TESTING OF LOUD SPEAKERS 



253 



are in phase. The phase angle, in degrees, between the output of the 

 two microphones is 



d 

 = - 360° 

 X 



11.7 



where d = distance between the two microphones in the direction of propa- 

 gation, in centimeters, and 

 X = wavelength of the sound, in centimeters. 



MICROPHONES 



Fig. 11.5. Schematic arrangement of apparatus for measuring the phase characteristic of a 



microphone. 



Phase distortion is of importance in combination microphones such as 

 the unidirectional microphone. See Sec. 9.4. 



E. Electrical Impedance Characteristic. — The electrical impedance char- 

 acteristic of a microphone is the impedance at the output terminals as a 

 function of the frequency. Any convenient method for measuring elec- 

 trical impedance may be used for determining the impedance character- 

 istic. 



F. Transient Response Characteristic. — For measurement of transient 

 response, see Sec. 11. 3G. 



11.3. Testing of Loud Speakers. — Many different measurements are 

 required to determine the performance of a loud speaker. The most 

 important characteristics which depict the performance of a loud speaker 

 are as follows: 



1. Response frequency characteristic. 



2. Directional characteristic. 



3. Nonlinear distortion characteristic. 



4. Efficiency frequency characteristic. 



5. Phase distortion characteristic. 



6. Electrical impedance characteristic. 



7. Transient response characteristic. 



