Paging Rod: Isolating Return boxes for HVAC

So the HVAC guy came out the other day and began putting in one of the returns for the studio. My first thought was to remove the screws attatching it to the wall framing, add some 2" neoprene to the side where it will attatch, and to the back where it touches the wall. I am attatching a picture of that. Upon further thought though I'm wondering if I need to do anything else to the box to stop it from becoming an unwanted "musical instrument" in my studio. It is lined with fiber-glass as you can see, but I am very concerned about the thought of using a steel product like this - that at some frequencies this thing will just start vibrating or making some kind of noise. What do you guys think? Is there something I can add or an alternative to using these kinds of boxes? Thanks, Andy

HOLEy mackerel! :shock: Is that rectangle the size of the register the room will have? The size of the hole in the drywall? :( Oh my, that's going to hurt isolation big-time... :roll: I'm worried! :? --Keith :Mrgreen:

okay so I'm right to be concerned and not getting overly critical here... so what do I do Keith?? :? What should I be having him do instead? What have others done for their returns? :?

Maybe Rod can help? I'm shining the big "R" light into the sky... I searched around on the forum as well as looking Rod's book but see no discussion of undersized returns. I do however agree with Keith - and this box is a real cause of concern to me... help!!!! :?

andy_eade wrote:

I'm shining the big "R" light into the sky...

:lol: That's hilarous! :lol: We need a graphic artist to illustrate what that would look like! :twisted: In my case, I took my cue from Paul Woodlock. I will be building large and beefy soffit-type silencer plenums for my supply and return ventilation ducts: two boxes inside the room, one inside the laundry room, and one inside the garage. These boxes will be made of two or three layers of MDF, plywood, or whatever else I decide to work with. Incoming air will get sucked from the outside, through duct #1 and into the heat recovery ventilator (HRV) where the fans are located. The air will then flow through duct #2 into the box in the laundry room, which will be lined with insulation and maybe a couple of baffles to "zig-zag" the air flow a bit. At the other end of the box will be a hole that goes through the wall and into a flex duct. The flex duct bridges the gap between outer and inner leaves. The other end of the short flex duct terminates to a similar sized and configured box on the inside of the room, with a register at the end. Outgoing air follows a similar path in reverse. I'm not saying you have to go to the same extent that I am, but hopefully this concept will help you understand the concept. I have yet to draw this plan up in detail -- and I'll need to do so soon. I don't think I'll have a chance to do so before you're ready to move forward though... :roll: In short, I think your existing plan is flawed, but I would think that even putting that box of yours inside the room, thus minimizing the size of the penetration in your wall, would be an improvement. How much of one, though, I can't say. --Keith :mrgreen:

How about if I covered the box with 2-3 layers of drywall on all 5 sides? Then it would be the same mass depth as the wall... Then I'm only really dealing with noise getting into the ductwork which I'm less concerned with...

That's the right idea... If you can afford the space in the cavity. And using lined ducts helps to absorb sound, or at least keep it from echoing around, along the duct path. This is not my strongest suit, mind you... My words are no replacement for Batman's. ;-) In the event of a dispute between my words and his, I lose, every time! (And I'm quite OK with that! :lol:) --Keith :mrgreen:

Just thought I would add some background info to help put everything into context here since I'm not sure if all this is in my thread. So my plan is basically what Rod talks about on page 134. I am modifying my existing HVAC system. We have added a fresh air intake system with a timer, as well as motor controlled dampers in the ductwork to push heating / cooling to the house or CR or studio based on the three thermostats in the house - one upstairs, one in the CR and one in the studio. Both CR and Studio have independant supply's and returns, both or which's ductwork contain multiple 90 degree turns and is done using flexduct (not popular with some I know). Returns are going low to the ground and supplies up in the ceiling I think that;s about it, but please let me know if there are any questions. Look forward to hearing your suggestions, Andy

Andy, buildinga 5 sided box to install the plenum in is the right way to go...... and it seemsto me that you have plenty of space to do this. I would(however) build it seperate from the duct and then slide the duct into it - this way you could set it on some neoprene spacers to keep it even that much more isolated from the wall assembly. I would (however) make the final connection through the use of 4 continuous pieces of bent metal - attached at the fron face - and installed over my last layers of drywall - This metal (made from the same gavanized duct metal) should be bent about 1 to 2 degrees past 90 - and then you can use it like a corner bead to finish tape (and seal) the opening before instaling your return air register. I hope that I am seeing what I see there because your contractor understood all of my comments regarding large volume and low velocity........ As long as noise in the adjacent space actually passing into the duct isn't a bit BIG concern - then you should be fine. I guess I am going to have to add substantially to that chapter when the 2nd edition comes down the pike. Sincerely, Rod