Elements of acoustical engineering (1940) Elements of acoustical engineering elementsofacoust00olso Year: 1940 252 MEASUREMENTS sound wave of very low distortion in free space. The arrangement^^^ shown in Fig. 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 no
Elements of acoustical engineering (1940) Elements of acoustical engineering elementsofacoust00olso Year: 1940 252 MEASUREMENTS sound wave of very low distortion in free space. The arrangement^^^ shown in Fig. 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. For the third harmonic the microphone is * AMPLIFIER^ '^ ANALYZER OSCILLATOR o. ^ AMPLIFIER a Fig. 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. , 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. 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
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