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BELL SYSTEM TECHNICAL JOURNAL 



pole-piece. With this change in construction, the microphone was 

 again calibrated. The results obtained in this case at the higher fre- 

 quencies are shown by curve B of Fig. 8. 



It is seen that the response of the microphone is quite uniform over 

 a wide frequency range, but that it decreases at the lower frequencies. 

 This decrease can be avoided by a reduction in the stiffness, 5o, but this 



Fig. 9 — Moving coil microphone. 



expedient has the practical disadvantage that it makes the trans- 

 mitter more delicate and increases its susceptibility to mechanical 

 vibrations. The response at these frequencies can be increased more 

 profitably by a simple modification which increases the force on the 

 diaphragm under the action of sound waves. If the air-space enclosed 

 by the magnet on the rear of the diaphragm is connected with the 

 outside air through a tube, then, under the action of sound, a pressure 

 will be developed within this space through the tube, differing in 

 magnitude and phase from that of the sound outside. This pressure 

 acts on the rear of the diaphragm. Under certain circumstances the 

 total force on the diaphragm will be increased by virtue of this 

 pressure. 



The microphone shown in Fig. 9 is provided with a tube for per- 

 forming this function. The acoustic impedance of a tube may be 

 calculated from the formula ^ 



Z = 



n r- 



1 



1 + 



2 J,{kr) 



k Joikr) 



(2) 



in which k — Vj/x/pw, / is the length and r the radius of the tube, ^t 

 the viscosity and p the density of air. At low frequencies, Z may be 

 ' I. B. Crandall, "Theory of Vibrating Systems and Sound," p. 237. 



