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Orono Substation Replacement Project Noise Assessment 1.0 INTRODUCTION The objectives of this noise assessment are to establish the existing ambient sound levels in the vicinity of the existing Orono Substation in Orono,Minnesota,and to assess the potential impacts on the surrounding residential area of the proposed Orono Substation replacement which would have larger transformers. The address of the Orono Substation is 3960 Sixth Avenue North,Orono,Hennepin County,Minnesota. Potential impacts are assessed with respect to the State of Minnesota(State)Nighttime Noise Standards(NNS)and the existing ambient sound level. For continuous sources such as a substation,the L50 standard,or that level exceeded for 50%or 30 minutes of an hour,is the applicable noise standard. Since residential land uses or receptor sites are involved and the substation will be in operation during the nighttime(10 p.m.to 7 a.m.) hours as defined in the Minnesota Rules,sound levels from the substation will be compared with the L50 50 dBA residential nighttime standard(Minn.Rules 7040.0040). The State noise standards are presented in Table 1.1. Sound levels in the table are specified in terms of the dBA or A-weighted decibel. It should be noted that,while the standards are receiving land use standards and not source standards,they have been interpreted by the State as applying to sources that impact the receiving land use. Thus,the receiving land use standards do not include ambient levels not associated with the source in question. This sound level metric or measure is commonly used throughout the world in evaluating environmental and community noise levels and is a single number that combines all frequencies of sound similar to how sound is perceived by the human ear. Table 1.1 Minnesota Noise Standards Daytime(7 a.m. to 10 p.m.) Nighttime(10 p.m.to 7 a.m.) Noise Area Classification L10 L50 L10 L50 NAC-1 (residential) 65 60 55 50 NAC-2(commercial) 70 65 70 65 NAC-3 (industrial) 80 75 80 75 This sound assessment analyzes the most sensitive receptor sites which are the closest residential structures to the Orono Substation site that would be exposed to sound by the one or two transformers in the replacement substation. The residences are identified on the aerial photograph in Figure 1.1. Ambient sound levels were measured between December 5 and December 6,2011,just west of the existing substation to establish the existing ambient level over a 24-hour period. Spot readings were taken on December 5,2011,close to the existing substation to establish a source sound level for the existing transformer and to provide a basis for confirming the noise prediction model for the existing and replacement substations. The continuous monitoring site locations,as well as the spot monitoring locations,are shown on the aerial photo in Figure 1.2. David Braslau Associates,Inc. Page 1