252 



MEASUREMENTS 



sound wave of very low distortion in free space. The arrangement^^^ shown 

 in Fig. 11.4 provides a simple means of obtaining a sound wave free from 

 distortion. A stationary wave is obtained in the tube by moving the 

 piston until the maximum pressure is obtained. A pressure of 1000 dynes 

 per square centimeter can be obtained with a fraction of a watt input to 

 the loud speaker. For the determination of the second harmonic the 

 microphone is placed at a second harmonic node. Under these conditions 

 the second harmonic component at the microphone is very small. The 

 second harmonic component is then measured by means of a harmonic 

 analyzer. See Sec. 11. 3C For the third harmonic the microphone is 



Fig. 11.4. Arrangement of apparatus for measuring the non-linear distortion generated by a 



microphone. (After Phelps.) 



placed at a third harmonic node. Either pressure or velocity microphones 

 may be tested, the only difference being in the position in the tube. 



D. Phase Distortion Characteristic. — The phase distortion character- 

 istic of a microphone is a plot of the phase angle between the voltage output 

 of the microphone with respect to some reference voltage as a function of 

 the frequency. A microphone such as the velocity microphone, see 

 Sec. 9.35, in which the output is in phase with the particle velocity (its 

 output is also in phase with the pressure in a plane sound wave), may be 

 used as the reference standard. The standard microphone and the micro- 

 phone to be tested may be placed side by side in a plane progressive wave 

 in free space, Fig. 11.5. The outputs of the two microphones are amplified 

 by separate identical amplifiers and connected to the vertical and hori- 

 zontal plates of a cathode ray oscillograph. The resultant Lissajou figure 

 indicates the phase relations between the output of the two microphones. 

 The two microphones are shifted relative to each other in a line parallel 

 to the direction of propagation until the outputs of the two microphones 



1=*^ Phelps, W. D., Jour. Acous. Soc. Amer., Vol. 11, No. 2, p. 219. 



