One of the most challenging aspects of building acoustics is that sound insulation and sound transmission paths are present in all three dimensions, when flanking transmission is included. If you have two rooms next to each other that requires a certain sound reduction index R´w in dB between them, the first thing that comes to mind is the directly separating construction, i.e. the wall between them or the floor structure. Most of the time, you will be correct, as this is often the most important transmission path. Hence, it is easy to miss that sound transmission occurs in many other places in a real building, which are not obvious at all when looking at a 2D drawing.