PRESSURE MICROPHONES 175 



rate masses. These masses consist of the central portion W5, the ribbed 

 intermediate portion m^ and the outer portion nii. The central portion in- 

 cludes the mass of the movable electrode and is coupled to the ribbed portion 

 by the compliance C^e which in turn is coupled to the outer portion by the 

 compliance Cm2- The paper books which support the edge of the dia- 

 phragm have a compliance Cma and a mechanical resistance rui- Their 

 mass is included in the outer portion of the diaphragm OT4. The internal 

 mechanical resistance of the portions which form the coupling compliances 

 Cui and Cm& are represented by rM2 and r^re respectively. A hole is pro- 

 vided in the diaphragm to permit rapid equalization of low frequency 

 pressures of high intensity and prevent damage to the diaphragm and other 

 parts. The mass and the mechanical resistance of this hole m^rMz is so 

 chosen that their effects on the response are confined to frequencies below 

 300 cycles. The controlling compliance Cms is that of the cavity between 

 the diaphragm and the die cast frame. The carbon granules are rep- 

 resented by a compliance Cmh and a mechanical resistance Tms- The mass 

 of the carbon granules is lumped with that of the central portion of the 

 diaphragm. The holes in the inner grid are sufficiently large so that there 

 is no reaction upon the response. The holes in the outer grill add the 

 mass niQ and the mechanical resistance Tmo. These holes are coupled to a 

 moisture proof membrane of mass mi and mechanical resistance Vmi by 

 means of the compliance Cmq of the enclosed cavity. The cavity com- 

 pliance Cm\ couples the membrane to the diaphragm. 



The response of this microphone computed from the equivalent circuit 

 is shown in Fig. 9.2. The response for constant sound pressure on the 

 diaphragm is also shown in Fig. 9.2. It will be seen that the agreement 

 between the computed and measured characteristics is very good and sub- 

 stantiates this type of analysis. The response is very much smoother 

 than in the case of the plate or disk type of diaphragm. 



The free space response shown in Fig. 9.2 indicates the diffraction effect 

 of the microphone as an obstacle in increasing the pressure on the dia- 

 phragm. See Sec. 1.11. 



In addition to the smoother response the sensitivity of the new unit is 

 higher because of the reduction in mass of the vibrating system. Due to 

 the shape of the carbon chamber the performance of the microphone is less 

 affected by angular position. 



2. Double Button Carbon Microphone. — For applications requiring both 

 high quality and large power output the single button carbon microphone 

 is not suitable due to the large nonlinear distortion. 



To obtain uniform response over a very wide range with very low dis- 



