60 



BELL SYSTEM TECHNICAL JOURNAL 



which have been discussed in considerable detail in connection with 

 the condenser microphone. 



The circuit employed in measuring the response of the carbon micro- 

 phone is shown on Fig. 11. Two steps are involved in the calibration 

 of the sound source. With the output terminals of the microphone 

 circuit and the sound source short circuited and the polarizing voltage 

 for the condenser microphone removed, the attenuator is adjusted 

 until the voltage applied to the measuring circuit is that developed by 

 the condenser microphone when a sound pressure of one bar is im- 

 pressed on its diaphragm. A record is made of the reading of the 



cni .MH CONDENSER 



c 



HIGH PASS 

 FILTER 

 60 CYCLES ANDn 

 135 CYCLES 



POTENTIAL 

 ATTENUATOR 



CARBON 

 MICROPHONE 



LOW 



PASS 



FILTER 



THERMO- 

 COUPLE 



^"X 



THERMO- 

 -COUPLE 



'C^ 



RECEIVER AND 



ATTENUATOR 



CURRENT 



METER 



>; 



POWER 

 OSCILLATOR 



Fig. 11 — Circuit emi)loyecl in calil)raling the 387-type carbon microphone. 



output meter in the measuring circuit. The polarizing voltage is 

 then applied to the condenser microphone. After the output terminals 

 of the attenuator have been short circuited an alternating current of 

 a known frequency is supplied to the sound source and the magnitude 

 of this current adjusted until the meter reading is the same as that 

 previously obtained with the attenuator. This completes the cali- 

 bration of the sound source for that frequency. After the carbon 

 microphone has been placed in the position previoush- occupied by 

 the condenser microphone, the polarizing \oltage is once more removed 

 from the condenser microphone and the output from the carbon 

 microphone circuit impressed on the measuring circuit. The reading 



