The sections of the IBC and IRC that deal with sound transmission are IBC Section 1206 and IRC Appendix AK.  These code provisions apply to wall and floor/ceiling assemblies separating dwelling units from other dwelling units, or from adjacent public areas, within a building.  These provisions do not deal with sound transmission from outdoors to indoors, through an exterior wall.  Although US model building codes do not contain provisions regulating minimum sound transmission performance of exterior walls, there are some cases where designers specify a minimum level of sound transmission performance for exterior walls on specific construction projects.  In such cases, the most direct and accurate way of assessing sound transmission performance of an exterior wall is through field testing in accordance with ASTM E966 and analysis in accordance with ASTM E1332.  The single-number ratings resulting from this test and analysis procedure are outdoor-indoor transmission class (OITC) and outdoor-indoor noise isolation class (OINIC(θ)). Conversely, sound transmission class (STC) is determined through laboratory testing in accordance with ASTM E90 and analysis in accordance with ASTM E413.  This testing and analysis is most commonly used for assessing the sound transmission performance of interior walls and floor/ceiling assemblies separating dwelling units from other parts of a building, as these are the only places where US model building codes require minimum sound transmission performance.  While it is possible to perform an ASTM E90 test on an exterior wall assembly in a laboratory and determine an STC rating for that assembly using ASTM E413, this is less commonly done because building codes do not place minimum performance requirements on sound transmission through exterior wall assemblies and because OITC and OINIC(θ) ratings provide in-situ assessments of exterior wall performance.