EFFICIENCY 



151 



of the air chamber to be zero. In general, it is impractical to design a high 

 efficiency loud speaker to cover a wide frequency range without an air 

 chamber, because the diaphragm area is usually larger than the throat 

 area. In order to eliminate interference the dimensions of the elements of 



Fig. 8.3. The efficiency characteristics as a function of the frequency of a horn loud speaker 

 system having a ratio of voice coil mass to diaphragm mass of 1 : 2 and 1 : 1, flux densities 

 of 22,000 and 14,000 gausses, and initial efficiencies of 20, 40, 60, and 80 per cent for an 

 aluminum coil. The above graphs may be applied to a copper voice coil by multiplying 

 the frequency by one-half. 



the air chamber are usually made small compared to the wavelength. 

 When these conditions obtain, the volume of the air chamber appears as 

 a capacitance. At the higher frequencies, the mechanical impedance at 

 the throat of the horn is resistive, the mechanical reactance of the suspen- 

 sion is very small and the mechanical impedance of the diaphragm system 

 is a mass reactance. The equivalent circuit reduces to a mechanical 

 resistance and compliance in parallel connected in series with a mass 

 reactance. It is the purpose of this section to show the effect of the air 

 chamber upon the efficiency from the standpoint of this equivalent circuit. 

 The mechanical impedance of a mechanical resistance and capacitance in 

 parallel, which is the equivalent circuit of the throat mechanical resistance 

 and capacitance of the air chamber, is given by 



Zm — 



run 



1 + jwrMHCux 



8.14 



