DISPERSION OF SOUND 



297 



locations, an intensity level of 70 db is more than adequate. Higher inten- 

 sity levels tend to produce annoyance in adjacent rooms. From a con- 

 sideration of the data of Fig. 12.7, it will be seen that the power require- 

 ments for the loud speakers will be small. To blend with the furnishings 

 of the room, it is desirable to mount the loud speakers flush with the wall 

 surface. Therefore, for these applications, a direct radiator loud speaker 

 of the permanent magnet dynamic or magnetic type is most suitable. In 

 this connection it should be mentioned that these loud speakers have a 

 very low efficiency, being of the order of 1 per cent as compared to 25 per 

 cent to 50 per cent for the horn loud speakers. 



n 



n 



H 



M 



fl 



Fig. 12.11. Two uses of call, general announce, and sound distributing systems. On the 

 left, a high efficiency horn is used to obtain a high sound level over a large floor area as in a 

 factory or warehouse. On the right, small direct radiator loud speakers are used to supply 

 the small rooms at a relatively low level as in paging, announcing, and centralized radio 

 installations used in hospitals, hotels, or schools. 



For large rooms requiring large acoustic outputs it is more economical 

 to use a high efficiency loud speaker and effect a corresponding reduction in 

 the power amplifier requirements. On the other hand, for an installation 

 of the type depicted on the right side of Fig. 12.11 and requiring a large 

 number of units, it is more economical to use relatively inefficient low cost 

 loud speakers and correspondingly larger amplifiers. 



I. Intercommunicating Systems. — Intercommunicating systems are loud 

 speaking telephones for use in communicating between two rooms. The 

 more elaborate systems are similar to the general announce system 

 described in Sec. 12.2/7 with the addition of microphone positions in more 

 than one room. The simplest system consists of two units for use between 



