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Noise is defined as any unwanted sound. Sounds are described as noise if they disturb <br />the person hearing them. Noise levels are measured in a logarithmic unit called a decibel <br />(dB). Humans are more receptive to middle- and high -frequency sounds than they are to <br />low -frequency sounds, so a weighted unit is used to reflect human perception more <br />closely. For the purpose of this study, sounds are measured using this adjusted scale, <br />called dBA. All references to decibels in the discussion of traffic noise impacts refer to <br />this scale. According to the MPCA publication -An Introduction to Sound Basics", a <br />sound increase of 3 dBA in an outdoor setting results in a barely perceptible increase in <br />noise, whereas a 5 dBA increase is clearly audible. An increase of 10 dBA is perceived <br />twice as loud as the original sound. <br />Under Minnesota Statute 116.07, Subdivisions 2 and 4, the Minnesota Pollution Control <br />Agency has developed Noise Pollution Control Rules (Minnesota Rules Chapter <br />7010.0001 — 7010.008). The noise criteria used in a noise analysis depends on whether <br />the land use is designated as Noise Area Category (NAC) 1, 2, or 3. NAC Category 1 <br />land use includes parks, single-family and multi -family residences, libraries, hospitals, <br />and other areas where nighttime sensitivity to noise is high. NAC Category 2 standards <br />are applied to commercial areas, hotels, and residences which have adequate acoustic <br />insulation, year-round climate control, and no accommodations that are intended for <br />outdoor use. NAC Category 3 includes industrial areas. Table 24.1 details the MPCA <br />noise level standards for each category. <br />Table 24.1: Minnesota Pollution Control Agency Noise Level Standards <br />Classification <br />NAC-1 <br />NAC-2 <br />NAC-3 <br />Land Use <br />Residential <br />Commercial <br />Industrial <br />MPCA Noise Level Standards <br />Daytime Noise Level [dBA] <br />(7a.m. —10 p.m.) <br />L10 of 65 <br />L10 of 70 <br />L10 of 80 <br />L50 of 60 <br />L50 of 65 <br />L50 of 75 <br />Nighttime Noise Level [dBA] <br />(10 p.m. — 7 a.m.) <br />L10 of 55 L50 of 50 <br />L10 of 70 L50 of 65 <br />L10 of 80 L50 of 75 <br />Traffic -generated noise can vary considerably over a relatively short period of time. <br />There are two analytical approaches which may be used for reporting traffic -related noise <br />levels, the first of which uses L10 and L50. For these values, the subscript value refers to <br />the percent of time during a one hour period that the noise level exceeds the specified <br />value. For example, an L10 value of 65 dBA during the peak hour indicates that the noise <br />level exceeded 65 dBA 10% of the time, or for 6 minutes during that hour. The second <br />approach, used in this report, uses Leq. This value represents the equivalent of a constant <br />sound level which, over a period of time, contains the same average amount of sound <br />energy as the varying level of traffic noise. According to the Federal Highway <br />Administration noise abatement procedures detailed in the Code of Federal Regulations <br />(23 CFR 722), Leq for typical traffic conditions is usually about 3 dBA less than the Llo <br />for the same conditions. This rule has been used to create an equivalent table of Leq <br />values based on the MPCA Noise Level Standards and is presented is Table 24.2. <br />23/24-2 <br />