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

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.