DISPERSION OF SOUND 



285 



directional loud speakers, the total sound energy density will be the same 

 in all parts of the theatre. Furthermore, the effective reverberation of 

 the reproduced sound (the ratio of generally reflected to direct sound) is 

 the same for all parts of the theatre. 



The distribution of a reproducing system in a theatre is usually checked 

 by means of a response measuring system. The plan and elevation view 

 of a typical theatre is shown in Fig. 12.3. The response characteristics for 

 positions yf , 5, C and D on the orchestra level and E and F on the balcony 

 level are shown in the respective graphs. These characteristics show that 



(^30 



§20 

 Q. 



in 10 



in 30 



O 20 



PLAN VIEW 



Fig. 12.3. A cross-sectional elevation and plan view of a theatre equipped with a loud speaker 

 for the reproduction of sound motion pictures. The graphs show the response frequency 

 characteristics in various parts of the theatre. 



it is possible to obtain uniform response in all parts of the theatre by fol- 

 lowing the procedures outlined in the preceding discussions. 



In sound motion picture reproduction the loud speakers are usually 

 placed behind a perforated screen upon which the picture is projected, Fig. 

 12.3. Sound is transmitted through the screen by actual vibration of the 

 screen and by the perforations. In general the transmission by vibration 

 is negligible. The perforations usually consist of small circular holes about 

 a millimeter in diameter. These holes form an inertance and acoustic re- 

 sistance. See Sees. 5.3 and 5.11. The resistance of the holes introduces 

 attenuation which is usually small. The acoustic reactance due to the in- 

 ertance increases with frequency, and therefore the attenuation increases 

 with frequency. The response characteristic of the screen shows more or 

 less constant attenuation in the low and mid frequency ranges due to the 



