EFFICIENCY 



149 



The efficiency may be written, employing equation 8.8, as 



52 



M = 



\ mcyir 



X 100 



8.9 



10^ + 52 



For a particular material qp is a constant. Equation 8.9 gives the efficiency 

 in terms of 5^, »Zc, and Ajy^l At- The efficiency as a function of Ad^/At 

 for aluminum voice coils of 0.1, 0.25, 0.5, 1, 2, 4, and 8 grams and flux 



0.5 



0.25 



14000 GAUSSES 



voice 



COIL 



mass] 



0.5 

 0.25 



Fig. 8.2. The initial efficiency of a horn loud speaker as a function o'i Aji^l At for aluminum 

 voice coils having masses of 0.1, 0.25, 0.5, 1.2, 4, and 8 grams and flux densities of 22,000 

 and 14,000 gausses. Aj) and At are the areas of the diaphragm and throat, respectively, 

 in square centimeters. The above graphs may be applied to a copper voice coil by multi- 

 plying the ratio ^^d^ -f- At by one-half. 



densities of 22,000 and 14,000 gausses is shown in Fig. 8.2. The charac- 

 teristics of Fig. 8.2 also apply to a copper voice coil if the abscissa are 

 multiphed by 0.5. Equation 8.9 and Fig. 8.2 show the factors which 

 influence the initial efficiency of a horn loud speaker. 



B. The Effect of the Mass of the Vibrating System upon the Efficiency. — In 

 the preceding section the mechanical reactance of the vibrating system 

 was assumed to be negligible compared to the mechanical resistance. The 

 mechanical mass reactance of the diaphragm and voice coil influences 

 the efficiency when this mechanical reactance becomes comparable to the 

 mechanical resistance. It is the purpose of this section to consider the effect 

 of the mechanical reactance of the vibrating system upon the efficiency. 



The real part of the motional impedance, equation 8.1, is 



Tem 





10-9 ohms 



8.10 



