614 BELL SYSTEM TECHNICAL JOURNAL 



Assuming the reflected sound to be uniformly distributed in the 

 room, it can be shown that: ^ 



.0038E 



Ir = 



aS 

 S — a 



where E = power emitted by source, in ergs per second, 

 6" = total surface area of the room in square feet, 

 a = absorption in square feet of equivalent open window. 



Introducing 7^ = aS/^S — a), the above becomes: 



J. .003SE 

 Ir - ^^-• 



Assuming the sound source to radiate hemispherically, as is frequently 

 the case because it is associated with a large surface acting as a baffle, 

 the direct sound intensity is: 



where r = distance from source, in feet, 



V = velocity of sound, in feet per second. 



In the above expression the direct sound intensity decreases in- 

 versely as the square of the distance from the source. This shows 

 that room absorption is effective mainly in reducing the noise from 

 sources at a considerable distance from the observing point, but has 

 relatively little effect on nearby sources. 



The curves in Fig. 3 give the variation in the total sound intensity, It, 

 with distance from the source for different values oi F = aS/{S — a), 

 as computed by means of the above expressions. 



Computation of Composite Noise 



In the following, the application of the statistical method outlined 

 above is discussed. Since noise measurements are usually expressed 

 in db sound level, it is necessary to change the form of the equations 

 given in the preceding sections. For this purpose equation (5) is 

 rewritten as follows: 



h MIo MIo'^ '" M lo' ^ ^^ 



where Iq = reference sound intensity. 



