dedicated hunter-killer groups—had pushed the U-boat campaign into severe decline. Between June and August of that year, merchant losses dropped to near insignificance while U-boat destructions climbed sharply, forcing submarines to largely abandon the Bay of Biscay. The acoustic torpedo briefly reversed the momentum again when the T5 struck Convoys ONS 18 and ON 202 in September 1943. On the Allied side, the Mark 24 proved equally decisive: a Catalina flying boat sank either U-657 or U-640 on 14 May 1943, and on the night of 24 June 1944 two Grumman Avengers from the escort carrier USS Bogue sank the Japanese submarine I-52 after it dove from a depth-charge attack. The engagement was captured on audio tape preserved in the U.S. National Archives.

Decoys, Maneuvers, and the Counter-Revolution

The Allies responded to the acoustic torpedo threat with a combination of acoustic decoys and tactical maneuvers. The British-built Foxer, and its American counterpart the FXR, consisted of one or two noise-making devices towed several hundred metres behind a ship. These units mechanically generated a cavitation roar far louder than the vessel's own propellers, luring the torpedo into a circling pattern around the decoy until it exhausted its fuel. The FXR was deployed on all transatlantic escort vessels at the end of September 1943 and was later superseded by the more effective Fanfare. The trade-off was significant: while the noisemaker blinded the enemy torpedo, it also rendered the ship's own ASDIC sonar useless and could mask the presence of another U-boat lurking nearby. Despite this drawback, the decoys proved remarkably successful—of roughly 700 G7es torpedoes fired by the Germans, only about 77 found their mark. British analysts also devised a tactical maneuver called Step-Aside, in which a warship would bait a U-boat into launching its torpedo prematurely, then execute a hard turn to slip outside the weapon's detection arc before bearing down on the submarine.