572 



BELL SYSTEM TECHNICAL JOURNAL 



wide freciuency range. Curve C, for instance, shows a variation of 

 less than 1.5 db from 200 to 10,000 c.p.s. It may be shown from 

 equations (1) that if the scale of frequencies is changed by a factor k 

 the form of the response curve will remain unchanged, provided r^ 

 and ro are multiplied by k, and ^i and 5o, by kr\ but the absolute value 

 of the velocity per unit force will be changed by the factor \/k at all 

 points on the curve. Thus, under these conditions, if the last value 

 of the abscissa in Fig. 7 is designated as 5000 instead of 10,000 c.p.s., 



S 25 

 £ 20 



i^ IS 



a o 



u ui 

 a. Q 10 



500 1,000 

 FREQUENCY IN CYCLES PER SECOND 



10,000 



Fig. 7— Theoretical response curves of moving coil microp:)hone. 



then k = 0.5 and the curves will remain unchanged in form but the or- 

 dinates will be raised 6 db. The form of any of the curves of Fig. 7 

 will, of course, not be changed if all the corresponding constants are 

 changed proportionally, although the absolute value of the velocity 

 per unit of force will vary inversely with the magnitude of these 

 constants. 



At zero frequency the velocity of the diaphragm per unit of force is 

 necessarily zero. In passing to the lower frequencies a point is there- 

 fore finally reached where the response decreases appreciably. This 

 point depends primarily upon the stiffness, .9o, of the diaphragm. A 

 method for overcoming this loss in sensitivity at low frequencies will 

 be discussed later. 



Construclion of the Microphone 



A microphone was constructed very similar in design to that of the 

 receiver just described, but with the cap omitted in order to expose 

 the diaphragm to the action of sound waves. The dimensions of the 

 various elements were changed so that the impedance of the diaphragm 

 with its associated network should have a substantially constant value 

 throughout a wide frequency range. The response as computed is 

 shown in Fig. 8. 



The moving coil microphone was calibrated experimentally by compari - 

 son with a calibrated condenser microphone. For this comparison each 

 transmitter was mounted with its face outward in an opening in the end 



