CHAPTER VI 



DRIVING SYSTEMS 



6.1. Introduction. — An electric mechanical or electroacoustic trans- 

 ducer or driving system is a system for converting electrical vibrations into 

 the corresponding mechanical or acoustical vibrations. The most common 

 driving systems in use to-day tor converting electrical variations into me- 

 chanical vibrations are the electrodynamic, the electromagnetic, the con- 

 denser and the piezoelectric. It is the purpose of this chapter to consider 

 the electrical and mechanical characteristics of these driving systems. 



6.2. Electrodynamic Driving System. — A moving coil or dynamic 

 driving system is a driving system in which the mechanical forces are de- 



LOADVl 



OJ I.COII- 



m 



^ 



ARMATURE^ r 



m 



^LOAD, 



MOVABLE 

 PLATE"* 



— rn;; 



OV. Pni AR!7lWr. 



I 



IXED 

 PLATE 



Fig. 6.1. Driving systems: ^^ electrodynamic, B electromagnetic, reed armature, Celectro- 

 magnetic, balanced armature, D condenser, E crystal. 



veloped by the interaction of currents in a conductor and the polarizing 

 field in which it is located (Fig. G.XA). The force, in dynes, due to the 

 interaction of the current in the voice coil and the polarizing field is 



Jm = Bit 6.1 



where B = flux density, in gausses, 



/ = length of the conductor, in centimeters, and 

 / = current, in abamperes. 



101 



