COMPUTATION OF THE COMPOSITE NOISE 621 



the noise from the new machine at the observing position is: 

 75 - - 7.5 = 67.5 db sound level. 



Adding this figure to the weighted value of the existing composite 

 noise {A -\- w = 69.3 db) on a power basis gives the new composite 

 noise value of 71.5 db sound level (the new weight factor being zero). 

 Hence, the composite noise at the listening position is increased 1.8 db 

 by the machine. 



(6) The machine produces noise intermittently. The increase in the 

 composite noise level will not be as great in this case as in the preceding 

 case. For example, assuming the rate to be 100 peaks per minute, 

 the new value of N will be 850 peaks per minute and the corresponding 

 weight factor from Fig. 5 will be — 0.3 db. For the noise from the new 

 machine, the weight factor (by Fig. 4) is — 5.0 db. The distance loss 

 as before will be — 7.5 db. The weighted value of the machine noise 

 at the observing position is then : 



75 - 5.0 - 7.5 = 62.5 db sound level. 



Adding this figure on a power basis to the weighted value of the exist- 

 ing composite noise, 69.3 db, results in a new weighted composite sound 

 level, A -\- w = lOA db. Since w = — 0.3 db, this gives A = 70.4 db. 

 Hence, the composite noise is increased 0.7 db by the intermittent 

 machine noise. 



Problem III 



What is the maximum permissible noise, measured at 2 feet, which 

 the machine considered in Problem H, may produce without raising 

 the composite noise in the typing room by more than 0.5 db? 



(a) The machine produces a steady noise. In this case, the composite 

 noise has 300 peaks and its weight factor is zero. The existing com- 

 posite noise was 69.7 db sound level (see Problem I). The maximum 

 permissible value of the new composite noise level is consequently 



A = 69.7 + 0.5 = 70.2 db sound level. 



Let the unknown machine noise be A^ (its weight factor is zero), and 

 since for the existing composite noise A -\- w = 69.3, equation (10) 

 gives : 



70.2 69.3 4j 



10^ = 10"^ + 101°. 



Entering the ordinate of Fig. 6 at the value of 70.2 — 69.3 = 0.9 db, 

 the chart indicates that A^ must be 6.3 db below 69.3. Hence ylg 

 = 63 db sound level at the observing position. 



