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<br />Addition and Subtraction of Decibel Levels <br /> <br />In many situations pertaining to noise control and monitoring, it is very useful to be able to add <br />and subtract noise levels. A doubling of sound energy yields an increase of three decibels. It is <br />important to note the characteristics of logarithmic addition or subtraction of decibel levels. <br /> <br />Addition and Subtraction of Decibel levels <br /> <br />1 SOURCE <br />50 dBA <br /> <br />2 SOURCES <br />53 dBA <br /> <br />4 SOURCES <br />56 dBA <br /> <br />8 SOURCES <br />59 dBA <br /> <br />~:.( ~- ~:.( ~- ~:.( ~- ~:.( ~-~:.(~- <br /> <br /> <br />.... ......... .......... ........ ...... , ..... <br />/, /, /, /, /, <br /> <br />~~~>- ~:.(~- ~:.L~-~:~>- <br /> <br />.... ........ .......... ...... .... ....... <br /> <br />/ <br /> <br />, <br /> <br />/ <br /> <br />, <br /> <br />/ <br /> <br />, <br /> <br />/ <br /> <br />, <br /> <br />,/ ,/, / <br /> <br />~-~- ~~~-~..~- <br />.... ........ .... .... ...... <br />/" /" / " <br /> <br />,/ ,/, / <br /> <br />~_~- ~_~-~..L~- <br />.... ........ ..... .... .... <br />/" / '" / " <br /> <br />Figure 2.1 <br /> <br />Background or ambient noise is present in any environmental noise-monitoring situation. <br />Background noise is considered to be all noise sources other than the noise source being <br />monitored. Wind may be a major source of ambient noise. The MPCA's noise test procedures <br />state, <br />Measurements must not be in sustained winds or in precipitation which <br />results in a difference of less than ten decibels between the background <br />noise level and the noise source being measured. <br /> <br />7 <br /> <br />1__ <br />