Chapter 6— PRINCIPLES OF SONAR 



Piezoelectric Process 



The piezoelectric transducer functions much 

 like the magnetostrictive type. An exception 

 is that crystals are used instead of nickel 

 laminations. Various kinds of crystals have 

 been employed, but the most commonly used 

 tjTJe is ammonium dihydrogen phosphate (ADP). 



The arrangement of the crystals on a dia- 

 phragm is similar to the method used in the 

 magnetostrictive transducer. One end of the 

 crystal is attached to a bakelite-covered steel 

 plate, and the other end is allowed to vibrate 

 freely. The free ends of the crystal block con- 

 stitute the transmitting and receiving elements. 

 The entire arrangement of crystals is connected 

 electrically to give the effect of a single large 

 crystal. The elemsnts are housed in a chamber 

 filled with castor oil, which has sound trans- 

 mission qualities similar to sea water. The oil 

 also protects the sensitive crystals from being 

 damaged by exposure to water or moisture. 

 When an electric current of the desired frequency 

 is passed through the crystals, they change size 

 as a unit, causing a vibration. Vibrations are 

 passed by the castor oil, through the "window" 

 of the sonar dome, into the sea water. When 

 outside energy is received in the form of an 

 echo, it exerts mechanical pressure on the 

 crystals, which produce an electrical current 



that is amplified and converted to visual and 

 audible signals. 



Nearly all transducers now being built are 

 of the ceramic type. Ceramic compounds have 

 high sensitivity, high stability with changing 

 temperature and pressure, relatively low cost, 

 and can be constructed in almost any reasonable 

 shape or size. 



Electrostrictive Process 



The most commonly used ceramic compound 

 is lead zirconate titanate. Such transducers are 

 known as electrostrictive transducers yet behave 

 in a piezoelectric manner, and now are more 

 widely used in modern sonar systems. Although 

 a ceramic material is essentially electrostrictive, 

 it can be mide to behave like a piezoelectric 

 material by polarizing it permanently. Polariza- 

 tion is accomplished by impressing an extremely 

 high voltage on the material for a period of 

 several minutes to align the molecules. Once the 

 molecules are aligned properly, the compound 

 can be treated similarly to that used with a 

 piezoelectric material. 



MODERN ACTIVE SONAR THEORY 



The theory of modern active sonar operation 

 may best be understood by breaking it down into 



71.51 

 Figure 6-5. — Sonar system— block diagram. 



95 



