As with all matters acoustical, things seem to come in waves. The latest revolution of problems to solve: private offices along window walls in office buildings. People have been complaining they hear everything that is said on the other side of the wall. While the problem might indeed be the wall itself, it is also likely to be the intersection of window and wall. The trick here is to get two ridged items to stay sealed airtight as they flex and move with the building during the heating and cooling cycles most notably found at the window.
From the viewpoint of an office designer, the best thing to do is to align the office walls with the window mullions. Ideally, the wall butts right up against the mullion so that we can then caulk it airtight. An airtight seal is your first priority in blocking the flow of noise. If necessary, we use a faux mullion between the wall and the window, and seal it to the back side of the window and to the wall. If you can run your business card between the closure and the window or the wall, your seal is not airtight and you might have a sound problem.
If the mullion-wall intersection is sealed airtight, and there still is a sound problem, then we likely need to add mass to the mullion system. After all, typically we are working with components of light-gauge aluminum that do not have the mass of the adjoining wall. This is where acoustical mass and damping products can be used. Netwell, a distributor, is one good source for obtaining such materials because they accept small-quantity orders.
Each situation is a little different, depending on the combination of wall, window and mullion. Remember the first goal is an airtight seal, and after that if necessary explore methods for beefing up the wall system to further block the noise.