CALIBRATION OF MICROPHONES 



251 



the directional characteristic of a microphone or loud speaker. The 

 apparatus of Fig. 11.3 is arranged to obtain the directional characteristic 

 of the microphone. The directional characteristic of the loud speaker may 

 be obtained by placing the loud speaker upon the rotating shaft and keep- 

 ing the microphone fixed in position. The sound is picked up by a micro- 

 phone and amplified. The output of the amplifier is detected and fed to a 

 low pass filter. The output of the filter is amplified by a d-c amplifier, the 

 output of which is fed to two potentiometers. The arms of the potentiom- 

 eters are spaced at 90°. The potentiometer arms and microphone shaft are 



CATHODE -RAY OSCILLOGRAPH 



POTENTIOMETERS 



Fig. 11.3. Schematic arrangement of the apparatus employing a cathode ray tube with a 

 long persistence screen as a polar directional characteristic indicator and recorder. 



rotated by a motor. The length of the radius vector of the spot is propor- 

 tional to the output of the microphone. The angular displacement of the 

 spot is synchronized with the microphone shaft. From this it will be seen 

 that the cathode ray beam traces the polar directional characteristic of the 

 microphone. In case it is desirable to record the characteristic, this may be 

 done photographically or by tracing the curve left upon the screen. 



C. Nonlinear Distortion Characteristic. — The harmonic distortion tests 

 are intended to show the spurious harmonics generated by the microphone 

 when it is actuated by a pure tone. The plot of the total distortion in per 

 cent of the fundamental is termed the distortion characteristic. It is also 

 common to plot the individual components in per cent as the distortion 

 characteristics. 



It is difficult to obtain a sound source which will generate an intense 



