NONLINEAR DISTORTION 



163 



F. Power Handling Capacity and the Voice Coil Temperature"^^. — The 

 maximum allowable distortion may determine the power rating for the 

 loud speaker. However, in certain loud speakers the maximum allowable 

 temperature of the voice coil determines the power rating. This is par- 

 ticularly true of high frequency loud speakers. 



By m^aking the efficiency a maximum, the dissipation in, and the resulting 

 temperature of, the voice coil for a certain acoustic output will be a min- 

 imum. Practically all the heat energy developed in the voice coil is 

 transmitted across the thin air film between the voice coil and the pole 



POWER INPUT 



Fig. 8.13. The temperature rise as a function of the power delivered to a voice coil for air 

 gap clearances as follows: A. 0.021 inch, B. 0.015 inch, C. 0.009 inch. Coil IJinch in 

 diameter and 0.25 inch in length. 



pieces, and from the pole pieces to the field structure and thence into the 

 surrounding air. In this heat circuit practically all the drop in temper- 

 ature occurs in the thin air film. The temperature of the voice coil 

 approaches the temperature of the pole pieces as the thickness of the air 

 film is decreased. The temperature rises as a function of the power dis- 

 sipated in the voice coil for various clearances between the voice coil and 

 pole pieces is shown in Fig, 8.13. These results are obtained for no 

 motion of the voice coil. When motion occurs, the thermal impedance of 

 the air film is reduced and the temperature of the voice coil is diminished. 



24 Olson, H. F., RCA Review, Vol. 2. No. 2, p. 265, 1937. 



