A "Hydrophone" is a device which will listen to, or pick up, the acoustic energy underwater. A hydrophone converts acoustic energy into electrical energy and is used in passive underwater systems to listen only. Hydrophones are usually used below their resonance frequency over a much wider frequency band where they provide uniform output levels.

Underwater echo detection systems were developed for the purpose of underwater navigation by submarines in World war I and in particular after the Titanic sank in 1912. Alexander Belm in Vienna described an underwater echo-sounding device in the same year. The first patent for an underwater echo ranging sonar was filed at the British Patent Office by English metereologist Lewis Richardson, one month after the sinking of the Titanic. The first working sonar system was designed and built in the United States by Canadian Reginald Fessenden in 1914. The Fessenden sonar was an electromagnetic moving-coil oscillator that emitted a low-frequency noise and then switched to a receiver to listen for echoes. It was able to detect an iceberg underwater from 2 miles away, although it could not precisely determine its direction. The turn of the century also saw the invention of the Diode and the Triode, allowing powerful electronic amplifications necessary for developments in ultrasonic instruments. Powerful high frequency ultrasonic echo-sounding device was developed by emminent French physicist Paul Langévin and Russian scientist Constantin Chilowsky. They called their device the 'hydrophone'. The transducer that Langevin used in the earlier days was a mosaic of thin quartz crystals glued between two steel plates (the composite having a resonant frequency of about 150 KHz), mounted in a housing suitable for submersion. In the underwater community the word transducer means a device that has the capability of both transmitting and receiving sound. A projector is a device that transmits sound underwater. Projectors are used in active systems. In active systems, after the sound has been generated, the sound waves travel to a target and return as echoes to be detected. Projectors are usually used near their resonance frequencies where they provide the highest acoustic output.

The first hydrophones, invented during World War I by British, American and French scientists, were used to locate submarines and icebergs. These were passive listening devices. The committee was named the ASDICS (for Anti-Submarine Detection Investigation Committee).

R.W. Boyle In 1912 returned to Canada from the University of Manchester and took up the position of the first head of the department of physics at the University of Alberta where he began his ground-breaking research on ultrasonics. With the outbreak of World War I, Boyle joined the staff of Britain’s Board of Invention and Research, working for the Royal Navy at Parkeston Quay. In 1916 he was placed in charge of top secret research on ASDICS. By the end of 1918, Boyle’s research team was obtaining ranges of 1400 yards with good bearing and the Royal Navy was ready to try out this new technology – which would later prove invaluable against German U-Boats. This breakthrough was achieved without the benefit of modern electronics. The first known sinking of a submarine detected by hydrophone was the German U-Boat UC-3, in the Atlantic during World War I on April 23,1916.

Richardson, M.L.F. (1912) Apparatus of warning a ship at sea of its nearness to large objects wholly or partly under water. Br. Pat. No. 1125.
Richardson, M.L.F. (1912) Apparatus for warning a ship of its approach to large objects in a fog. Br. Pat. No. 9423.
Boyle. R.W. and Rawlinson, W.F. (1928). Theoretical notes on the passage of sound through contiguous media. Trans. Roy. Can. 22:III:55-68.
Boyle, R.W. and Taylor, G.B. (1925). The small effect of high frequency on the velocity of longitudinal waves in liquids. Trans. Roy. Soc. Can. 19:III:197.
Boyle, R.W. and Reid, C.D. Practical experiments on the detection of icebergs and on sounding by means of an ultrasonic beam. Trans. Roy. Soc. Can. 20:III:233-244.

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