Chapter 6 — PRINCIPLES OF SONAR 



In the searchlight type of sonar, several 

 hundred nickel tubes are arranged in a circle 

 and mounted on one side of a metal plate called 

 a diaphragm. Each tube is wrapped with a coil 

 of wire to prevent frequency doubling. When 

 an alternating current is applied to the coll, 

 the tubes shorten and lengthen at the rate of 

 the alternating current. Each tube is polarized 

 by a constant magnetic field, usually supplied 

 by a permanent magnet. During one half cycle 

 of the applied signal, the a-c voltage and the 

 polarizing field add; during the next half cycle, 

 they oppose each other, and the magnetic field 

 is always in only one direction. The contractions 

 of the tubes thus are fixed to the a-c frequency. 



As the tubes contract and expand, the dia- 

 phragm vibrates and produces an acoustic signal 

 of the sams frequency as the applied alternating 

 current. A magnetostrictive transducer used in 

 searchlight sonars is shown in figure 6-2. 



Transducers employed with some scanning 

 sonars also operate on the principle of magneto- 

 striction. Instead of nickel tubes, however, the 



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transducer elements have nickel laminations 

 pressed in a thermoplastic material. Each element 

 contains a permanent magnet for polcirizing the 

 nickel. Several elements are mounted vertically 

 to form 1 stave, and 48 vertical staves are 

 arranged circularly to form the transducer. A 

 scanning transducer is shown in figure 6-3. An 

 exploded view of a portion of a stave is seen 

 in figure 6-4. 



Many types of operational scanning sonar 

 transducers are magnetostrictive. Except for 

 variations in dimensions, they are similar in 

 design. The type shown in figure 6-3 is a 

 cylindrical unit approximately 19 inches in dia- 

 meter and 27 inches long, and has an operating 

 frequency of 20 kHz. 



71.48 

 Figure 6-2. — Searchlight magnetostrictive 

 transducer. 



71.49 

 Figure 6-3. — Scanning magnetostrictive 

 transducer. 



93 



