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EXTERIOR BUILDING SHELL NOISE REDUCTION <br />OVERALL NOISE REDUCTION LEVEL REQUIREMENTS <br />The overall Noise Reduction Level (NRL) required within a given noise zone can <br />be determined by subtracting the desired interior level (45 dBA) from the <br />highest noise level within that contour. For example, in Noise Zone 4 (65 to <br />70 dBA), the required reduction is 70 - 45 = 25 dBA. The structure performance <br />standards adopted by the Metropolitan Council are shown in Table C-1. <br />TABLE C-1 <br />STRUCTURE PERFORMANCE STANDARDS1 <br />Land Use <br />Residential <br />Educational/Medical <br />Cultural/Entertainment/Recreational <br />Office/Commercial/Retail Services <br />Industrial/Communication/Utility <br />Agricultural Land/Water Area/Resource Extraction <br />Typical Interior2 <br />Sound Level <br />45 dBA <br />45 dBA <br />50 dBA3 <br />50 dBA <br />60 dBA <br />65 dBA <br />1These performance standards do not apply to buildings, accessory buildings, <br />or portions of buildings that are not normally occupied by people. <br />2The noise description used to delineate the appropriate noise policy zone is <br />annualized Ldn. <br />3Special attention is required for certain noise sensitive uses, such as <br />concert halls. <br />Each local government that has land within an airport's noise zones will be <br />responsible for implementing and enforcing these performance standards within <br />its jurisdiction. The Metropolitan Council will review the adequacy of these <br />mechanisms as part of its review of amendments to each community's <br />comprehensive plan. <br />This Builder's Guide has been developed to assist in preventive treatment. <br />Corrective treatment can be more complex, but could be addressed with this <br />Guide, providing that appropriate simplifying assumptions can be made. <br />EVALUATION OF COMPOSITE EXTERIOR SURFACES <br />Since a building is made up of a number of different elements, i.e., walls, <br />windows, and doors, the "composite" noise reduction of this combination must be <br />determined. <br />This evaluation is made in terms of energy passing through the wall. The <br />energy is then summed and converted back into the Noise Reduction Level in <br />dBA. Figure C-1 is a graph permitting the evaluation of such a wall with two <br />separate elements, each with different transmission loss values. <br />43 <br />