622 BELL SYSTEM TECHNICAL JOURNAL 



The distance loss for 20 feet is — 7.5 db. The machine then could 

 produce a noise of: 



63.0 + 7.5 = 70.5 db sound level 



at 2 feet distance without raising the composite noise level by more 

 than 0.5 db at the observing position. 



(b) The machine produces noise intermittently. The solution of the 

 problem in this case follows the same lines as in Part (a) except that 

 the weight factors are changed. The rate for the new machine noise, 

 Aq, is assumed to be 100 peaks per minute, so that Wg = — 5.0 db, as 

 in part (b) of Problem II. The maximum permissible value of the 

 new composite noise is 70.2 db sound level (as before) but its weight 

 factor now is — 0.3 db as in part (b) of Problem II. The weighted 

 value of the existing composite noise as before is 69.3 db sound level. 

 Equation (10) then gives: 



(69.9) 69 3 U9-5.O) 



10""io~ = lO"!"" + 10"^^*^. 



From Fig. 6 it is found that for a value of 69.9 — 69.3 = 0.6 on the 

 ordinate, the abscissa is 8.3 db. Hence, ^9 — 5.0 must be 8.3 db below 

 69.3 or ^9 = 66.0 db sound level at the observing position. Applying 

 the same distance loss as before, the machine could produce a noise of 

 73.5 db sound level at 2 feet without increasing the composite nqise 

 level by more than 0.5 db at the observing position. 



From the computations, then, it may be expected that adding a 

 steady noise will increase the general noise level more than adding an 

 intermittent noise having the same average value, when there are a 

 number of sources operating. That this is actually so can readily be 

 verified by sound level measurements. As has been stated, sound 

 level measurements under most conditions are directly related to the 

 effects of noise upon the individual exposed to it, and the method 

 described provides a convenient and reasonably reliable way of com- 

 puting such readings and thereby makes possible the engineering 

 analysis of noise problems. 



Bibliography 



1. "Environment and Employee EfiSciency" by Harold Berlin and others — 

 Office Management Series No. 81 — Copyright 1937, American Management 

 Association. 

 "City Noise" — Report of Noise Abatement Commission 1930, Dept. of Health, 

 New York City. 



