EFFICIENCY 



147 



where B = flux density, in gausses, 



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

 Zm = mechanical impedance of the vibrating system, in mechanical 

 ohms, at the point/^ Fig, 8.1. 



T^aTma 



' T hJ 



VOICE COIL 

 CUECTRICAL CIRCUIT 



EQUIVALENT CIRCUIT 



OF THE 

 MECHANICAL SYSTEM 



CROSS-SECTIONAl. VIEW 



Fig. 8.1. Cross-sectional view of a horn loud speaker, the electrical circuit and equivalent 

 electrical circuit of the mechanical system. In the voice coil circuit: e the internal voltage 

 of the generator, teg the internal resistance of the generator. L the inductance of the 

 voice coil. rsD the damped resistance of the voice coil, zem the motional impedance. 

 In the equivalent electrical circuit of the mechanical system: mA and tma the mass and 

 radiation resistance due to the air load on the back of the diaphragm, mc and mD the 

 mass of the voice coil and diaphragm. Cms and Cmi the compliance of the suspension and 

 air chamber, zmh the mechanical impedance at the throat of the horn, zme the mechan- 

 ical impedance due to the electrical circuit, /m the force generated in the voice coil, /mo 

 the force of the mechanical generator. 



The efficiency jjl, in per cent, is 



fEM 



M = 



rED + rEM 



X 100 



8.2 



where rsM = resistance component of the motional resistance, in ohms, and 



rsD = damped resistance of the voice coil, in ohms. 

 In the equivalent electrical circuit, Fig. 8.1, the impedance Zm ^.t /m is 

 given by 



Zm = jo^rriA + Tma + jo)?nc + jc^mo + -. 



1 



+ 



Zmh 



JwCms J(^CmiZmh + 1 



8.3 



where pja = mass of the air load on the back of the diaphragm, in grams, 

 mc = mass of the voice coil, in grams, 

 jnr, = mass of the diaphragm, in grams, 

 rMA = resistance load on the back of the diaphragm, in mechanical 



ohms, 

 Cms — compliance of the suspension, in centimeters per dyne, 

 Cmi = compliance of the air chamber, in centimeters per dyne. 



