Replacing 6" duct with flex duct to decrease sound

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Old 08-29-03, 04:41 PM
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Replacing 6" duct with flex duct to decrease sound

I'm trying to minimize sound transmission through the ductwork since many of the ducts are overhead in my soon to be basement theater. If I replace 6" sheetmetal duct with 6" flex duct will I get a decrease in flow because of the increased surface area of the flex?

Besides using flex duct, is there any way to treat the existing ducts to minimize sound transmission (ie wrapping them with some sort of insulation)?

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Old 08-29-03, 05:06 PM
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flex duct

You dont say how long the runs would be. To try and insulate the pipe that you have now would still have sheet metal pipe inside so the sound would still be there. Yes you will have a little more drag with the flex but I dont think you will feel it. As this is inside. You could like put flex from the take off to the boot if you need a 90o bend use a metal L for it. Hang the flex up as flat a run as you can. ED
 
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Old 08-29-03, 05:35 PM
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i would

agree with ed. be sure to pull the inner liner tightly to reduce air friction
 
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Old 08-29-03, 07:20 PM
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Ed, all of the runs are less than 12 feet and are all straight. So I can easily replace the 6" duct with flex duct.

Any suggestions on handing the rectangular trunk duct? Someone suggested using that spray on sound dampener that one can get at an autopart store.
 
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Old 08-30-03, 07:55 AM
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NO NO

To put something on the out side of the duct wont help with the metal inside.Now if you have room like at the blower bottom in the furnace and the cold air return drop . You can get some insulate duct board that we make ducts out of and cut this to line them and this will help. But dont put it in if it will cut down on your inside duct size. ED
 
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Old 08-30-03, 07:26 PM
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Thanks again for the advice Ed. One final question (I hope): will zip ties be adequate for attaching the flex duct to the boot?
 
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Old 08-31-03, 02:16 PM
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noise...

flex will increase the turbulance of the air in the duct and turbulance is noise. Your most likely missing the flex collar that connects the furnace to the duct system. This will transmitt the noise into the duct system much like the way thermal breaks on windows break the heat transmission to the outdoors. You should have them on both the return and supply.
Are you saying that your metal supply duct isn't insulated??? If so, then the entire duct must be exposed inside of the conditioned space, even then the first five feet or so must be insulated for noise reduction. Also your fan speed may be too fast, or the duct work is too small. Properly sized duct work makes no noise at all...I use a .10 static pressure but some duct designers use a .05 static pressure which is more expensive, BUT you can barely hear it at all...you just might need to install more supply drops...
 
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