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passive sonar[′pas·iv ′sō‚när]
a device that detects the source of acoustic vibrations or noise and determines the direction, or bearing, to the source. It is used primarily as a hydroacoustic direction finder.
Passive sonars operate in the range of sonic, ultrasonic, and infrasonic frequencies. They include fixed devices, installed on the floor of the ocean or a sea on special support structures or anchored buoys, and mobile devices, which are used on submarines, on certain types of surface vessels, in containers lowered into the water from helicopters, and on free-floating buoys.
Passive sonars consist of sonic detector, compensator, filter, amplifier, and display device. The detector is usually an acoustic antenna with the required directivity pattern (seeDIRECTIVITY OF ACOUSTIC TRANSMITTERS AND RECEIVERS),equipped with up to several hundred hydrophones. The compensator turns the axis of the directivity pattern of the antenna toward the noise-producing object and determines the bearing. When a rotating antenna is used, the passive sonar usually has no compensator. The filter passes a specific band of acoustic frequencies. Filtering out low frequencies helps increase the operating range of the direction finder, and filtering out high frequencies improves the ratio of signal to background noise and increases direction-finding accuracy. The amplifier increases the amplitude of the signals being received to the level necessary for display. The display may be a telephone, loudspeaker, or cathode-ray tube.
Special devices can be connected to passive sonars to analyze the spectrum of the acoustic signal being received and identify the objects emitting the noise by type (submarine, surface ship, torpedo, and so forth). The accuracy of target direction finding of modern passive sonars may be within fractions of a degree, and the operating range may be in the tens of kilometers.
REFERENCESTiurin, A. M., A. P. Stashkevich, and E. S. Taranov, Osnovy gidroakusiiki. Leningrad, 1966.
Kolesnikova, I. K., and I. A. Rumynskaia. Osnovy gidroakusiiki i gidroakusticheskie stantsii. Leningrad, 1970.
S. A. BARCHENKOV