The New Encyclopaedia Britannica in 30 Volumes - Macropaedia 17 [antikvár]

Sonar Sonar is the name of a technique for detecting the presence of objects underwater by acoustical echo. Having been developed during World War I, it antedates the better known radar, which uses electromagnetic echo. "Active sonar" employs an apparatus for radiating acoustical energy to bounce off underwater objects; "passive sonar" consists merely of the passive reception of acoustical energy generated by another source. DEVELOPMENT OF THE TECHNIQUE Interest in a means of detecting underwater objects was originally aroused by the problem of icebergs, dramatized by the sinking of the "Titanic" in 1912. The first proposal was put forward by a British meteorologist, L.F. Richardson, and the first successful application used in iceberg detection was made by the American radio pioneer, RA. Fessenden. Development was stimulated by the outbreak of World War I and the impact of submarine warfare. A French physicist, Paul Langevin, played the leading role in research in which first British and then American scientists joined. A passive system of submarine detection, operational by 1916, employed a hydrophone (underwater microphone) and amplifier to pick up the noise emitted by submarine engines. By 1918 scientists had developed an active system in which a pulse of sound was transmitted and its rebounding echo used to detect a submarine even when its engines were shut down. The original term, "asdics," is said to have been derived from 'Anti-Submarine Division-ics,' although other explanations have been given. The name was long retained in the United Kingdom; the term sonar, from sound navigation and ranging, a United States acronym from World War II, is now used widely. In the years between the wars, British and United States researchers refined techniques to such a point that the Allies enjoyed a substantial advantage over Germany in World War II underwater detection. They developed two types of beams, one vertical to bounce off the sea floor for depth determination, the other horizontal and capable of detecting underwater objects and obstacles near the surface. The depth detector, or echo sounder, was in widespread navigational use in the 1930s. A major American wartime contribution was the development of a system that could rapidly scan with a narrow beam either a sector or all around, without mechanical motion of the acoustic transmitters or receivers, making possible swift and methodical search for submarines. In the years since 1945 a great deal of work has been done in naval acoustics by the United States, the United Kingdom, and the Soviet Union, and other maritime nations, but virtually all of it is secret and so has had little influence on peaceful applications. The most important of these is the use of echo sounders to detect shoals of fish, a potential discovered in the 1930s. By 1950 specialized equipment was being installed on fishing vessels, with notable success. In the early 1970s the simple echo sounder remained the basic sonar device used by fishing fleets. A type of sonar has been developed with a horizontal scan that locates fish at a distance of one kilometre (about 1,100 yards), greatly facilitating purse-seining (large net) operations by trawlers. The beam's direction can be changed by mechanical rotation of the underwater transmitting and receiving equipment, permitting a thorough search of a fair-sized area. The technique is slow, owing to the slow rate at which sound travels through water (1.5 kilometres per second). Another problem in fish sonar is that there is as yet no technique for detecting fish close to the seabed in front of the ship, as the bottom-trawling method of fishing used by the British and other fishermen requires. There are, however, systems that accurately indicate the depth of a net towed behind the trawler, so that adjustments can be made to take advantage of any fish shoals detected on the main sonar. Known military applications in the 1970s include the detection and location of submarines, control of antisubmarine weapons, sonar-equipped homing torpedoes, and mine hunting. Detection ranges for civil and military sonar systems vary from 100 metres to 10 kilometres. Wave lengths for the acoustic signal range between 0.5 centimetres and 30 centimetres, corresponding to frequencies of approximately 300 kilohertz and 5 kilohertz. basic physical principles Principles of a simple sonar system. As with most radar systems, sonar systems generally use the transmission and reflection of a pulse of energy as their basis of operation. The arrangement in Figure 1 is typical. IndividFrom B.K. Gazey and D.G. Tucker, Applied Underwater Acoustics (1966); Pergamon Press; reprinted by permission Figure 1: Schematic arrangement of a typical pulsed sonar system. ual systems vary, with some, for example, using a single transducer both for generating the acoustic waves in the water and for detecting the reflected waves. The pulses they send out are always bursts of the transmitted, or carrier, frequency, although the term sonar may also include marine seismic work, using the sound pulses generated by a small explosion. Display. The distance from the transducers to a reflecting target is indicated by the time elapsing between transmission of the pulse and the reception of an echo. The display, usually a pattern on a moving strip of paper, always includes a time base, the traverse of which is initiated by the transmitted pulse. For simple systems, the commonest display is a chemical recorder which records the received information on sensitized paper by means of a recording stylus drawn across paper. Since the echo actuates the marking mechanism, the position of the mark across the traverse indicates the range of the target. The