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Dan McCarthy 7 July 1993 Page 2 For the MintakeM, both sides should be lined with TECI'UM sound absorption to minimi7.e noise leakage from within the partial enclosure to the outside. Since this is susceptible to water and humidity effects, it should be wrapped in a thin weatherproof vinyl which should have minimal impact on its ability to absorb sound at the fre- quencies of interest. IT the structure were more permanent, perforated metal absorption panels might be recommended. The access door should be tightly titted to prevent the leakage of noise around the door. A typical hollow-core wood door should be sufficient for purposes of the barrier. It should be emphasized that the recommended design is conceptual and can be modified to ensure adequate function of the equipment. However, I would like to review any changes in the design to ensure that the noise barrier effectiveness is not compromised. Noise barriers to shield roofto,p units An alternative to rooftop barriers is the installation of quieter equipment. However, assuming that no change in equipment is made, the installation of noise barriers to shield the community would be most appropriate. Several problems must be addressed for rooftop barriers: - Attachment to the roof to accommodate wind loading - Sufficient distance from the equipment so as not to adversely affect ventilation - Absorbing or tilted surfaces to minimize reflection from nearby sources - Snow deposition patterns behind the barriers The construction of modular triangular barriers may be the simplest approach. This would simplify construc- tion, reduce wind loading, provide a larger base for attachment to the roof, and provide the tilted surfaces needed to minimize reflection of noise from other sources. Such a barrier could be constructed of wood or me- tal, depending upon cost and availability of materia1s. A triangular barrier made of 114M plywood or similar material would provide sufficient mass for the barrier. While the sources vary in height, it appears that a 6' height would be a reasonable compromise for these barriers. A simplified schematic of this type of barrier is shown in Attachment C. The barrier should be long enough to provide shielding of the neighborhood to the north as shown in Attach- ment D. There is still some concern that barriers near the Cyclone exhaust, even if sloped to the vertical, might reflect noise under some atmospheric conditions into the neighborhood. Therefore, the barriers near the Cyclone should not be oriented parallel to the edge of the building. IT a modular design is used, the sections should be closely titted and sealed at the joints to eliminate any openings. It should be emphasized that the equipment and barrier locations shown in Attachment D are approximate only and may have to be changed depending upon the exact placement of equipment and roof construction. An alternative to the triangular barriers is the installation .of vertical metal walls with integral sound absorption. If you are interested in this altc:rnative, I can provide information on appropriate materials.