I recently purchased and installed a CV1800 with 16" impeller. I knew the noise was going to be a problem, as my shop is the first level of the house, so I built a 2x4 frame enclosure with 24"x48" interior space, and placed it on some old gym/weightlifting rubber tiles to isolate it from the floor. The cabinet is also 3" lower than the ceiling. The exterior is about 50% solid core doors (pockets I replaced with swinging doors), with sheetrock added on the visible sides. Rather than installing a door, I made two, 48" high hatches in front of the cyclone body out of 1/2" ply and 3-1/2" sides of 1/4" ply, attached to 3/4x3/4 braces glued to the hatch. The inside of the cabinet I lined with rockwool insulation, and stapled some moving blankets (bought off CL) to the studs. The back end of the closet is open, and covered with 3 "curtain rods" of doubled moving blankets (stacked and staggered) so that the slightest overpressure will allow return air. The top 2" of that area is open, so the motor can breathe. Leaving the end "open" allows full access to the filters, and the sound levels are the same there as to the side of the enclosure (with blankets in place).
dB was 91 around 6 feet from door without hatches. With hatches in place but a 12" opening above them (no soundproofing in the hatch) it dropped to 78. Getting somewhere. I added 3 layers of moving blankets to the hatches, and closed off the very top of the cabinet with some plywood leaving a 5/8 gap at the top for more venting. Sound levels are now down below 75dB - the wind noises at the machines are louder than the DC.
Total materials were the 2x4s for the frame, 2 sheets of 5/8" sheetrock, 1 bundle of rockwool batts, little over 1/2 sheet of 1/2" ply, and around 1/4 sheet of 1/4" ply. As I said, I used 3 doors, and various OSB scraps around the house, and 10 moving blankets. It was some work, but what an amazing difference. Not annoying at all now.