OPERATION AND APPLICATION OF SONAR DEVICES 



TO ELECTRICAL TERMINALS 



FIXED CONDENSER 

 PLATE 



TO ELECTRICAL TERMINALS " 

 Figure 3. Electrostatic transducer. 



side of the diaphragm. When the instrument is placed 

 in a sound field, the diaphragm and coil are set into 

 vibration relative to the magnet. This vibration 

 changes the flux linked by the coil, thus inducing an 

 alternating emf in it. Conversely, the force due to the 

 interaction of an alternating current through the coil 

 and the field of the magnet gives rise to vibrations of 

 the coil and diaphragm, generating a sound wave in 

 the medium. 



Electrostatic, piezoelectric, and magnetostriction 

 transducers are generally pressure rather than pres- 

 sure-gradient instruments. Thus, a condenser-type 

 transducer uses an electrostatic field to convert acous- 

 tic into electric energy and vice versa. An example of 

 the condenser-type instrument is shown in Figure 3. 

 Here a constant potential is applied between the 

 plates of the condenser. When the instrument is 

 placed in a sound field, the movable plate is set into 

 vibration with respect to the fixed plate. This vibra- 

 tion changes the distance between the plates, and con- 

 sequently the capacity, of the condenser. Since the 

 voltage across the plates is inversely proportional to 

 the capacity, an alternating voltage is generated. Con- 

 versely, the application of an alternating electric po- 

 tential to the plates changes the force between them, 

 and consequently causes vibration of the movable 

 plate. 



The other two types of reversible transducers de- 

 pend on less familiar physical phenomena. The first 

 of these is the piezoelectric effect. It has been found 

 that certain crystals, when subjected to compression, 

 exhibit electric charges on their faces: under tension 

 the charges are reversed. The inverse piezoelectric ef- 

 fect also exists: the crystals expand when a potential 

 of one sign is applied across the faces and contract 

 when an opposite potential is applied. 



WATER 



WATER 



PIEZOELECTRIC 

 CRYSTAL PLATE 



Figure 4. Piezoelectric transducer. 



The faces on which the charges appear when the 

 crystal is subjected to stress depend on its structural 

 properties. A tourmaline crystal, for example, is one 

 of the simplest insofar as piezoelectric effects are con- 

 cerned. Tourmaline possesses a single piezoelectric 

 axis such that a stress in the direction of this axis pro- 

 duces charges on the faces normal to it. Thus, in Fig- 

 ure 4, if a stress is applied in the direction of the piezo- 

 electric axis Z, charges appear on the faces a and a'. 

 Conversely, applying a potential between a and a' 

 causes the crystal plate to expand or to contract in 

 thickness (depending on the sign of the potential) 

 in theZ direction. b 



The simplest type of piezoelectric transducer, there- 

 fore, comprises a tourmaline crystal in contact with 

 two metal condenser plates. Application of an alter- 

 nating potential to the plates causes the crystal al- 

 ternately to expand and contract. Upon immersing 

 the system in water, a sound field is generated by the 

 vibration of the plates. Conversely, vibrations pro- 

 duced by placing the crystal and condenser plates in a 

 sound field give rise to an alternating voltage on the 

 plates. 



The fourth type of transducer, shown in Figure 5, 

 operates on the principle of the magnetostrictive ef- 

 fect, which bears certain similarities to the piezo- 

 electric effect. If a rod or tube of ferromagnetic mate- 

 rial (iron, cobalt, nickel, or various alloys containing 



b The changes in length involved are small. For tourmaline, 

 the fractional change in length per unit electric field (1 volt per 

 cm) is 1.93 X lO-io C m' 4 gm- 1 ' 6 sec. 



<• It may he remarked that the amplitude of vibration for a 

 given impressed alternating voltage and the magnitude of the 

 induced alternating voltage for a given impressed sound field 

 are both maximized when the frequency of the impressed volt- 

 age, or sound field, coincides with the natural mechanical reso- 

 nance frequency of the crystal. 



