During the Atlantic convoys of WWII supplying vital materials to Britain, Allied intelligence discovered the creation of a new German acoustic torpedo being used by Uboats. The British referred to them as GNATs. (German Naval Acoustic Torpedoes)

Acoustic decoys were derived from minesweeper pipe noisemakers and modified to deal with the new technological threat. The British FOXER, Canadian CAAT and U.S. FXR helped minimize the effectiveness of the GNAT.

Much research and intelligence was compiled trying to find the technical features of the G7es T45 and how to create countermeasures through to the end of the war. This 1946 U.S. Navy declassified report details some of those efforts.

OEG REPORT No. 51
ANTISUBMARINE WARFARE IN WORLD WAR II
Charles M. Sternhell and Alan M. Thorndike

https://www.ibiblio.org/hyperwar/USN/...