RECEIVERS AND MICROPHONES 



571 



same figure are given values of the response as determined by com- 

 putation of the mechanical impedance from the constants of the 

 receiver. The ordinates were so adjusted arbitrarily as to bring the 

 computed and observed values into coincidence at the lower frequen- 

 cies. There is a general agreement between the computed and ob- 



10 50 100 500 1000 5000 lO.OOC 



FREQUENCY IN CYCLES PER SECOND 



Fig. 6 — Response of moving coil receiver. 



served curves, yet the variations are larger than can be accounted for 

 on the basis of experimental errors. It is probable that the quantities 

 used in the calculations are not strictly constant up to the higher 

 frequencies, where the diameter of the diaphragm becomes comparable 

 to the wave-length of sound. However, except for a depression in 

 the neighborhood of 4000 c.p.s., the measured is better than the com- 

 puted characteristic. 



A receiver of this general character was supplied for the Master 

 Reference Systems for Telephone Transmission ^ in Europe and in 

 America where it has been in service since 1928. 



The Moving Coil Microphone 



It has been pointed out that in an electrodynamic microphone of 

 high quality the diaphragm with a rigidly attached coil should have 

 the same velocity per unit of force throughout the frequency range. 

 If the dynamical system of the microphone is represented by the 

 mechanical circuit of Fig. 2, this condition requires that the constants 

 of the various elements of this circuit be so chosen that the magnitude 

 of the impedance, z, is the same at all frequencies. It is evident that 

 these values will differ materially from those of the high quality 

 receiver. 



In Fig. 7 the impedance expressed in db as determined by equations 

 (1) is shown as a function of frequency for several sets of values of the 

 constants of the impedance elements. They show how, by the proper 

 choice of these values, a uniform response may be obtained over a 



* " Master Reference System for Telephone Transmission," by W. H. Martin and 

 C. H. G. Gray, Bell Sys. Tech. Jour., July, 1927. 



