DISPERSION OF SOUND 



283 



at the two points would be inversely proportional to the ratio of the squares 

 of the distances from the reproducer. In this particular case, the difference 

 in level would be 13 db. Obviously, such a large variation in sound in- 

 tensity precludes the possibility of satisfactory reproduction over the entire 

 area to be supplied. Therefore, a compensating means must be provided 

 to counteract the variation of intensity with the distance from the repro- 

 ducer. The directional loud speaker furnishes a solution of the problem. 

 The directional characteristics of the loud speaker are shown in Fig. 12.2. 

 In this particular case, the difference of level for a point 40° from the axis, 



--•^^E D C B A 



POSITION ON LISTENING LEVEL 



Fig. 12.2. Arrangement of the loud speaker for a sound motion picture reproducing system 

 in a theatre. The graph shows the intensity level at points along the Hstening level. 



as compared with the level at a point on the axis, is 13 db. The loud 

 speaker is adjusted until the axis of the characteristic passes through the 

 point E. Then the position of the loud speaker is adjusted until the angle 

 6 is. 40°. The distribution over the distance under consideration is shown 

 in Fig. 12.2. Summarizing, the variation of the sound pressure with the 

 angle between the axis and the line joining the observation point and the 

 reproducer has been employed to compensate for the decrease of the sound 

 energy with the distance. 



From the response frequency characteristic of the loud speaker the pres- 

 sure at any distance r centimeters on the axis may be obtained from the 

 following equation. 



p=p. 



Xq 



12 A 



