Sound Through Forced Air Ducts

Discussion in 'Home Theater Projects' started by Tom_K, Apr 7, 2004.

  1. Tom_K

    Tom_K Auditioning

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    I have a forced air heating and cooling system which would be used for a basement HT. How do I stop sound from travelling through this type of system to the rooms attached to the main truck lines?

    How do you stop this? Thanks
     
  2. Frank Zimkas

    Frank Zimkas Supporting Actor

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    Almost impossible to eliminate the sound entirely, but you can cut down the noise quite a bit by using Flex-duct and putting in a couple long radius 90deg. bends. Short radius bends can restrict the air flow too much and then you've got inadequate Htg/Cooling issues to deal with.
     
  3. Tom Kay

    Tom Kay Stunt Coordinator

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    Hi, from one Tom K to another;

    Here's a thought about the sound transmitted through ducts:

    I have not tested this yet, but it seems likely to work. I have attached two 5 inch diameter ducts to the end of the main (big)duct, with tapered metal cones, so that the air can flow nice and straight out the end of the big duct. Then, the air in both 5 inch ducts, passes through a soft coupling, turns a nice smooth 90 degrees, down a foot, through another soft coupling, another 90 degrees and out into my HT. I have observed the concept of making the airflow turn 90 degrees, nice and smoothly to minimize airflow losses, but hopefully these two turns will kill some sound bouncing back into the ducts, to the furnace and the rest of the house.

    But wait ! What's this soft coupling, you ask ?? I'll tell you !

    Just as an experiment as much as anything, I cut out a one foot chunk of each 5 inch duct, almost like a bridge with a center portion collapsed and missing. Then I used artificial leather ( I was a hockey equipment maker in a past life) to essentially create a tube to take the place of
    the missing metal, and hose-clamped each end of the soft leather tube at the cut areas of the metal duct. If you didn't know the metal was missing, it would look like a normal round duct with a big band-aid wrapped around it.

    I did this because I had a thought that if you create a disconnect at one or two places in the duct, any sound that is bouncing around inside would hit this soft coupling and die right there. The soft coupling is still very smooth inside, and allows the air to flow very well, but should help in reducing the amount of sound energy that makes it all the way back to my furnace.

    My idea was based on the whole resilient channel thing, of creating a sound disconnect from the wall to the framing behind it. I wish my HT were done, so I could brag if it worked, or moan if it didn't, but I figured I had nothing to lose. The soft coupling is made of soft but durable material used to make hockey goalie pads, so I know it's tough. It won't dry out and crack, even when blasted with warm air. Real leather might crack after years of that.

    I will certainly let the forum know how this works when tested. I hope you get something useful out of this idea.

    Cheers, Tom Kay, Ottawa Canada.
     
  4. Tom_K

    Tom_K Auditioning

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    Thanks -- I am still in the planning phase so you are ahead of me. I understand what you did nad looking forward to hearing the results
     
  5. John Sayers

    John Sayers Auditioning

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    Tom - it can be done but you will need acoustic ducts to do it. By acoustic ducts I mean ducts that are lined internally with acoustic absorption - typically a foil faced rigid fibreglass on all four surfaces or you can just box in insulation lined flexible ducts.

    You can build you own ducts out of MDF or Plywood. Basically it means building rectangular ducts and lining them with absorption. The more corners you bend around the better and the bigger they are and the slower the air movement the better also.

    Unfortunately as this is only my 3rd post here so I'm not allowed to give you a link to more info but it you click on my web site link and follow the links to "studios under construction" and check out "long sought for studio" you can see photos of what others have done in this situation using plywood boxes and flexible ductwork.

    cheers
    john
     

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