I built a soundproof equipment closet to house my compressors, phase converters, a/c, and dust collector. I'm in the process of building the return air plenum. I have a 30x14 filter grill on the closet side and an identical 30x14 filter grill on the shop side at the same elevation which I will be connecting with 30x14 ducts made from 1-1/2" fiberglass duct board.

There will be about 24 linear feet of duct comprising of a 90° elbow at each grill and (2) 90° elbows to comprise a 180° bend. I was originally planning on making 3-piece elbows to increase efficiency but now I'm wondering if the extra turbulence with hard 90°s would be better for soundproofing. Which type of elbow would be better to use for soundproofing?

workshop return air plenum v1.jpg workshop return air plenum v2.jpg


In addition, I'm considering expanding the width of the last long section of duct so that a pair of baffles could be added but I'm concerned about restricting the opening which ends up being about 20x14 at each baffle. Thoughts?

workshop return air plenum baffles.jpg