A NON-DIRECTIONAL MICROPHONE 



415 



determination of the field calibration of this reference micro]>hone pre- 

 sented an interesting problem and an original solution is described in 

 Appendix B. 



At low frequencies where the wave-length of sound is very large 

 compared to the dimensions of the microphone, the coupler shown in 

 Figure 9 is used. At the higher frequencies a steady state sound field 

 is set up in the damped test room and the pressure at a given point is 

 measured by means of the small reference condenser microphone; then 



/yy/////^y/ //////^//y//////////77 ; 



MOVING-COIL 

 MICROPHONE 



Q 



CONDENSER 

 MICROPHONE 



DAMPED TEST ROOM 



SOUND 

 SOURCE ' 



I 



y////////////////////////y/77A 



ATTENUATOR 



LOW-PASS 



FILTER 



HIGH -PASS 

 FILTER 



SOUND 

 SOURCE 



zzzzzzzzzz. 



MOVING-COIL 

 MICROPHONE 



°^ 



TO ^ y = 



iCILLATOR V,,,,,,,,,^,,,,T>J ^t^ 

 CIRCUIT rniJPI FR FOR 



CONDENSER 

 COUPLER FOR " MICROPHONE 

 LOW-FREQUENCY 

 MEASUREMENTS 



OSCILLATOR 



Fig. 9 — Response measuring circuit. 



the instrument to be tested is substituted at this point and its generated 

 voltage recorded. The response is obtained in terms of decibels below 

 a reference level of one volt per bar of undisturbed sound pressure. 



In conclusion we wish to acknowledge our indebtedness to Mr. L. W. 

 Giles and Mr. R. C. Miner of the Bell Telephone Laboratories who 

 have aided us greatly in this work. 



APPENDIX A 



Theory 

 Since in most papers on moving coil microphones an equivalent 

 circuit of this instrument is given without deriving it, it does not seem 

 superfluous to indicate here the method of obtaining such a circuit and 

 the calculation of its constants. 



