TELEPHONE RECEIVERS 



231 



network for controlling the response. A cross-sectional view of a typical 

 dynamic telephone receiver is shown in Fig. 10.4. The equivalent electrical 

 circuit of the mechanical system is also shown in Fig. 10.4. 



The electrical impedance, in abohms, due to the mechanical system is 

 given by 



Bl 

 zem = — 10.2 



Zm 



where B = flux density in the air gap, in gausses, 



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

 Zm = total mechanical impedance at/iif, in mechanical ohms. 



DIAPHRAGM EAR CAP 6 



mo ^ 



VOICE COIL CIRCUIT 



EQUIVALENT CIRCUIT 



CASE 



CROSS -SECTIONAL VIEW 



1000 

 FREQUENCY 



Fig. 10.4. Cross-sectional view, the voice coil circuit and the equivalent circuit of the mechan- 

 ical system of a dynamic telephone receiver, e the voltage of the generator, teg the 

 resistance of the generator, yed the damped resistance of the voice coil. L the inductance 

 of the voice coil, zem the motional impedance. In the equivalent circuit: m^ the mass of 

 the diaphragm. C>fo and ta/o the compliance and mechanical resistance of the suspension. 

 7n\ and rM\ the mass and mechanical resistance of the slit. Cm\ the compliance of the cavity 

 behind the diaphragm. Jm the force generated in the voice coil. The graph shows the 

 pressure response frequency characteristic. A. Receiver feeding a closed cavity. B. Re- 

 ceiver feeding an artificial ear. (After Wente and Thuras.) 



In dynamic telephone receivers the flux density is relatively low and %em 

 is small compared to Veb and may be neglected. 

 The force /m, in dynes, is given by 



/a 



Bli 



10.3 



where /, the current in abamperes, is obtained from the electrical circuit. 

 In general, the force /j^- is practically a constant and may be considered a 

 constant in the equivalent circuit. 



The response feeding a plain cavity is indicated by A, Fig. 10.4. The 

 response measured on an artificial ear indicated by 5, Fig. 10.4, shows 

 that the response at the low frequencies is reduced due to the leak. 



