PIEZOELECTRIC DRIVING SYSTEM 109 



Substituting equation 6.40 in 6.38 the generated current, in statam- 

 peres, is 





47r;c2 



From equations 6.41 and 6.35 



e 167rV/^ 



6.41 



6.42 



I ^0 a; 



167rV 



where %em — motional impedance, in statohms, and 



z^j, = total mechanical impedance presented to the vibrating sur- 

 face inchiding the vibrating surface. 



The condenser driving system has been employed as a loud speaker, in 

 which case the moving electrode radiates directly into the air. Means 

 must be provided to keep the electrodes separated without, at the same 

 time, adding a large stiffness. In a bilateral or push-pull arrangement the 

 movable electrode is placed between two stationary plates and the large 

 steady forces are balanced out. 



The impedance of a condenser loud speaker is inversely proportional to 

 the frequency. This makes it very difficult to provide efficient coupling 

 to a vacuum tube over a wide range. Another undesirable feature is the 

 high polarizing voltage required in order to obtain high efficiency. 



6.5. Piezoelectric Driving System. — A piezoelectric driving system is 

 a driving system in which the mechanical forces result from the deformation 

 of a crystal having converse piezoelectric properties. The bimorph ele- 

 ment in use to-day is an assembly of a plurality of electroded and properly 

 oriented plates of Rochelle salt cemented together face to face. Two types 

 are available, namely: " benders " and " twisters." The driving system 

 in Fig. 6. 1£ depicts a " twister " element. A bender element will be con- 

 sidered in the chapter on microphones. See Sec. 9.2C. 



The deformation in centimeters at the end of the lever arm produced in 

 a crystal by the application of an electromotive force is 



X = Kxe 6.44 



where Kx = constant of the crystal, and 

 e = electromotive force, in volts. 



