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Enterprise Park to Crooked Lake 115 kV Transmission Line March 2012 Environmental Assessment Table 5.3: Conductor Noise Estimates — Heavy Rain Case Measurement Location Edge of right-of-way Directly under the line L5 L50 17.7 dBA 14.2 dBA 18.8 dBA 15.3 dBA BPA developed general guidelines based upon public response to alternating current (AC) transmission line audible noise. The guideline indicates that numerous complaints can be expected if the line noise exceeds approximately 58.5 dBA and that few complaints should be expected if audible noise is limited to 52.5 dBA, values relatively consistent with the Minnesota noise rules. The calculated values for the proposed transmission lines are well below the state's standards and BPA's guidelines. The audible noise will be barely perceptible during fair weather. Noise associated with substations includes the operation of transformers and switchgear. The transformers produce a constant low -frequency humming noise, while the switchgear produces an impulsive or short duration noise during infrequent activation of the circuit breakers. Due to the infrequent operation of the switchgear, the noise generated will be considered temporary in nature and not predicted to exceed the MPCA Noise Limits. Modifications at the Crooked Lake Substation will not include the installation of a new transformer. Accordingly, the only new noise source will be from new switchgear. Modifications at the Enterprise Park Substation will include the installation of a new 115- kV step-down transformer. A typical transformer of this type would have vendor noise level guarantees measured one meter from the equipment of 75 dBA when the fans are running To conservatively predict future noise levels and compliance with the 50-dBA limit, the 75-dBA noise level was treated as a point source at the transformer and modeled to determine the distance where the noise levels would be reduced to 50 dBA. A simplified, conservative model was created to determine the distance at which the noise would attenuate to 50 dBA (the NAC 1 nighttime L50 noise standard). Noise propagation through the outdoor atmosphere typically decreases in level with increasing distance between the source and the receiver. The noise attenuation is the result of several mechanisms, including geometrical spreading of the sound waves, shielding provided by physical structures, atmospheric absorption of the acoustic energy and ground effects on the sound waves. In general, the noise or sound pressure levels emitted from a substation will decrease approximately 6 dB for each doubling of distance from the source to the receiver. The simplified model was prepared based on this 6-dB reduction with a doubling of distance. The model is conservative in that it does not factor in geometric spreading or any attenuation from shielding or ground effects. 27