Axiom Home Page Forums General Discussion Home Theater sound deadening return air ducts

I was talking with a friend about the setup of a home theater sound room in a basement and he advised me about isolating the return air duct.

He had used something like dynamat that has a sticky side and ran it along the exposed bottom side of the return air to deaden metal. It doesn't really remove the sound, but gets rid of the ringing pitch that tends to travel along the air ducts.

Have any of you experts in the HT room building trade used this, or is there a better product that I should be using?

In my installation, I have a two story house. The HT/Sound room is directly below the front sitting room in the house. This is essentially a non used room in daily life at my house. (it gets used when guest come over and we have a party). My thought was that distance reduces sound, and having all the bedrooms on the second floor, sound from the basement will be greatly reduced just by distance from anyone watching a movie or listening to tunes by the fact that there is a complete first floor between them.

I was told the one problem to this is that sound can get into the air ducts then bounce back and forth along the length of the forced air ducts and come out the other end as a tinny amplified version of what is playing in the basement.

Any advise would be much appreciated.

I have read many helpful, but conflicting opinions over at HVACTalk.com so I'll relate mostly what I read in my sound construction book.

There are 2 things you can do to damp out sound transmission in an existing rectangular duct run. First, cut a piece out and replace it with a flexible coupling. A short section of a few inches or so is what is required to isolate mechanical noise/vibration. This should occur immediately after exiting the noisy space.

Second is linacoustc or similar product inside. Acoustically lined ducts are available for new construction, but I assume your post relates to a retrofit. If not possible to line the duct on all sides inside, insert a long piece from corner to corner diagonally with the ends cut tapered to improve airflow. Make sure it is material backed on both sides and put foil tape on leading and trailing edges to contain fibres.

This creates 2 smaller parallel ducts. As far as sound goes smaller is much better. Each of these "new" small ducts will share the acoustic damping material wall. Airflow will not be affected really.

I would not look at Dynamat or NoNoise products. The acoustic sound energy is most harmful inside the duct and enters/emits from other ports in the system. Assuming you are framing out and drywalling it in, I would just opt to line it on the outside with fiberglass insulation if anything.

If you had the option to bring a dedicated return into the room from the main trunk using flexduct, the sound would have to travel all the way around the "circuit" to affect other areas. Flexduct does not transmit sound like metal ducting. Best option is NOT to cut a takeoff into the return duct for a register right in the room.

Here you go for further explanation:

It is super easy. I have 2 supply ducts and now a return duct in my theater. ZERO sound transmits through the duct. How? Just used simple flexible, insulated ducting for the ducts in the room. There is still a little rigid duct between the theater room and the rest of the basement, but it is mounted up in the floor joists and has normal fiberglass insulation between it and the ceiling of the theater. It definitely takes more that a couple of inches though. You need some good length and at least one 180 degree bend (don't squish the duct though). The sound bounces into the side wall of the insulation at that point and basically destroys its ability to continue down the duct.

The people at The Soundproofing Company have diagrams buried in their website somewhere, but I know from actually doing this that it worked. I have a YouTube video from my construction where I talk about it and you can see it. Unfortunately, I am being summoned to go bowling like *right now* with the in-laws.

Typical Christmas get together activity.

I will look it up later, or look for my "I should start a build thread" or whatever I called it topic in this Home Theater part of the forums. Towards the last few pages, I list the videos and give a brief description. It would be from before all of the drywall is done.

At about 4:09 into this video, you can see my supply ducts. Not tucked away, but you see the bend and length.

[video:youtube]http://www.youtube.com/watch?v=EppTFwrS3vg#t=249[/video]


Edit: Hmm.. I have no idea why the link isn't working correctly. Just click it and it will take you to YouTube.

The HT room is right beside the furnace area. The issue is not that I have a return air duct inside the room, rather the whole of the return air trunk runs down the left side of the HT room (bulk head).

Now I have boxed around it and stuffed it with about 2" of safe'n'sound, but I can't get away with 6-8" or I would be bashing my head into it as it's right beside the entrance to the room.

The issue is not that sound from the room is travelling down the trunk, but the metal trunk duct itself is resonating and giving a high pitched version of what is going on in the two bedrooms upstairs that have a return duct.

The trunk duct is a good 16" wide and 8" deep. Sized for the whole house with smaller ducts branching off of it. The idea i was told that the dynamat would deaden the induced vibration ringing that causes the noise. The idea is like if you have a sheet of metal and bang it with a stick, it gives a noise followed by a persistent ring for a bit afterwards. With a deadening coating on it, it will go clank, but no ring afterwards.

I just want to make sure that we are talking about the same thing. There is no option to try and replace this trunk duct with a flexible piece as it's simply too big. I don't think that sound it getting into the duct, but is being induced along it.

My bad. I was looking at the diagrams from the book shown above and thought that was the problem. Dynamat is pretty amazing stuff. That is what is used to dramatically quiet down road/engine noise in cars. You would have to line the whole outside, which would be pricey, but I am sure that some other dense and similar material would be effective with some loose, regular insulation. Double up on the drywall, or enclose the area around the duct with one layer before putting up the drywall would help as well.

Finished 2 basements with your situation. Once drywalled and insulated as you are with roxul you are good to go.

People are assuming here that it is sound traveling up the ducting.
Keep in mind that the ducting through one's house, in behind walls, is held in place with screws, strapping, etc. ANY vibration, whether it be from the HT or from a garbage truck rumbling up your road, can create noise from any and all of the HVAC parts which are not secured 100% and, additionally, connective parts being secured with a dampening material.
e.g.
The metal strapping around ducting would need to have at least some thin rubber or foam material between it and the metal ducting to reduce vibration noise and i highly suspect NO ONE on these forums, except for those presently building a new room, has this on the HVAC through their home.
Most major ducting would be secured with metal screws, metal strapping and tight to a solid 2x4/2x6 joist/stud, etc.

The HT may be the source of heavy vibration, but the ducting immediately around the HT area may not have anything to do with the 'ringing' being heard upstairs. For all you know that could be from loose strapping somewhere along the chain for which insulating around the HT room won't do a darn thing.