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BELL SYSTEM TECHNICAL JOURNAL 



ceiver is then operating from an idealized resistance source having an im- 

 pedance which matches that of the receiver approximately, over the range 

 of interest. 



The receiver is coupled acoustically to a small-diameter condenser micro- 

 phone by means of a closed coupler^. The condenser microphone has a 

 substantially uniform characteristic up to a frequency of 10 kc. The 



SQUARE WAVE 

 GENERATOR 



•TEST RECEIVER 



STRING 

 OSCILLOGRAPH 



CONDENSER MICROPHONE 

 -6 CO. COUPLER 

 Fig. 4 — Circuit diagram of apparatus for indicial response measurements. 



10,000 





(B) 

 Fig. 5 — Frecjuency response (A) and indicial response (B) of measuring apparatus. 



microphone voltage is then amplified to the point where it can be measured 

 by an oscillograph. 



Either the cathode-ray oscilloscope or a rapid-recording string oscillo- 

 graph^ may be used, but in the latter case it is necessary to equalize the 

 string oscillograph to a frequency of about 10 kc in order to cover the audio 

 frequency range. The choice of these instruments depends somewhat upon 

 whether a permanent record is desired or whether a visual indication is 

 sufficient. 



