Most of the sound is generated by the blower impeller -- the small square block of wood in the outlet reduces the dB output by close to 10 dB, so be certain that it is in place. The best strategy for noise reduction is to locate the cyclone in a closet with sound-deadening insulation and to return the air to your shop through an insulated plenum. The return air does carry some of the noise, but a plenum should reduce that significantly. Remember that when you are running many of your shop tools, you wear hearing protection anyhow; unless your concern is the baby in the crib immediately above your shop!
hey guys, jumping in late here, I had a question in regards to McRabbets post about the small piece of wood in the blower outlet, I dont see that anywhere in my unit, i have the straight transition that will be going into filters via 8" insulated pvc. I am currently in works and planning stages of building a closet and any and all advice is welcome (types of insulation, quietrock vs sheetrock, anything to get the noise of this thing down. I have a cv1800 with a 16" impeller in a 30x30' detached shop with 10' ceilings and this thing is loud, so loud that I cant carry on a conversation at all. I expected it to be loud but it sounds like a jet engine. Just wondering if this piece of wood may have something to do with it. Thanks guys.
Search "Noise Cancellation" and you will find "Quieter Blowers" started by K9-mal on 08/01/2009. Both Matt and Ed responded and Ed posted an attached drawing showing the location of the stick. They measured the noise reduction at 10 dB which helps but is no where near as much as you want / need. Search for noise reduction closets and you will find a lot of useful ideas.
Take a look at a muffler you can build, see http://billpentz.com/Woodworking/Cyclone/MufflerBelliveau.cfm This will drop the noise level about 10 db, e.g. cut it in half.
Also if you build a small room to enclose the unit use blue denim to insulate with. I used this for my 50 gallon air compressor and it was a huge improvement. I just measured the noise level, with the door open 107 db, with the door close 98 db. Thus I cut the noise level almost in half. I used a solid wood door left over from my house build and used 2x6 for the walls. The outside of the wall has sheetrock, the inside of the wall has NO sheetrock.