416 BELL SYSTEM TECHNICAL JOURNAL 



In deriving the theory of this instrument it is convenient to speak of 

 the contour pressure response of a microphone.* We shall define the 

 contour pressure response at a certain frecjucncy as the generated 

 open-circuit voltage per bar of uniform pressure over the face of the 

 microphone. On the other hand the field response at a certain fre- 

 quency is defined as the generated open-circuit voltage per bar of 

 pressure of the undisturbed sound field. The difference between the 

 contour pressure response and the normal incidence field response is 

 caused by diffraction around the microphone, as explained earlier in 

 this paper. We may approximate the normal incidence field response 

 by adding to the contour pressure response the diffraction of the corre- 

 sponding sphere and circular plate. In the considerations that follow 

 the influence of the screen on the response will be omitted. 



To obtain this contour pressure response we assume, then, a uniform 

 pressure over the microphone face. We further require that no wave 

 propagation shall occur within the microphone. Let a small alternat- 

 ing pressure be applied and consider the motion of the system when the 

 pressure is positive. Then the air in the grid holes moves as a whole 

 and imparts an excess pressure to the grid chamber and to the di- 

 aphragm. The central portion of the latter moves as a rigid piston. 

 The air volume underneath the diaphragm is compressed, and some air 

 is forced through the coil slot into a very small chamber just in front 

 of the damping ring. We shall assume again that the air in the coil 

 slot moves as a whole. From here the air flows through the acoustic 

 resistance into the larger case chamber. The outside pressure instead 

 of acting upon the diaphragm in the described manner may enter the 

 case through the equalizing tube. On its travel through the tube it is 

 attenuated and its phase is changed. At low frequencies this property 

 of the tube is used to increase the response of the instrument. In the 

 theory that follows, the tube circuit is omitted at first, but its position 

 in the general arrangement will be shown later. 



Let us use the following notation for the elements of the vibrating 



structure: 



r_i = equivalent mechanical resistance of all holes in grid, 



m_i = equivalent mass of all holes in grid, 



n — number of holes in grid, 



A-\ — area of all holes in grid, 



* The term contour pressure response is useful when the microphone has acoustic 

 circuits in front of its diaphragm. Pressure response is a term which has been re- 

 served specifically for the condition where, uniform pressure is applied directly to the 

 diaphragm. (See the report of a subcommittee on fundamental sound measurements 

 on the calibration of microphones in the journal of the Acoustical Society of America, 

 Vol. 7. April, 1936, p. 301.) 





