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Background noise <br />Background, or ambient, noise is present in any environmental noise - monitoring situation. Background noise <br />is considered to be all noise sources other than the noise source being monitored. This can include traffic, <br />animals, machinery, voices, and other sounds. <br />Wind may be a major source of ambient noise. The MPCA's noise test procedures states that measurements <br />should not be made when noise from wind or precipitation results in a difference between the background <br />sound level and noise source being measured that is less than 10 dBA. In practice, this means that wind speeds <br />must be below 11 mph, and rainy weather conditions should be avoided. <br />Background noise correction <br />Background noise could impact monitoring results. The background noise must be more than 10 decibels <br />below the noise level of the source being monitored to have confidence in the accuracy of the measurement. <br />In certain instances, when a single noise source is analyzed along with other noise sources, correction factors <br />can be used to isolate the noise source being monitored and calculate its individual noise level. This is done by <br />measuring and recording the total noise level of all sources. Next, the noise source to be isolated is turned off <br />and a noise level reading is taken with all the other existing noise sources in operation. Total noise level is then <br />subtracted from the background noise level. <br />10 <br />9 <br />8 <br />7 <br />6 <br />Rs <br />0 <br />4 <br />3 <br />2 <br />1 <br />B <br />0 1 <br />Background noise correction <br />2 3 4 <br />Difference Between Total Noise Level and Background Noise <br />Figure 6 <br />Figure 6 is a graph used to estimate the amount of background noise influencing a measurement, and the <br />corresponding decibel level to be subtracted from the total measurement to determine the decibel level of the <br />noise source being monitored. <br />For example, if the total noise level is 84 dBA, and then falls to 80 dBA when the source of interest is turned <br />off, the difference of four decibels between the total noise level and background noise indicates that two <br />decibels should be subtracted from the total. This means that an 82 dBA noise level can be attributed to the <br />monitored source in the absence of background noise. <br />A Guide to Noise Control in <br />Minnesota • October 2008 Minnesota Pollution Control Agency <br />6 <br />6 7 8 9 10 <br />