-heavy duty metal bars and L joints to hold the window plugs in place

Step 1 - Start with a sheet of drywall approximately the same depth or slightly deeper than the inset between the metal frame and your window panel and cut it to fit into perfectly into the metal window frame to act as the first reflective surface that covers the window glass.

Step 2 - Cut you acoustic fiber sound board to fit the area against your windows metal frame leaving enough room for for the acoustic barrier to be tightly wrapped around the sides of the sound board and still have a firm fit within the window. Mark the area where the drywall will attach onto the the acoustic fiberboard so that it will line up with the window glass area, but do not attach it.

Step 3 - Time to attach your acoustic barrier material to the acoustic soundboard. The acoustic sound board is not a solid wood material, more like a lightly compressed layered fiberboard which does not hold glue or staples well. (You will be attempted to use screws to ensure a stable build, which could be a good idea for the outside frame of a full sized window box with 4-6 inches of open space in the middle. However, a large amount of sound is transferred through even very minute penetrations between sound barriers or absorption levels.) I went with the heavy duty staples and industrial adhesive. This was effective but it required a relatively large quantity of staples to hold the relatively inflexible acoustic barrier down after it was wrapped around the corners. While you are probably thinking that multiple staples are penetrating the acoustical barrier enough to weaken the reflective properties of the barrier anyway, here is how I fixed that problem. The acoustic barrier is fully attached to the acoustic fiberboard only by adhesive on the side facing the room and the corners keeping the barrier material 100% intact and working at full potency.

Due to the propensity of the acoustical fiberboard to shred off its outside layers when glued, it is best to use a liberal amount of glue, apply it with the acoustic barrier down, apply weights during the adhesive drying process, allow extra drying time and not move it until the overlapping corners and sides are glued and stapled heavily to keep it tight and in place. The overlapping area should be as close as possible to a perfect boarder for attaching the drywall inner window reflective surface. Keep in mind that the acoustic barrier around the boarder of the acoustic fiberboard may alter the placement of the drywall into the inner window slightly. (The raised boarder caused by the acoustic barrier is why I said your inner drywall insert can be slightly wider than the depth of the window frame in Step 1, as the raised boarder will pull it out a bit.)

In order for the staples to hold it is best to have them attached half on the acoustic barrier and half on the acoustic fiberboard otherwise they will tend to pull out. The overlapped stapled areas should also have a liberal use of adhesive to help seal the holes in the materials as well as share the load. Also, keep pressure on the overlap because the stiffness of the material will pull out the staples until the glue and a large number of staples can support it with enough strength. Be careful to cut the acoustical barriers to at matching angles so they meet flush and adhesive / staple them together as well.

Facing Outside

Step 4 - The area of attached overlap should form a boarder for attaching your drywall sound barrier with the industrial adhesive. The drywall can still attach with a little overlap over the acoustic barrier material, but it will not hold as solid and will require a large amount of adhesive to create an even seal. Once everything has has a chance to a full dry time, (I personally wait longer than the adhesives instructions by at least 4 hours because of differences in temperature and humidity), place this base barrier into the window to ensure a solid fit and that the drywall fits appropriately into the inset window glass area. If the dry wall does not fit flush make sure that you did not simply attempt to place it in upside down, if needed you can trim the drywall to fit as well as possible by using a box knife. Be careful not to damage the acoustical barrier material in the process.

Step 5 - Attach a second layer of drywall to the inside acoustical barrier using only adhesive. Be sure that the drywall comes all the way to the base of your window plug. Remember that this side of your acoustical barrier is attached only by adhesive and if the drywall does not rest on your window sill then it will certainly rip out the current connection between your acoustical barrier and the encompassed acoustical fiberboard. This should also dry with considerable weight on top of it. I also walked the board to ensure a tight seal.

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Step 6 - Move the base window plug carefully into place being careful to support the boards from the sides to ensure that they do not pull on your adhesive bindings. Now it is time for the light test... This is best done in a pitch black room at a time of day when the window gets direct sunlight. Hold the window plug in place and start hunting for light leaks. If there are light leaks, then there are sound leaks! Unless you are very lucky, talented or have a window shape that naturally helps lock the plug in place tightly then there is a bit more work to do. My solution to the light/sound penetration problem was to attach strips of sound barrier that had about a 3 inches of overlap around the drywall that faced the window. (Once again raising the boarder around your inner drywall insert and decreasing its depth as mentioned in Step 3/Step1) The pressure of the stiff overlapping material helped to hold it in place, blocked the light and made a considerable difference in the sound penetration. The overlapping sections will take considerable pressure from the window pane and will require a good amount of adhesive and heavy duty staples to hold firmly in place. The staples should be done while the adhesive is still wet in order to help fill the holes made by the staples and help to support their holding power.

Step 7 - Unfortunately something must be used to ensure a firm pressure against the window. I choose a metal bar that is screwed into the wooden beam between the windows and held the plug into place with pressure, and a 90 degree metal joint to hold the bottom in place. To avoid low frequency transfer between the window plugs and the wall through the bar/wood/screws, I added acoustic barrier to each point of contact of the metal bar. For a more easily removed version you could attach a sliding bolt locks instead.

Step 8 - I built another absorbing layer to help make up for the lack of 4 to 6 inches of open space. It is quite simply 1" thick acoustical foam with linoleum flooring glued to one side and insulation baffling glued to the other side and then the insulation is covered with a heavy knit fabric and attached in the back by adhesive and glue. this gives a loosely packed area for low frequency collision and provides another absorptive and reflective surface to help block the remaining frequencies.

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Step 9 - I covered both window openings with a few of audimute's sound absorption sheets that I got on a clearance sale because of the sound reflection coming off of the window plugs. I know that those blankets are not the best solution, but they have definitely helped improve the higher frequency room reflection problems and I needed something to cover those window plugs.

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Total project time including construction time and trips to the hardware store was about 11 minutes. Ok, it was more like 2 full days, but a lot of that was because of drying time and hardware hunting. They have proven to be extremely effective against low frequency road noise, motorcycles and aircraft (those are my ultimate test sounds as they are amongst the hardest to block) as well as the general city sounds.