The forerunner of the Zaunkönig was the G7e/T4 Falke, codename "Falcon," which was introduced in March 1943, but saw limited use for trial and was rapidly phased out of service in favor of the later model that received a number of significant enhancements.
The acoustic homing torpedo was specifically designed as to be attracted by the pitch of an escort's propellers and would — even if aimed inaccurately — explode under the ship's stern.
[1] There were three variants: The acoustic homing torpedo required a minimum distance of 400 metres (1,300 ft) to lock onto the target after launch.
However, despite some initial success, in particular sinking destroyers and corvettes, the Zaunkönigs effectiveness was quickly nullified by the introduction of a noise making decoy known as Foxer.
Grossadmiral Dönitz, not satisfied with the development of countermeasures, realized that his brief success against the North Atlantic trade convoys was only a flare up and recorded: [it is] finally clear that surface warfare for U-boats had come to an end.