Hydrophones were the first acoustic antisubmarine sensor system. In essence, they were instruments to permit an operator to hear, as clearly as possible, sounds of engines and machinery being transmitted through the water.
They existed first in static land-based installations near harbours – non-directional installations were being deployed in Auxiliary Patrol bases in late 1916. In March 1917, the S.N.Os. of the A.P. areas were asked to appoint an officer in charge of the hydrophone from those trained at Hawkcraig Experimental Station. In April, directional equipment started to arrive and in May, gramophone records of various engine and propeller noises were distributed for training.
Other land-based installations coordinated the use of electrically-triggered mine defences. As their value in antisubmarine warfare became clear, efforts to create versions that could be used from a moving ship platform and which afforded the user a sense of in which direction the undersea quarry was to be found commenced. Progress in this field was modest during the war.
- Admiralty, Technical History Section (1919). The Technical History and Index: The Anti-Submarine Division of the Naval Staff. Vol. 1, Part 7. C.B. 1515 (7) now O.U. 6171/7. At The National Archives. ADM 275/19.
- a section in Annual Report of the Torpedo School for 1924 on 295 on hydrophones in submarines.
- The Technical History and Index, Vol. 1, Part 7. p. 11.