NONLINEAR DISTORTION 



161 



The equivalent circuit of the mechanical system, Fig. 8.10, shows the 

 effect of the nonUnear element. When the stiffness of the suspension 

 system increases with amplitude, the third harmonic is the preponderant 

 distortion. The wave shape under these conditions is shown in Fig. 8.10. 

 A distortion frequency characteristic of a diaphragm coupled to a large 

 throat horn is shown in Fig. 8.10. 



D. Distortion Due to a Nonunijoryn Magnetic Field in the Air Gap. — 

 Inhomogeneity of the flux density through which the voice coil moves is 

 another source of distortion. The result is that the driving force does not 

 correspond to the voltage developed by the generator in the electrical 

 system. 



The force, in dynes, developed by the interaction of the current in the 

 voice coil and the magnetic field is 



/ = Bli ~ 8.20 



where B = flux density, in gausses, 



/ = length of the voice coil conductor, in centimeters, and 

 i = current, in abamperes. 



20 



?:!3i6 



m It) 

 o u 



X38 



-1 = 



li. ^ 4 



-A -.2 .2 .4 



AXIAW LENGTH INCHES 



Fig. 8.11. Graph of the flux distribution in an air gap. A. Typical distribution of the flux 

 lines in an air gap. B. A voice coil longer than the air gap. C. A voice coil shorter than 

 the air gap. 



Equation 8.20 shows that the force Is directly proportional to the current 

 if Bl is a constant. If the Bl product varies with the position of the voice 

 coil the force will not be proportional to the current and distortion will 

 result. A typical flux distribution in an air gap is shown in Fig. 8.11. A 

 consideration of the flux distribution shows that the Bl product will be 



