sonar(redirected from Fessenden transducer)
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sonar(sō`när), device used underwater for locating submerged objects and for submarine communication by means of sound waves. The term sonar is an acronym for sound navigation ranging. The main component of sonar equipment is an electroacoustic transducer that is in direct contact with the water. It is suspended from the hull of a ship or on a cable from a low-flying helicopter. The transducer converts electric energy into acoustic energy (thus acting as a projector), much as does a loudspeaker, and converts acoustic energy into electric energy (serving as a hydrophone), as does a microphone. A pulse of electric energy vibrates the diaphragm of the projector, sending sound waves through the water. These waves are concentrated into a sound beam, which scans the water when the projector is rotated. After the sound wave is emitted, the projector is converted into a hydrophone and listens for an echo. The cycle is repeated periodically. A returning echo is converted into an electric current by the transducer and may be interpreted (for range, bearing, and the nature of the target) aurally through a loudspeaker or visually represented on a display screen, as is done with radar signals. The various types of sonar in use can be put into three classes: direct listening, communications, and echo ranging. In direct listening, also known as passive sonar, the object under observation generates the sounds that are received. In communications and echo ranging the sonar must generate its own signals. Sonar operates in the 10- to 50-kilocycle acoustical frequency range. It is used for communication between submerged submarines or between a submarine and a surface vessel, for locating mines and underwater hazards to navigation, and also as a fathometer, or depth finder. Sonar is widely used by commercial fishermen for locating shoals of fish. Research has indicated that sonar used for echo ranging can affect some dolphins and porpoises and especially beaked whales. In some instances it may startle them and cause them to surface too rapidly, producing a disorder similar to decompression sicknessdecompression sickness,
physiological disorder caused by a rapid decrease in atmospheric pressure, resulting in the release of nitrogen bubbles into the body tissues. It is also known as caisson disease, altitude sickness, and the bends.
..... Click the link for more information. (in which nitrogen bubbles form in body tissues); this may be linked to strandings of those species.
See J. W. Horton, Fundamentals of Sonar (1957); D. G. Tucker, Underwater Observation Using Sonar (1966).
a hydroacoustic system (or instrument) for determining the location of underwater objects with the aid of sound signals. In addition to the distance to the submerged object, some sonar systems also determine the depth of its submergence by the projection of distance and angle of direction to the object in a vertical plane. For the methods of determining the location of an object and the uses of sonar, See SONAR TECHNIQUES.
Sonar operates in the following manner. A pulse of electric voltage produced by a generator is fed through a receive-transmit switch to electroacoustic transducers (oscillators), which produce an acoustic pulse of 10 to 100 msec length into the water at a certain solid angle or in all directions. When emission ends, the oscillators are switched into a heterodyne amplifier to receive and amplify the acoustic pulse signals reflected from objects. Then the signals come into indicator instruments: a recorder, an electrodynamic loudspeaker, telephones, or a cathode-ray tube (CRT). The recorder measures and electrochemically records the distance to the object on tape. With the aid of the telephones and the electrodynamic loudspeaker, the received signals are heard at a frequency of sound and are classified, and the direction is determined from the maximum sound. On the CRT the signal from the object is illuminated and the distance to it and the direction (bearing) are measured. The pause between adjacent emissions of pulses measures several seconds.
Depending on the technique of searching for an object, there are step-by-step sonar, sector searching sonar, and scanning sonar. In the step-by-step searching and direction finding, the acoustic system is rotated, according to the maximum signal, in a horizontal plane over an angle of 2.5°-15°, followed by a pause equal to the time it takes the pulse to travel from the sonar to an object located at the maximum possible distance and from the object to the sonar. Then the next rotation is made. In direction finding by the phase method, the acoustic system is constructed in the form of two separate systems that are changed by a contactless switch-gear from an emitting regime to a receiving regime and back. The signal sums and differences, taken from a two-channel compensator, are fed, after being amplified and phased, to the CRT and recorder, where the distance is read off. This method is characterized by a comparatively high precision of direction finding, the great amount of time (several minutes) it takes for surveying the underwater area, and the ability to track only one object. Under sector searching, acoustic energy is emitted simultaneously in all elements of a certain sector, and the receiving and direction finding of reflected signals are carried out during rapid scanning of the directivity index within this sector. Under the scanning method, which is the most widespread, there is nondirectional (circular) emission and directional (within the limits of a narrowly rotating directivity diagram) reception, which assures the detection and direction finding of all objects surrounding the sonar system. The acoustic system (antenna) is constructed in the form of a cylinder or sphere consisting of a large number of individual oscillators and is located in a vertically movable apparatus or in a stationary radome. The advantages of this method include rapid scanning of the entire horizon and the ability to detect and track several objects.
Most sonar systems operate in the sonic and ultrasonic frequency ranges (4-40 kHz). This is based on the necessity of obtaining a sharp directivity for the antenna (despite its relatively small size) and of achieving the required resolving power. Sonar systems of various designations have a range capability from hundreds of meters to tens of kilometers and provide a direction-finding precision of about I°. To reduce the unfavorable influence of hydrologic factors on range capability, sonar is used with an acoustic system located in a container that is towed by the ship at a depth of several tens of meters (sonar installation with a variable depth of submergence).
S. A. BARCHENKOV