teE RING ARMATURE TELEPHONE RECEIVER 125 



network which has been used extensively in the study of the ring armature 

 receiver. The cross-sectional drawing of the receiver and associated handset 

 handle and cap is labeled to indicate the various acoustical and mechanical 

 elements which are represented by the electrical circuit in the lower portion 

 of the figure. Thus Sd , Md and Rd represent the stiffness, mass, and me- 

 chanical resistance of the diaphragm; Mh and Rh represent the mass and 

 resistance of the hole in the diaphragm which provides the low-frequency 

 cut-off in the receiver, etc. One item of interest in this circuit representation, 

 which differs from that of previous receivers, is the division of the chamber 

 in back of the diaphragm into two parts, Sb and Sa , connected by the air 

 passageway RaMa of the magnetic air-gap between the armature and the 

 pole-piece tip. Under certain conditions, particularly those representing the 

 receiver with some of the acoustical controls removed, the acoustical con- 

 stants of the air-gap have been found to be of sufficient magnitude to war- 

 rant this division of the total back chamber into two connected parts. An 

 approximation is involved in this representation of the back chamber in that 

 the force applied to the coil chamber, Sa , by the motion of the armature is 

 ignored, but this approximation is justifiable through a consideration of the 

 relative magnitudes of the effective areas and volumes involved, and the 

 representation has been found to be in good agreement with measurements 

 on the actual physical structures. 



The constants of the equivalent circuit are determined by various physical 

 measurements and computations. For example, the effective mass of the 

 diaphragm, Md , is estimated from the weights and the integrated vibratory 

 kinetic energy of its various parts. The diaphragm stiffness, Sd , is then 

 computed from its resonant frequency. The diaphragm resistance, Rd , is 

 determined from a circle diagram analysis. The various chamber stiffnesses 

 are computed from their air volumes, knowing the integrated effective area 

 of the diaphragm. The acoustical resistances and masses are obtained from a 

 combination of theoretical computations and special tests on the network, 

 using circuit conditions in which these constants play the predominant role. 



In setting up this equivalent circuit for analysis and study, mechanical 

 resistances are replaced by electrical resistances; masses are replaced by 

 inductances; and compliances, the reciprocal of the stiffnesses, are replaced 

 by capacitances. Response-frequency characteristics may then be deter- 

 mined by applying a constant voltage to the input terminals and noting the 

 voltage across Sc , which is proportional to the pressure generated in the ear 

 coupling chamber for constant force applied to the armature. The effects 

 of changes in any of the elements can be determined by simply changing 

 the electrical value of the equivalent network element and repeating the 

 measurement. In this manner, optimum values or combinations of values 

 jmay be determined to provide the desired response-frequency character- 



