THE RING ARMATURE TELEPHONE RECEIVER 



131 



At low frequencies, the reactance of the receiver as seen from the electrical 

 side is X = o}L{].-\-K). Hence the last factor of the above equation becomes 



RqX 

 (R -\- RY + X^ Takmg the case of a pure reactance receiver first, we may 



place R = 0, and if we further match Rq to X at the midband frequency 

 /o , and then denote the value of X at/o as Xq , we have 



R,X XqX f/fo 



~ v2 



{Ro + RY + X' xi -^ X' 1 + {f/foY 



95 



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 Z^< 80 



<^ I- 

 Ziup 

 oak 75 



ct UJ o 

 crz 70 



(i< 



65 



COUPLING LOSS=7.0DB 



£1 



RES. L0SS = 1.1DB 



(a) 



^U ACTUAL RESPONSE 



CORRESPONDING LEVEL 

 AND RANGE LIMITS 



^CALCULATED RESPONSE 



(b) 



400 600 1000 2000 

 FREQUENCY IN CYCLES PER SECOND 



4000 6000 10,000 



Fig. 16— (A) Low frequency loss distribution of ring armature receiver. (B) Theoretica 

 limits of response level and frequency range, for receivers with uniform response down to 

 zero frequency. 



The latter step assumes that the low frequency reactance X is a linear func- 

 tion of frequency. For the pure reactance receiver, the response then becomes 



2tPo.10^ Vc Sf K 



10 log 77 = 10 log 



(-» "•" 



Fc7r/o( - , r 



Vc Sr-hSf 1+K 



(9) 



Thus it is seen that the expression for the efficiency may be factored into 

 four terms, each of which has a significant physical interpretation. 



A plot of these factors versus frequency is shown in Fig. 16(a). Curve (1) 

 represents the first term only, the others are assumed to be equal to unity. 

 This may be called the ideal response, where no loss occurs and it has a 

 level of 91.7 db vs. (1 dyne/cm2)2 per watt for a 6 cc. front volume and a 

 matching frequency of 1000 cps. Curve (2) represents the product of the 

 first two terms; added is the volume loss Vc/Vg -\- Vc , which is the effect of 

 introducing additional front volume between the diaphragm and cap, thus 

 increasing the total front chamber volume of the load. Curve (3) includes 

 the effect of the third term of the equation, which may be called the stiffness 



