RECEIVERS AND MICROPHONES 575 



represented by a resistance in series with a mass reactance, and the 

 whole dynamical system of the microphone by the circuit diagram of 

 Fig. 10, in which Fe^"'' is the pressure in the sound wave multiplied 



HI WW — ^W^ — WW — WW T ^W^ WA^ 



jut 



Fig. 10 — Simplified circuit diagram of moving coil microphone at low frequencies. 



by the effective area of the diaphragm ; s' , the stiffness imposed upon the 

 diaphragm by the air enclosed within the magnet, if the tube were 

 closed; r' and m' are the acoustic resistance and mass respectively of 

 the tube multiplied by the square of the area of the diaphragm. The 

 other symbols of Fig. 10 have the same significations as before. 



Substituting numerical values for the various impedances of the 

 circuit shown in Fig. 9, and solving this circuit for the velocity of the 

 diaphragm per unit of force, we obtained the low-frequency values given 

 by the curve B of Fig. 8. The circles give the corresponding values ob- 

 tained experimentally. The agreement between these values and those 

 computed is within the experimental errors with which the constants of 

 the microphone were determined. The addition of this acoustic 

 network has increased the response at the low frequencies so that 

 there is no loss in sensitivity down to a frequency of 45 c.p.s., even 

 with a diaphragm of comparatively high stiffness. 



The absolute sensitivity of this microphone is approximately 

 9.5 X 10^5 volts per bar. However, in practical operation a trans- 

 former is used between the microphone and the vacuum tube of the 

 initial stage of the amplifier. The transformer that has been used 

 for this purpose has a voltage ratio of 100 with a variation of less than 

 2 db between 45 and 10,000 c.p.s. Under this condition the voltage 

 delivered to the vacuum tube is 9.5 millivolts per bar. This value 

 compares with approximately 3 millivolts per bar for the Western 

 Electric Company 394 Condenser Microphone, which was designed for 

 maximum efficiency for frequencies up to 7,000 c.p.s. The electro- 

 dynamic microphone thus has a sensitivity about 10 db higher, and 

 covers a wider frequency range. 



The condenser microphone commonly used has a cavity in front of 



