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BELL SYSTEM TECHNICAL JOURNAL 



Adding the values of Column 7 successively on a power basis by- 

 means of the curve in Fig. 6 gives A -^ w from which is obtained the 

 total composite noise A = 69.7 db sound level. This differs by ap- 

 proximately 1 db from the average of the measured composite noise 

 distribution shown by Curve B of Fig. 1. 



The effect of sound treatment on the walls and ceiling in reducing 

 the typing room noise may readily be calculated by means of the 

 curves in Fig. 3. Supposing that the added absorption raises the 

 value of F from 650 to 2000 units, this figure shows that noise pro- 

 duced by sources 20 feet or more away from the observing point would 

 be reduced by approximately 5 db. At shorter distances the reduction 

 would be less. For the observing position here considered, a com- 

 putation similar to that carried out above indicates that the composite 

 noise level would be reduced about 3 db by the added absorption in 

 the room. 



TABLE 



Total Source Peaks: N = 750 



w = — 0.4 db 

 (Fig. 5) 



Power Addition A + w = 69.3 db 

 (Fig. 6) 



A = 69.7 db sound level. 



Problem II 



A piece of office machinery, such as an addressing or copying 

 machine, which produces an average sound level of 75 db at 2 feet 

 distance, is to be installed in the typing room considered in Problem I, 

 20 feet away from the observing position. How much will the com- 

 posite noise level be raised? 



(a) The machine produces a steady noise. The new value for the 

 number of composite noise peaks is then 300 and the weight factor 

 of this noise is zero. The distance loss of the machine noise (for 

 F — 650) is — 7.5 db from Fig. 3. Hence, the weighted value of 



