COMPUTATION OF THE COMPOSITE NOISE 609 



On the assumption of discrete integration intervals, furthermore, a 

 source can produce no more than one peak every 0.2 second. The 

 maximum number of peaks per minute that can be obtained from a 

 single source, consequently, is 300. 



Definition 3 



The noise from all sources as measured by the indicating meter is called 

 composite noise. 



Room noise measured at a given observing position in the room is 

 an illustration of composite noise. The peaks of a composite noise 

 are called "composite peaks." Composite peaks have similar charac- 

 teristics to source peaks as regards duration, frequency weighting, etc. 



Statistical Method of Combining Noise Sources 



In developing this computation method the principal aim of the 

 authors has been to provide a practical, working method which is 

 easy to handle yet is sufficiently reliable for engineering purposes. In 

 accordance with this objective, a number of simplifying assumptions 

 have been made. Some of these have been indicated in connection 

 with the discussion of the assumed characteristics of noise peaks. The 

 division of the time into discrete 0.2 second intervals is another ap- 

 proximation which has been made. The statistical treatment, in 

 addition, includes approximations which are usual in probability 

 mathematics of this type. Practical experience has shown that these 

 approximations do not lead to errors which affect the usefulness of the 

 method. 



In the following an expression is derived for computing the average 

 intensity of the composite noise from the average intensities of the 

 source peaks and their number. Consideration is first given to the case 

 when only a single source peak may occur in each 0.2 second interval. 

 The consideration is then extended to cover the general case when 

 more than one source peak may occur in a 0.2 second interval. 



When only one source peak may occur in a 0.2 second interval, the 

 average intensity I of the composite noise for these intervals is the 

 arithmetic mean of the intensities of the source peaks, weighted by 

 their frequency of occurrence. 



