ELECTROMAGNETIC DRIVING SYSTEM 105 



where %em — motional impedance, in abohms, and 



"Z-MT — total mechanical impedance including the armature with 

 reference to a point on the armature directly over the 

 pole piece. 

 From equations 6.7 and 6.18, assuming Rx = R^^ 



'Z'EM = —z 6.19 



Equation 6.19 is similar to equation 6.6 for the electrodynamic system. 



This driving system is not generally used in loud speakers. The most 

 common example of this driving system is the bipolar telephone receiver 

 where the diaphragm is the armature. 



B. Balanced Armature Type. ■ — There are innumerable possibilities in 

 the design of a magnetic driving system. The preceding section consid- 

 ered the simplest magnetic driving system in which both the steady flux 

 and the alternating flux flows through the armature. It is the purpose 

 of this section to consider the balanced armature type of driving system 

 in which only the alternating flux flows longitudinally through the arm- 

 ature. 



A typical balanced armature driving system is shown in Fig. 6.1C. The 

 steady field is usually supplied by a permanent magnet. The armature 

 is located so that it is in the equilibrium with the steady forces. The 

 alternating current winding is wound around the armature. The steady 

 force in dynes at 1, 2, 3 or 4, Fig. 6.1C, due to the magnetic field, is 



where </)i = total flux, in maxwells, at each pole due to the permanent 

 magnet, and 

 A = effective area, in square centimeters, of the pole piece at 

 1, 2, 3 or 4. 



The flux, in maxwells, at 1, 2, 3 or 4 due to a current in the coil is 



47rM 



R, 



6.21 



where A^ = number of turns in the coil, 



i = current in the coil, in abamperes, and 

 i?2 = reluctance of the magnetic circuit, in oersteds, which the coil 

 energizes. 



