Sonar
ALTERATIVE NAME/S: Sound Navigation and Ranging
DETAILS/ DESCRIPTION: Sonar is an active detection system in which a sound beam is used underwater in much the same way as a searchlight or radar beam in air - the beam is trained on a target, and a receiver detects any sound energy reflected back by this target (the beam is transmitted in short pulses, and the range to a target is calculated by multiplying the velocity of sound by the time interval between transmitted pulse and received echo, and dividing by two in order to account for the round trip of the sound pulse).
During World War I, the havoc to Allied shipping created by the German U-boat campaign had by 1916 inspired Allied use of an early passive system of sonar, the hydrophone - essentially a microphone (often used in towed lines) able to convert sound waves from an underwater source (such as a submarine) into electrical signals.
But as submarine development progressed during World War I the U-boat became quieter, and during the final stages of the war the French and British navies realized that the only effective answer to the submarine problem would be an active detection system based on echo-ranging.
Over a year earlier, in January 1915, a Russian electrical engineer named Constantin Chilowski had approached the French navy with an idea for an ultrasonic echo-ranging device which he believed might be used for the detection of submarines. A month later the project was passed to the famous Sorbonne physicist Langevin*, who (using some of Rutherford’s* earlier work) agreed to experiment with underwater echo-ranging in collaboration with Chilowski.
After many difficulties he developed a mica-dielectric transmitter, with one electrode a circular metal plate, insulated by a thick coating of resin, on which was laid a thin sheet of mica dielectric which was free to vibrate, with a specially designed carbon hydrophone as a receiver. The sea water in contact with it acted as the radiating electrode, the necessary vacuum (between the plate electrode and the mica sheet) was maintained by a rotary mercury air pump, and the high-frequency electric power was generated by a wireless telegraphy arc transmitter.
The first successful underwater signals (which made the familiar "ping") were sent across the Seine at the end of 1915, and the preliminary results led to a joint patent application by Langevin and Chilowski in May 1916. Further improvements were made, including mounting the hydrophone in the focus of a parabolic reflector, and output was increased. With this apparatus, in April 1916, signals were transmitted and received at sea at a range of 2 km and, for first time, echoes were obtained from sea bed and also from an armour plate suspended in water at a distance of 200 metres.
Langevin and Chilowski parted company but Langevin continued his experiments at the naval base at Toulon, and early in 1917 he was able to take up the study of | piezoelectricity* seriously, owing to the recent development of the Radiotélégraphie
Militaire's new R6 eight-valve high-frequency amplifier that could cope with the extremely feeble piezoelectric charges produced by his transducer, without masking the signals by self-noise generated within the amplifier.
Having persuaded an optician named Werlein to cut some large slices from a 10 inch quartz crystal on display in his Paris shop, Langevin was then able to build his first quartz receiver by embedding one of the quartz slices in wax. He then placed a protective sheet of mica on the front face, and fixed a foil electrode to the other side, forming, in own words, "the internal armature of a condenser in which the quartz forms the dielectric, and the salt water the external armature."
By November 1917 the mica-transmitter and quartz receiver were installed on Langevin's yacht, the 'Vigoureux', and gave a signalling distance of up to 6 kilometres, without having to place the receiver in a parabolic sound reflector.
In April 1917 Langevin had also begun developing his first quartz transmitter using a single, large, perfect crystal. Spectacular results were soon achieved, but the voltage required for this projector proved far too high, nor could bulk supplies of quartz of the necessary size and purity be obtained.
Later in 1917, Langevin came up with his famous steel-quartz-steel sandwich construction (a quartz transmitter which became known as the "Langevin Sandwich") a four-millimetre layer of quartz crystal sandwiched between two three-centimetre layers of steel.
When an electric charge of the correct frequency was applied to the quartz, it caused the crystal to change shape and eventually to oscillate at its own resonant frequency, which in turn caused the steel plate to vibrate. When the "sandwich" was placed within the hull of a ship the vibrating steel would send out high-frequency pulses into the surrounding water which, if they hit an object, would bounce back and be picked up by the quartz receiver, which apply the returning vibrations to another quartz plate. The effect of these vibrations would cause the quartz to emit electrical signals that could be processed to show the size and range of the object.
Sea tests of Langevin's quartz transmitter began in February 1918, during which signalling distances of up to 8 km were achieved, and clear echoes were obtained from a submarine 600 metres away for the first time. The British and Americans, in cooperation with the French, began to develop and improve Langevin's work.
Since the end of World War II, sonar has proved to be a highly versatile technology, subsequent developments including the echo sounder, or depth detector, rapid-scanning sonar, side-scan sonar, and WPESS (within-pulse electronic-sector-scanning) sonar.
The uses of sonar are now many: in the military field are a large number of systems that detect, identify, and locate submarines, while sonar is also used in acoustic homing torpedoes, in acoustic mines, and in mine detection; non-military uses of sonar include fish finding, sonic depth sounding (using the fathometer, invented by Fessenden* in 1921), mapping of the sea bottom, Doppler navigation, and acoustic locating for divers.
ONELINER: Sonar
OTHER CONNECTIONS: None
BIBLIOGRAPHY:
EB, Collier's Encyclopedia
Hackmann, Willem,Seek and Strike: Sonar, anti-submarine warfare and the Royal Navy, 1914-54, London: HMSO, 1984
Hezlet, Sir Arthur,The Electron and Sea Power,London: Peter Davies, 1975.