Great thread! I've got one of the diaphram compressors and it's so noisy it does drive me out of the garage, so I've been thinking about a soundproof enclosure, too. I've got all my materials for a good plumbed air suppy system, but I need to find a permenant place to quiet the thing down.

The wall ideas so far sound great. I've been trying to figure out a good vent system for the closet. The pegboard posted idea kind of clicked. If I was to frame an opening and then build a baffle inside, using multiple layers of pegboard, staggered, with about a 3/8 airspace between, it might do the trick. The pegboard I have has 3/16 holes on 1 inch centers. Anyone know how to calc. how small that opening can be? Like a rough square meter/cfm or something?