286 



ARCHITECTURAL ACOUSTICS 



acoustic resistance of the holes. However, the attenuation in the high fre- 

 quency range increases with frequency due to the acoustic reactance of the 

 holes. The inertance increases with the thickness of the screen and de- 

 creases as the ratio of the open to closed area of the screen increases. For 

 example, for 3 db attenuation at 10,000 cycles the hole area is usually 15 

 to 20 per cent of the screen area. If the hole area is 7 to 15 per cent the at- 

 tenuation is about 6 db at 10,000 cycles and about 3 db at 5000 cycles. 

 These examples show that the screen is an important problem in wide range 

 reproduction. 



LOUD 



MICROPHONES 



^ 65 



> 60 



t45, 



POSITION ON 



C 

 LISTENING 



B 

 LEVEL 



Fig. 12.4. Arrangement of the components of a sound re-enforcing system in a theatre. The 

 graph shows the intensity level due to direct sound at the points indicated on the orchestra 

 floor. Curve 0, the intensity due to the original source of sound. Curve L the intensity 

 level due to the loud speaker. Curve T the resultant intensity level. 



C. Sound Re-enforcing Systems ^^. — A large theatre equipped with a 

 sound re-enforcing system is shown in Fig. 12.4. Microphones are con- 

 cealed in the footlight trough for collecting the sound on the stage and 

 others are placed in appropriate positions in the orchestra. The loud 

 speakers are located above the stage in the proscenium arch. The volume 



" Olson, H. F., RCA Review, Vol. 1, No. 1, p. 44, 1936. 



