# Duct Mufflers



## mnye00 (Jan 29, 2010)

Has anyone had experience with Duct Mufflers and care to comment on their efficacy? Thanks


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## bpape (Sep 14, 2006)

They can be very effective for mid and high frequency sound transmission control. Most of them don't do much for bass transmission though. There are ways to build your own if you have the space to do it. A box with mass (MDF) surrounding flex duct that's preferably at least 15' long with 3 90 degree bends and some distance between them will give you a lot of control.

Bryan


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## mnye00 (Jan 29, 2010)

Thanks for the suggestion Bryan. Looking at my system, I can probably accommodate the turns in the heat vent, but the return may cause some problems as it runs parallel to another line inside the same joist.

Would the box go around the whole of the flex vent or even further?


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## bpape (Sep 14, 2006)

15' from the time it enters the theater. If the return is just tin on the bottom of a joist cavity, that definitely needs to be changed. It's also best if your supply and return are not shared with other rooms for a reasonable distance from their openings into the HT.

Bryan


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## mnye00 (Jan 29, 2010)

Thanks again Brian. I'm thinking of building the box in place around the heat vent by green gluing mdf to the joists - any issues here. I am also installing Quietrock on the whole ceiling as well as using resilient channel to attach the drywall and acoustic insulation in all cavities. It seems like my only solution to the return problem is to put in a duct muffler due to the space limitations that I have.


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## bpape (Sep 14, 2006)

The MDF box is fine. Use flex inside and fill the rest with fluffy insulation. You'd be better using channel of some sort to hang the boxes in the joist cavities rather than attaching them hard (Green Glue won't 'glue' anyway)

Just using flex, distance, and bends will give you a more effective solution over a broader range.

Bryan


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## mnye00 (Jan 29, 2010)

I just spoke with my HVAC guy and he clarified that what I thought was my return is actually not. My actual return is, right now, just a hole in the side of some rectangular ducting on the ceiling of the basement which leads down the middle of a wall joist. Once this is drywalled over, my return will start on the floor where there is a plate now. He wasn't able to give any suggestions as to how to sound proof this empty cavity as he said that I have to leave it open once the drywall is in place so as to not limit the air flowing to my furnace. In that case, I am going to have to just to do my GG wall with double drywall attached to clips and furring channel. I will insulate the wall joists other than this one.

Also, he suggested stepping up a size for the flex ducting. So, if I have a 6" duct line running right now, he said to go from this 6" line to 7" flex, back to 6" to the termination.

Thanks again for your help


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## Ted White (May 4, 2009)

This is a common duct muffler, shown with 6" flex


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