The Biber (German for “beaver”) was a German midget submarine of the Second World War. Armed with two externally mounted 21-inch (53 cm) torpedoes or mines, they were intended to attack coastal shipping. They were the smallest submarines in the Kriegsmarine. The Biber was hastily developed to help meet the threat of an Allied invasion of Europe. This resulted in basic technical flaws that, combined with the inadequate training of their operators, meant they never posed a real threat to Allied shipping, despite 324 submarines being delivered. One of the class’s few successes was the sinking of the cargo ship Alan A. Dale. A number have survived in museums, including one example that has been restored to operational condition. [Wiki]
Acoustic devices had been used for finding and monitoring the position of enemy aircraft and their flight direction since World War I.Although radio detection and ranging (radar) was already massively used, the Luftwaffe and heavy anti-aircraft artillery control units were still using the RRH acoustic devices (which were developed in the 1930s) for early warning against the approaching enemy bomber streams until the end of the Second World War.