DISPERSION OF SOUND 287 



control and microphone mixing system is usually located in a box or booth 

 in the balcony. 



In this system there are two sources of direct sound, namely: the original 

 sound and the augmented sound from the loud speakers. Usually the in- 

 tensity of the original sound issuing from the stage will be quite satisfactory 

 on the orchestra floor near the stage and, as a consequence, it is not neces- 

 sary to augment the sound in this portion of the theatre. As the distance 

 from the stage increases, the original sound intensity decreases. To make 

 up for this loss the sound energy from the loud speakers must progressively 

 increase towards the rear of the theatre. It is the purpose of this section 

 to describe and analyze a sound re-enforcing system. 



Consider the system shown in Fig. 12.4. If the distance between the 

 source of the original sound and the point of observation is r centimeters, 

 the energy density, in ergs per cubic centimeter, due to the direct sound is 



where Pdi = power output of the sound source, in ergs per second, and 

 c = velocity of sound, in centimeters per second. 



The sound intensity or intensity may be obtained from the energy den- 

 sity by multiplying it by the velocity of sound. The reference intensity 

 for intensity level comparisons is 10~^ ergs per second or 10~^^ watts per 

 square centimeter. The intensity level of a sound is the number of decibels 

 above the reference level. 



The intensity level on the orchestra floor resulting from a sound source 

 as, for example, a speaker or singer on the stage, is given by the curve 

 of Fig. 12.4. It will be seen that the intensity level of the direct sound in 

 the rear of the house is inadequate for good hearing. The arrangement 

 and characteristics of the sound re-enforcing system should be chosen so 

 that the resultant intensity level, due to the direct sound from the com- 

 bination of the original source and the loud speakers, is the same for all 

 parts of the audience. 



The energy density, in ergs per cubic centimeter, at a distance r centi- 

 meters due to direct radiation from the loud speaker, from equation 12.5, is 



Em = ^W 12.9 



where po = pressure, in dynes per square centimeter, obtained at a dis- 

 tance Xo centimeters, 



