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BELL SYSTEM TECHNICAL JOURNAL 



diaphragm. This microphone, when provided with a coil having an 

 electrical resistance of 20 ohms, will generate 10~^ volts per bar of 

 sound pressure. The smallest voltage that can be measured at the 

 terminals of a resistance is limited by the voltage due to thermal 

 agitation of the electrons,'^^ which under normal conditions and for a 

 frequency band of 15,000 c.p.s. is equal to 7 X 10~^ volts for a resist- 

 ance of 20 ohms. Hence the smallest pressure that it is possible to 

 measure with this microphone is about 7 X 10"** bars. However, 

 over a narrow band of frequencies, or at a single frequency, measure- 

 ments may be made down to still lower pressures if the circuit is pro- 

 vided with a band-pass filter. The sensitivity of this instrument is 

 higher than that of any other microphone of comparable frequency 

 range at present available. In evaluating some of the other micro- 

 phone principles we shall, therefore, use its sensitivity as a reference, 

 without meaning to imply that sensitivity is the sole criterion of the 

 merit of a microphone. There is also an upper limit to the sound in- 

 tensities that may be measured with this instrument. This is governed 

 by the maximum amplitude of excursion that the diaphragm can make 

 without the generation of appreciable harmonics. The upper and 

 lower limits at the various frequencies are shown by the curves in Fig. 2. 



200 



D 100 500 1000 6000 10,000 20,000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 2 — Operating range of moving coil microphone. 



The upper limit is taken as the pressure at which the higher harmonic 

 components of the voltage are equal to 3 per cent of the fundamental. 

 The lower limit represents the pressure at which the signal voltage is 

 just equal to the voltage of thermal agitation. For comparison the 

 29 J. li. Johnson, Phys. Rev. 32, 97 (1928). 



