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Distance attenuation of noise levels from a point source <br />° s 's <br />2 ;.1 <br />95 dBA 89 11R <br />100 fi <br />Figure 4 <br />Addition and subtraction of decibel levels <br />4d <br />&I fI13A► <br />200 11 <br />In many situations pertaining to noise control and monitoring, it is very useful to be able to add and subtract <br />noise levels. This can be done with principles similar to how sound attenuation over distance is estimated. It is <br />important to note the characteristics of logarithmic addition or subtraction of decibel levels. <br />A doubling of sound energy yields an increase of three decibels. For example, each generator at a factory <br />produces sound that is measured at 50 decibels, so running one generator would create sound measured at 50 <br />dBA, turning on a second generator would increase sound by 3 dBA to 53 dBA, and doubling again to four <br />generators would increase sound levels to 56 dBA. Figure 5 illustrates this principle. <br />1 SOURCE 2 SOURCES 4 SOURCES <br />50 dBA <br />Doubling sound energy <br />increases sound levels by <br />three decibels <br />Addition and subtraction of decibel levels <br />53 dBA 55 dBA <br />Figure 5 <br />8 SOURCE=S <br />59 AU. <br />A Guide to Noise Control in <br />Minnesota • October 2008 Minnesota Pollution Control Agency <br />5 <br />