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necessary to contact window and door manufacturers or their representatives <br />directly if published test data are not readily available. STC estimates for <br />custom-built windows must be determined by comparing these windows with other <br />manufactured ones for which such data are available. <br />Detailed test data are generally available for acoustic -rated windows, which <br />are specially constructed to minimize sound transmission. These windows are <br />generally constructed of metal because of the need to build in the sound and <br />vibration isolation details that are essential to achieving a high STC rating. <br />Nonacoustic-rated windows, for which STC data are not generally available, are <br />commonly proposed for use in residential construction. For Noise Zones III and <br />IV, the careful use of nonacoustic-rated windows may be sufficient to meet the <br />Metropolitan Council guidelines. However, as the need for greater window <br />attenuation increases, it may be necessary to employ only acoustic -rated <br />windows to meet the guidelines. <br />OTHER SINGLE -NUMBER TRANSMISSION RATINGS <br />Exterior Wall Rating (EWE) is another single -number rating that has <br />been developed specifically to address the problem of exerior-interior noise <br />transmission. The EWR standard curve is similar to that of STC except that the <br />lower frequencies, normally associated with transportation noise, are given <br />more emphasis. An analysis of selected building partitions suggested that a <br />standard'relationship can be established between EWR and STC, at least within <br />the range of typical building materials used in cold -climate residential <br />construction. Since most partitions are still rated using the STC rating, <br />selected EWR data have been adjusted to an STC rating for use in this Guide. <br />REASONS FOR EMPLOYING STC VALUES IN THIS GUIDE <br />In the development of the Model Ordinance for Aircraft Noise, the decision was <br />made to use STC, rather than other single- or multiple -figure ratings, <br />because extensive STC data are available to architects and building code <br />officials. A subsequent evaluation of other single -figure ratings, including <br />the Exterior Wall Rating (EWR) and Shell Isolation Rating showed the need for <br />establishing a data base, especially for new products. The participants in the <br />drafing of the ordinance felt that it should require the minimum amount of <br />additional effort and cost for builders and that additional laboratory testing <br />should be avoided where possible. Thus, it was decided that STC ratings would <br />provide the simplest alternative. <br />42 <br />