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david br~ b associates. incorporated 1313 5th street s.e.. suite 322 · minneapolis. mn. 55414 . telephone: 612-331-4571 7 July 1993 Dan McCarthy Facility Manager Advance Circuits, Inc. 1633 Terrace Drive St. Paul, MN 55113 RE: Noise Control Recommendations Dear Dan: Based upon sound level measurements at your facility and in the adjacent neighborhood, several recommenda- tions for reducing noise levels are proposed. It is hoped these will reduce the noise level in the community by 5 of 10 dBA. We are continuing to evaluate the community data which show an increase in background noise level at night and some higher noise levels in the 5 am to 7 am time period. However, this should not affect the recommendations contained in this letter. Our sound level measurements showed that the three noisiest sources affecting the adjacent community are: (1) Central Vacuum Cyclone (2,250 cfm) (2) 3,000 dm scrubber (3) Open freight door at the west end of Building 1633 The 3O-ton condensor, 31,000 cfm scrubber and the.po' Unit on Building 1633 are only several dBA lower than these other sources and should probably also be shielded to reduce noise in the community. A comparison of the overall A-weighted sound levels due to these sources when projected at the Anderson residence is shown in Attachment A. Three types of recommendations to control these sources are described below. Partial Enclosure around Central Vacuum Cvclone A schematic drawing for this partial enclosure is shown in Attachment B. Since this equipment will be replaced within about a year, the proposed structure is not considered to be permanent. However, sufficient mass and strength will be needed to shield noise emissions from this equipment. The barrier is designed to allow air flow from the bottom while minimizing transmission of noise to the outside of the barrier. It is shown with access to air on the west side. Depending upon the volume of air needed by the eq~pment, a similar .intake. on the east side next to the stairway could also be included. The barrier should ex- ten'd 6' above the roofline to shield the vacuum exhaust which is itself a major noise source. The barrier can be constructed of 3/8. plywood or similar weight material. Studs should be spaced no more than 24. on center to minimi7.e vibration of the plywood itself.