DISPERSION OF SOUND 



295 



quiet intervals. The noise level may be determined by means of a noise 

 meter. The power available should be sufficient to produce a minimum 

 sound level of 80 db or, for very noisy conditions, 20 to 30 db above the 

 noise during the relatively quiet intervals. In the two examples cited 

 above and, in fact, for all outdoor public address work, the only considera- 

 tion is direct sound. The problem is to select amplifiers and loud speakers 

 with characteristics which will deliver the required sound level over the 

 distances and areas considered. The steps in the selection of a system 

 may be as follows: First, the directional characteristics should be deter- 

 mined, as outlined in the preceding discussion, so that uniform response is 

 obtained over the audience area. Second, either a single or a group of loud 

 speakers having the desired directional characteristics should be selected. 



AUDIENCE 

 AREA 



PLAN VIEW 



PLAN VIEW 



LOUD SPEAKERS 



ELEVATION VIEW 



ELEVATION VIEW 



Fig. 12.10. Tw^o arrangements of sound re-enforcing systems for an outdoor theatre. The 

 arrangement on the left employs a single loud speaker having suitable directional charac- 

 teristics to produce a uniform intensity level over the audience area. The arrangement on 

 the right employs a number of loud speakers, each covering a small portion of the audience. 



Third, the response characteristic of the system on the axis at a specified 

 input and distance should be available to show the amplifier power re- 

 quired to supply the desired intensity level. Fourth, the power handling 

 capacity of the loud speakers and amplifiers should be adequate to supply 

 the required intensity level. 



Two types of sound re-enforcing installations for an outdoor theatre are 

 illustrated in Fig. 12.10. The system depicted on the left employs a single 

 loud speaker station. The same procedure for obtaining uniform sound 

 coverage and adequate intensity level of the direct sound as used in the 

 preceding considerations is applicable in this case. The system depicted 

 on the right employs a large number of loud speakers, each one supplying a 

 small portion of the audience. The directional characteristics of the loud 



