TELEPHONE RECEIVERS 



229 



Fig. 10.2. The mass of the diaphragm is represented by mo. The com- 

 pliance and mechanical resistance of the diaphragm are designated as 

 Cmo and Tmo- The back of the diaphragm is enclosed, forming the com- 

 pliance Cmi due to the resulting cavity. This cavity is connected to the 

 recess in the receiver handle by a hole in the plate. A special silk covers 

 this hole, forming the mechanical resistance r^i and the mass mi. The 



ELECTRICAL 



CROSS - SECTIONAL VIEW 



Fig. 10.2. Cross-sectional view, electrical circuit, and equivalent circuit of an improved bi- 

 polar telephone receiver. The graph shows the pressure response frequency characteristic 

 of the receiver feeding a closed cavity. The dots represent response computed from equiv- 

 alent circuit. (After Jones.) 



volume due to the recess in the receiver handle forms the compliance Cm2' 

 The holes in the ear cap form the mechanical resistance rMs and the mass 

 mz. The compliance Cms is due to the cavity between the ear cap and the 

 diaphragm. The response of this receiver was taken by measuring the 

 pressure generated in a plain cavity. This cavity is designated by the 

 compliance Cma- The holes in the grid covering the receiver proper are 

 large enough to have no reaction upon the response. A resilient screen of 

 silk is mounted on the back of this grill. The mass of this screen is very 

 small and is lumped with the diaphragm mass mo. 



The electrical portion of the circuit consists of the winding resistance rsi 

 and inductance Z-i. The eddy current elements are designated as rE2 and 

 L2. The electrical impedance due to the mechanical system is designated 

 by the motional impedance Zem- See Sec. 6.3^4 and equation 6.19. The 

 force /m can be obtained from equation 10.1. 



The response computed by means of the equivalent electrical circuit is 

 shown by the dots on the graph of Fig. 10.2. The measured response is 

 given by the curve on this graph. The agreement is very good and shows 



