So here's how I've accomplished 22 dbs of sound reduction...
Nicely done! I've always said that a good enclosure can be made on almost any budget.
Sheetrock is more than acceptable, especially when there is a flexible layer in between (Green Glue is commonly used).
The kirby is 101 dbs
Vacuum cleaners are effective but the fine particles will often dramatically reduce their life. It's very important that the exhaust be filter otherwise this can INCREASE human exposure to dangerous particulates. If one cannot filter, consider exhausting the air outside the building.
I built a baffled intake/ exhaust
Air going and air going out "mufflers" are very important for compressor and vacuum sound reduction. Sounds like you're approach is very good.
(1 micron filter bag)
HEPA filters are on the order of 0.5 microns. A 1 micron filter is quite good... and often cheaper. Remember that your recycler system is going to quickly fill the filter. Carefully and slowly, bag it, take it outside and wash it out. Let it dry thoroughly before using it again.
Do you (whoever is reading) think encasing the vacuum's enclosure with the foam on the outside will benefit me at all.
Foam that works well for sound is different than many other types of foam. My opinion is that using furniture foam would be a small improvement at best.
I have a sheet of plexiglass and plan to triple pane the door, why not?
Windows are often an easy way for sound to escape. Double and triple panes are an excellent idea to reduce the easy of sound escape. Switching to solid walls and a camera - given that cameras aren't expensive now-a-days, is a better way to keep the sound in.
I'll try it in a couple weeks. Also (hey Mark) I supported the bottom of the enclosure with an extra vertical 4x4 in the middle of the bottom to eliminate the "drum" problem you mentioned earlier.
The Dirichlet problem is very real, but it's affects vary with the materials used, the scale involved (distances), and frequencies involved.
Not long ago I had someone contact me about a CNC enclosure not being anywhere near as quiet as expected, particularly at the lower frequencies. I reviewed the enclosure and asked that two cross members go under the deck that the CNC rested on. Once done, the results were quite satisfactory, many decibels less.