568 



BELL SYSTEM TECHNICAL JOURNAL 



discussion of the mechanical design of the instruments which is to 

 follow. The actual values of the constants are to be so chosen, if 

 possible, that the mechanical impedance, z, of the whole network is 

 constant with frequency in the case of the microphone and inversely 

 proportional to the freq uency f or the receiver. The absolute value 

 of this impedance, z, is Vr- + x^ where 



r = 



X = 



Si'ri 



5icj[m i^(coi^ — CO-) — rr] 

 r{-ur 

 s 



+ ^0, 



^0 



+ miHcoi^ - a;2)2 



+ mow , h 



(1) 



mi 



The Electrodynamic Receiver 



If the mechanical system of the receiver can be represented by the 

 circuit diagram shown in Fig. 2, then, as the amplitude per unit force 

 is a measure of the receiver response, we may calculate the product of 

 frequency and impedance and so get a response-frequency character- 

 istic for any specified set of values of the constants. Such character- 

 istics are graphically shown in Fig. 3 for several sets of values. Curves 



2 20 50 100 500 1000 SOOO 10,000 



"^ FREQUENCY IN CYCLES PER SECOND 



Fig. 3 — Theoretical response curves of moving coil receiver. 



of identical character but of different level would, of course, be ob- 

 tained if the magnitude of each of the corresponding impedance ele- 

 ments were changed in the same proportion. It is seen from these 

 curves that, theoretically at least, it is possible to obtain a uniform 

 response over a wide frequency range. Curve C, for example, shows 

 a variation of less than 1.5 db for frequencies up to 10,000 c.p.s. As 

 might be expected, the wider the frequency range of uniform response 



