Sonar decoy


A sonar decoy is a device for decoying sonar. One may be released from a submarine or a surface vessel. A decoy acts as false targets for human operators or sonar-homing weapons such as acoustic torpedoes. Many count as a type of torpedo defence.

Submarine decoys

The first submarine decoys were the German Bold fitted to U-boats of World War II. These were a pellet of calcium hydride in a simple metal container. On contact with sea water, the calcium hydride decomposed to produce a trail of hydrogen gas bubbles that acted as a bubble curtain and reflected ASDIC impulses to produce a false target. The container trapped hydrogen and floated, with a crude spring valve to maintain buoyancy to keep it at a constant depth.
Later decoys, such as Sieglinde, were motorised and could deploy their false target away from the host submarine, increasing safety.

Surface ship decoys

Decoys were also used by surface ships to decoy the developing acoustic torpedoes. These were usually towed behind the host.

Example decoys

Bubble decoys

Reflective bubble targets that produce an active return

Sonar jammers

  • Siegmund - German "hammer and explosive decoy", intended to swamp the listening device with noise
  • Acoustic Device Countermeasure

Signature decoys

Combination