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WSB Engineering Carwash Noise Impact Assessment David Braslau Associates, Inc. Page 16 7.0 NOISE MITGATION OPTIONS 7.1. Minnesota Noise Standards As noted in Figure 5.1, the daytime L50 level of 60 dBA is predicted to be exceeded at residential receptor sites closest to the carwash entry and exit. The simplest and most effective way to bring these levels down to or below the 60 dBA standard is to close the entry and exit doors when initial power washing and final drying takes place. Even a clear vinyl with a surface weight of 1 lb/sq can provide significant sound reduction. Figure 7.1 shows the reduction in the sound level spectrum that could be achieved with a closed door of even this material. The A-weighted decibel could be dropped at receptor A1 from 63 dBA down to 36 dBA. Also, the level at Homes 3 through 6 could be reduced from 63 dBA associated with the dryer exit down to 36 dBA as well. Therefore, to ensure the daytime noise standard is achieved, it is recommended that the entry and exit doors be closed during initial washing and final drying of vehicles. 7.2. Background Noise Impact Figure 7.1 also compares the door open and door closed spectra with the background level estimated at A1. However, closing the doors will have no impact on the exterior vacuum units. Figure 7.2 shows the sound level spectra for three individual vacuum units, the combined level with three vacuum units operating at the same time, and the combined level with six vacuum units operating at the same time and compares this with the quieter background sound level at receptor A1. Similar comparisons, perhaps not as severe, could occur at all of the Kenzington receptor sites. In Figure 7.3, the City has recommended shielding the vacuum units with some type of a partial enclosure open only to the south to allow for vehicle parking while vacuuming. This is shown schematically in Figure 7.4. This recommendation is problematical for a number of reasons, some of which are the noise paths to Kenzington that could still occur with the partial barrier. In addition, to be effective, the barrier would have to be sound absorptive on interior surfaces to keep levels down to acceptable levels for users and Kenzington receptors. A more preferred option to this partial barrier is to put the vacuum motors in an acoustical enclosure with only the hoses coming out of the enclosure. This alternative was mentioned in a document on the website carwash.com. Some air suction noise at the cars will still be audible at Kenzington but the higher frequency sound will decay faster in the atmosphere. A canopy could be helpful to shield this noise but might still expose the Kenzington units closest to Kenzie Terrace to some noise. Another way to minimize vacuum sound level impact on Kenzington would be to provide barriers with sound absorptive surfaces between each vacuum position. How to shield residents from vacuum noise will require some further analysis. 82