Sonar (an acronym for Sound, Navigation and Ranging) is a system for underwater detection and location of objects by acoustical echo. The first Sonars, invented during World War I by British, American and French scientists, were used to located submarines and icebergs and were called ASDICS (for Anti-Submarine Detection Investigation Committee) in Britain. These were passive listening devices. Sonar is an American term dating from World War II. The first Active Sonars were mechanical devices, most of the advances made in sonar technology occurred during the "cold war" and continue to this day.


April 23,1916 First known sinking of a submarine, detected by hydrophone was the German U-Boat, UC-3, in the Atlantic, during WWI.
1934 First echo-ranging equipment installed in the American destroyers of DesDiv 20, one of which was USS Rathburne (DD-113).
1939 West Coast Sound School opened at San Diego Destroyer Base.
1939 Atlantic Fleet Sound School opened at the sub base in New London, Connecticut. First sound class graduated March 1940. Later that year, the school was transferred to Key West, Florida.
Summer 1941 A British invention, the Attack Teacher was installed in Portland, Maine for training destroyer sailors in ASW tactics.
September 1941 USS Kearny torpedoed by U-Boats. Later the USS Rueben James (DD-245) was sunk by torpedo fire, the first destroyer to be lost in WWII.
October 1941 Bathythermograph introduced to the Fleet. Officer Specialists were first trained and assigned to take observations aboard ships.
January 1942 ASW unit established in Boston to collect and evaluate ASW data for recommendations to the Navy.
Dates uncertain The Key West Fleet Sonar School closed all operations leaving the Fleet ASW Training Center in San Diego and the Submarine Sonar School in New London as the last two U.S. Navy Installations where basic sonar training was conducted. Sonar A School graduated it's last class in 1974