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BELL SYSTEM TECHNICAL JOURNAL 



and for the average composite noise in db sound level : 



A = lOlogio-T- . 

 It is usually convenient to use logarithmic weight factors 



ini ^1 



^i-101og,0 3QQ 



(7) 



Wi = 10 log] 



W2 



300 



Wn = 10 logio 

 W = 10 logio' 



nin 

 300 



M 

 300 



(8) 



Figures 4 and 5 permit ready computation of these logarithmic weight 

 factors. The chart in Fig. 5 is based on the relation between M and 

 N given in equation (4b). It should be recalled that the derivation 

 of this equation involved a number of approximations. This ex- 

 pression especially does not apply when one or more of the noise 

 sources are continuous, in which case the exact expression (eq. 4) gives 

 M = 300 (for to — 0). Hence w = in this case. 



The terms of equation (5b) then can be rewritten in logarithmic 

 form by using equations (6), (7) and (8), as follows: 



^i + z.. = ioiog.o^^f; 



A2 -\- W2 — 10 logic 



nil h 

 300 To 



^„ + ^„= 10 logio l^j^ 

 vl+z. = 10 logio 300 j^ 



(9) 



This gives the following formula: 



(A+w) (Ai-\-W\i 



10"^o~ = 10""i^ 



(Ai+Wii 



+ 10~^^~ + 



10 



10 



(10) 



from which the average composite noise A can be found. 



The application of this expression is materially simplified by the 

 use of the chart shown in Fig. 6. Power addition of a number of 



