Where single-number sound performance ratings, such as sound transmission class (STC), are available for individual components of an assembly, can these values be added together to determine an STC rating for a complete assembly?

As stated in Section 1.1 of ASTM E90, the scope of this standard is limited to building partitions such as walls, operable partitions, floor-ceiling assemblies, doors, windows, roofs, etc.  Although it is possible to use ASTM E90 to assess the sound transmission performance of individual components that would be used within an assembly, such use is not within the scope of the standard and is typically only done for the purpose of research.  Furthermore, sound transmission class (STC) values derived for individual components of an assembly are not necessarily additive.  In other words “component-additive methods” (CAM) do not necessarily yield accurate estimates of sound transmission through a complete assembly.  This is especially the case when dealing with assemblies that have a cavity or void through the thickness (e.g., a stud wall with cavities between the studs).  That said, there are other models (besides CAM) for estimating sound transmission performance that yield more accurate results.  For example, AWC’s Technical Report 15 (TR15) provides models that can be used to estimate STC and impact-insulation class (IIC) ratings for light-frame floor/ceiling assemblies (https://awc.org/publications/tr-15-calculation-of-sound-transmission-parameters-for-wood-framed-assemblies).