EFFICIENCY 



155 



t = temperature of the voice coil, in degrees Centigrade, and 

 Tem = motional resistance of the voice coil. 

 The efficiency as a function of the temperature for various values of initial 

 efficiency at 0° Centigrade is shown in Fig. 8.6. These characteristics 

 show that the relative losses in efficiency with increase in temperature is 

 considerably greater for a loud speaker with low efficiency. 



F. The Effect of the Radiation from the Unloaded Side of the Diaphragm 

 upon the Efficiency. — In the consideration of the efficiency, usually very 

 little cognizance is taken of the radiation from the back of the diaphragm 



50 100 150 200 250 



TEMPERATURE RISE IN DEGREES CENTIGRADE 



Fig. 8.6. The efficiency as a function of the temperature of the voice coil for various values 

 of initial efficiency at 0° Centigrade. 



of a horn loud speaker. In view of the large amount of sound that is 

 radiated from the back of the diaphragm, some consideration should be 

 given to the effect of this radiation upon the efficiency. Since this radia- 

 tion cannot be used, it must be considered as a loss the same as the resistance 

 loss in the voice coil. The loss due to the reactive component is usually 

 small compared to the reactance of the remainder of the system. 



The radiation from the back of the diaphragm may be assumed to be 

 the same as that from a piston in an infinite baffle. See Sec. 5.7 and 

 Fig. 5.1. The percentage of the total radiation which is lost due to the 

 radiation from the back is given by 



Efficiency loss = 





X 100 



8.18 



