184 MICROPHONES 



The equivalent circuit of the mechanical system consisting of the dia- 

 phragm coil and suspension system is shown in Fig. 9.6. 



The velocity, in centimeters per second, of the voice coil is given by 



-^" 9.17 



1 



JCjoCmi 



where Tmi = mechanical resistance of the suspension system, in mechanical 

 ohms, 

 mi = mass of the diaphragm and voice coil, in grams, and 

 Cm] = compliance of the suspension system, in centimeters per dyne. 

 The generated internal voltage, in abvolts, is 



e = Blx 9.18 



where B = flux density in the air gap, in gausses, 



/ = length of the voice coil conductor, in centimeters, and 

 X = velocity of the voice coil, in centimeters. 



Equation 9.18 shows that the microphone will be uniformly sensitive 

 with respect to frequency if the velocity Is Independent of the frequency. 

 The characteristics 1 and 2 in Fig. 9.6 were computed by employing equa- 

 tion 9.17. These characteristics show that a uniformly sensitive dynamic 

 microphone, with respect to frequency, must be essentially " resistance 

 controlled." 



The characteristic marked 2 shows some falling off in velocity at the high 

 and low frequencies. This can be corrected by the use of some additional 

 elements. The major portion of the resistance is the silk cloth m^rui- 

 Resistance in the case of silk cloth is due to the high viscosity introduced 

 by the small holes. See Sec. 5.3. Slits have also been used for the re- 

 sistance element. See Sec. 5.4. The mass reactance of the diaphragm 

 is reduced at the higher frequencies by the compliance Cm2 formed by the 

 volume between the silk and the diaphragm. The addition of the elements 

 Cm2, fM2 and mi changes the characteristic at the high frequencies from 

 that marked 3 to that marked 5. An Increase In response over an octave 

 Is obtained by the addition of these elements. A corresponding Increase* 

 in response can be obtained at the low frequencies by means of the case 

 volume Cmz and the addition of a tube m^rMZ- The equivalent circuit shows 

 the action of the additional elements in changing the response from the 

 characteristic 3 to the characteristic 4-5. 



The most common materials used for the diaphragms of pressure micro- 



