262 



MEASUREMENTS 



axis of the system. The characteristics may be plotted as a system of 

 polar characteristics for various frequencies or as response frequency char- 

 acteristics for various angles with respect to the reference axis. 



The directional characteristics of a direct radiator loud speaker in a 

 very large baffle may be obtained at a distance of five feet. For a small 

 baffle or cabinet the distance should be at least three times the largest 

 linear dimension of the system. The directional characteristics of a horn 

 loud speaker should be obtained at a distance three or more times the 

 largest dimension of the mouth. 



Obviously, very slight reflections will introduce considerable error for 

 angles in which the response is very low. For this reason, it is almost 



w> 



Fig. 11.11. Schematic arrangement of the apparatus for measuringthe non-linear distortion 



of a loud speaker. 



imperative that the measurements be made outdoors in free space. Appa- 

 ratus for obtaining directional characteristics is described in Sec. 11.25. 



C. Nonlinear Distortion Characteristic. — The nonlinear distortion 

 characteristic of a loud speaker is a plot of the total distortion in per cent 

 versus the frequency at a specified input power. A plot of the individual 

 components of the distortion in per cent versus frequency is also used to 

 depict the distortion characteristic of a loud speaker. 



The apparatus and circuit in schematic form for measuring the distor- 

 tion produced by a loud speaker are shown in Fig. 11.11. Great care must 

 be taken to avoid appreciable harmonics in the sound generating and 

 sound measuring equipment. To reduce the already low harmonic con- 

 tent in the signal generator to a negligible amount a variable cutoff low 



