Duct Liner vs. MLV - AVS Forum
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post #1 of 14 Old 06-26-2008, 09:25 AM - Thread Starter
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What would be more effective around round duct for sound proofing 1" duct liner or mass loaded vinyl? The room will be approx 20X25 and all the round ducts that lead to the second floor is located within the room as well as the main supply & return. I was thinking of wraping the round ducts with duct liner. The mlv would be more expensive than duct liner. Also I'm using a drop ceiling and insulating the joists with r-19 or r-30.
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post #2 of 14 Old 06-26-2008, 09:48 AM
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If you can bury the ducts in the ceiling or soffits and can thereby shield them from sound, then the biggest remaining issue is sound bouncing around inside the ducts. This would mean duct liner.

MLV wrapped on the outside won't help much with sound bouncing inside the duct.

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post #3 of 14 Old 06-27-2008, 11:46 AM - Thread Starter
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Thanks Ted. What would be better to seal up the ducts plywood or drywall?
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post #4 of 14 Old 06-27-2008, 11:48 AM
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From the outside drywall is generally used. Cheap and heavy. You would build a soffit around the duct to shield it.

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post #5 of 14 Old 06-27-2008, 12:03 PM
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Another FYI if these round ducts are critical part of a sound containment strategy you might want to think about replacing them with acoustical duct. Acoustical duct has a INNER fiberglass layer covered with a fabric and reduces the sound transfer into and along the duct. The insulated flex duct sold at HD helps as well but it has a plastic inner sleeve and is not as effective in controlling the propagation of sound along the length.

product 6b here:

http://www.flexmasterusa.com/pg/fdpp.php
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post #6 of 14 Old 06-27-2008, 12:11 PM
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Equally useful, is to increase the size of the duct by one size, have your HVAC contractor balance the system so that the VELOCITY of the air at the registers is 250 feet per minute or less. (Don't let them get confused between VELOCITY (fpm) and volume (CFM)). In other words, you want them to deliver the required CFM but at an FPM of 250 or less.

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post #7 of 14 Old 06-27-2008, 01:18 PM
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Geez Dennis, thanks for that. I learned something today.

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post #8 of 14 Old 09-15-2008, 02:45 PM
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Sorry for resurrecting an older post, but the WAF is increasing as I delay in our build and try to gather as much info as possible about HVAC and figured it better to resurrect than start new. My HT has the main trunk running 1/2 way into the room, then with 4 supply branches (5" round, metal) coming off the end (3 to feed other rooms and one for the HT. There is a RA in the back of the HT room as well. Here are my questions:

1. If the entire system (trunk and branches) is encased in a soffit with insulation, then RSIC/DD/GG, is that enough to dampen the outside, or should MLV or equivalent be wrapped around the outside of the duct work before insulating?

2. Without replacing the current duct work with acoustic insulated flex, is the only way to deaden the sound entering into the HT supply run to take apart the current metal, wrap the inside with duct liner (as Ted mentioned) and put back together? Would an HVAC guy be able to do this better (ie quicker and easier) than me (with no experience)? Would cutting apart and using a duct muffler be easier?

3. If I were to use flex duct, would all of the branches coming off the main trunk need to be replaced, or just the supply in the HT?

4. If the RA is nothing but a hole at the bottom of the wall, running up between two studs (with drywall on each side of course) and entering the main trunk, does the inside of the drywall need to be lined with MLV or equivalent to deaden the sound going back into the main line?

5. The main trunk actually has an additional supply cut right into it for the HT. I know this is a BIG no-no for sound reduction from what I've read, but my HVAC guy thinks I should keep it just in case, and fill it with styrofoam or something if/when it's not needed. Does this make sense, or should I just have him seal it off now, even though he doesn't think one supply will adequately heat the room (11x17)?

6. Please tell me this is all worth it.

Thanks.
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post #9 of 14 Old 09-15-2008, 03:19 PM
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Welcome to the forum!

I'll take a stab at your questions.

#1 I would hold on the MLV wrapping

#2 Best to replace with flex and be done with it. Duct cunducts a vibration through the metal body, and also by bouncing the sound on the inside. Duct liner stops some of this bouncing at high frequencies only, but does not help the conduction through the metal. So the duct liner is only a partial solution.

#3 Just the supply and return to the theater.

#4 You might find that in the theater your return would be better placed up high. Cooling the room is 90% of the HVAC efforts, and so you'd efficiently pull hot air out. You would like a longer run before entering the main system. I would suggest looking to pull the air from the opposite side of the room and have it run through 10' or 15' of flex, then enter the main trunk. If you only run 8', there's not much absorption happening.

#5 You might want to use this if the guy is telling you you'll need to. Again, consider running long runs of flex for supplies and returns.

#6 If you want isolation, you'll have to do these things. Ducts and doors are the biggest sources of flanking noise: http://www.soundproofingcompany.com/...icles/flanking

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post #10 of 14 Old 09-15-2008, 05:29 PM
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You can check out what I did to my ducts to see if that would work out for you... I had custum couplers made that would not transmit vibrations, I also replaced the round with oval ducts and decoupled BOTH ends.. so none going back to the airbox and none traveling upstairs.

I am poor, so I did what I could


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post #11 of 14 Old 09-15-2008, 05:59 PM
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Thanks for the replies. I should have mentioned that the HT is in a basement, so the concern is more about heating than cooling. That's why the return is at the base of the wall. I should be able to replace the one supply with acoustic flex duct (it's about a 10' run with a couple of bends in it).

I'm thinking my bigger problem now is the return air. From the base of the wall, it utilizes two wall joists to go up to a 1x3 metal box attached to the floor joists. From there, they've put metal sheets across the floor joists which end up connecting to the main return (which ends in the HT room). So there is no option for me there to use flex duct to attach to the return. Would a baffle box inside the wall joists work in this case? MLV lined inside the wall joists?
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post #12 of 14 Old 09-15-2008, 06:03 PM
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Actually, MLV anywhere in this situation isn't going to do anything. It simply adds mass... like drywall. What's needed is a long duct run to give some chance to absorb.

Heat WILL be your #1 issue. Why? You'll be building a bunker that is super insulated. Then sparking up a heat producing PJ and humab bodies. Theaters get hot quick, basement or not.

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post #13 of 14 Old 09-15-2008, 06:22 PM
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Thanks, Ted. I understand the heat issue, which is why I'm thinking of sealing off the one supply right from the main trunk, because I don't think it will be needed. With the return at the base of the wall, would the floor joists its running through be considered a 'run'? Would anything inside that dampen the noise (wrapped insulation, like in the flex duct, or the sound dampening liner put in car speakers)? Or does that defeat the purpose of the return?
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post #14 of 14 Old 09-15-2008, 06:24 PM
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If you are talking about sheets of metal like this....



I used a product called ThermaPan that is basically cardboard... will not vibrate back much. If you are concerned about it even more, you could recess the ThermaPan and put sheetrock over it.


The return should not be too big of a deal, heck I am mounting my air return in my equipment rack to suck out the hot air and pump it back into the room.

I guess pics in your situation might help out. Any reason why you could not even put the return in a smaller duct so you could insulate around it in the wall?

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