Chapter 2 



OPERATION AND APPLICATION OF UNDERWATER 



SOUND DEVICES 



By Leslie L. Foldy 



Calibration work in underwater sound is con- 

 cerned primarily with devices which com tit elec- 

 tric energy into acoustic energy (projectors) and, con- 

 versely, those which convert the energy of a sound 

 held into electric energy (hydrophones). Generically, 

 these devices are known as electroacoustic transducers. 

 To provide a suitable background for the detailed 

 discussion of the problems involved in calibrating 

 transducers, this chapter presents a brief discussion ol 

 (1) the physical principles underlying the action ol 

 transducers, and (2) their uses. 



2i PRINCIPLES OF OPERATION 



OF TRANSDUCERS 



As already noted, energy conversion is the funda- 

 mental purpose of transducers." Most transducers can 

 convert energy in either direction, that is, they are re- 

 versible. According to the nature of the physical proc- 

 ess used in the energy conversion transducers may be 

 classed under four general headings, namely electro- 

 dynamic, electrostatic, piezoelectric, and magneto- 

 striction. The principles involved in each are dis- 

 cussed here in a qualitative manner. A more quantita- 

 tive treatment is given in Chapter 3. 



The simplest example of an electrodynamic trans- 

 ducer is a moving-ribbon instrument (Figure 1) 

 which consists of a rectangular metallic strip or rib- 

 bon suspended in a magnetic field. When the instru- 

 ment is in a sound field, there is generally a difference 

 in pressure established between its front and back. 

 Since the mass of the ribbon differs from that of the 

 magnet, relative motion of the two results, inducing 

 an electromotive force in the moving ribbon. Con- 

 versely, if an alternating current flows through the 

 suspended ribbon, the forces on it due to interaction 

 of the current and the external magnetic field pro- 

 duce vibration. This vibratory motion in turn creates 

 a sound field in the medium. Since the process de- 



TO ELECTRICAL 



'TERMINAL 



ELECTRICAL TERMINAL 



Figure 1. Electrodynamic transducer (pressure-gradient 

 type). 



pends on the pressure difference between the front 

 and bac k of the ribbon, such an instrument is called a 

 pressure-gradient hydrophone. 



Electrodynamic transducers in which operation de- 

 pends on the value of the pressure at a point in the 

 medium, rather than on the gradient of the pressure, 

 may also be considered. A simple example is a trans- 

 ducer containing a diaphragm mounted in a water- 

 tight housing in such a manner that only one of its 

 faces is exposed to the water (see Figure 2), and with 

 the pressure in the housing adjusted to compensate 

 for the external hydrostatic pressure. A coil encircling 

 a fixed magnet is rigidly fastened to the unexposed 



TO ELECTRICAL TERMINAL 



DIAPHRAGM, 



" These matters are discussed in detail in the Division 6 vol- 

 umes on magnetostriction and crystal transducers. 



-TO ELECTRICAL TERMINAL 

 Figure 2. Electrodynamic transducer (pressure type). 



