296 ARCHITECTURAL ACOUSTICS 



speakers should be selected so that each individual area is adequately sup- 

 plied. Cognizance must be taken of the energy supplied from adjacent 

 loud speakers. 



There are certain advantages in each system. In the case of the single 

 loud speaker system, better illusion is obtained because the augmented 

 sound appears to come from the stage. On the other hand, the intensity 

 level outside the audience area in a backward direction falls off very slowly. 

 At a distance equal to the length of the audience area the level is only 6 db 

 lower than that existing in the audience area. In certain locations the 

 sound levels produced by such systems will cause considerable annoyance 

 to those located in the vicinity of the theatre. By dividing the theatre 

 area into small plots, each supplied by a loud speaker, and by directing the 

 loud speakers downward, the sound intensity level outside the audience 

 area will be considerably lower than in the case of the single loud speaker 

 station and usually eliminates any annoyance difficulties. The short 

 sound projection distance is another advantage of the multiple loud speaker 

 system. 



The above typical examples of outdoor public address and sound re- 

 enforcing systems illustrate the principal factors involved in this field of 

 sound reproduction. 



H. General Announce and Paging Systems^'°. — General announce sys- 

 tems are useful in factories, warehouses, railroad stations, airport terminals, 

 etc. A typical installation is depicted on the left portion of Fig. 12.11. 

 For this type of work intelligibility is more important than quality. The 

 deleterious effect of reverberation upon articulation can be reduced, and a 

 better control of sound distribution can be obtained, by reducing the low 

 frequency response of the system. Furthermore, the cost of the amplifiers 

 and loud speakers is also reduced by limiting the frequency range. To 

 find the power required, the sound intensity level under actual operating 

 conditions should be determined. The system should be designed to pro- 

 duce an intensity level 20 to 40 db above the general noise level. Under 

 no conditions should the system be designed to deliver an intensity level 

 of less than 80 db. The loud speakers should be selected and arranged 

 following an analysis similar to that outlined in the preceding sections, so 

 that uniform sound distribution and adequate intensity levels are obtained. 



For certain types of general announce, paging and sound distributing 

 installations, as is used in schools, hospitals, department stores, hotels, etc., 

 the intensity level required is relatively low and the volume of the average 

 room is usually small. For most installations of this type, save in noisv 



" Olson, H. P., RCA Review, Vol. 1, No. 1, p. 49, 1936. 



