Reduce sound transmission through air duct

[Sparky4598]

Im looking for some suggestions for ways to reduce noise transmission through ductwork. My furnace is only about 2ft below the return register in my house and it is connected by rigid metal ductwork. I don't think the noise is coming from the metal ductwork itself, it seems to be turbulence noise from the blower itself.
I was wondering if there any thing I can put in the ductwork to reduce the noise transmission some, assuming it is coming from the blower fan? I was thinking like a small cell honeycomb grid a few inches thick may help some. I had thought about sound absorbing foam panels on the inside walls of the ductwork also, but was afraid that may disrupt the airflow too much or collect too much dust.
Any thoughts or suggestions would be much appreciated! Thanks!

 

[GregLocock]

Use a spectrum analyser on your phone to get an idea of what frequencies you are objecting to. Or at least is it a hum (single frequency) or a roar (I suspect the latter).

 

[Sparky4598]

Use a spectrum analyser on your phone to get an idea of what frequencies you are objecting to. Or at least is it a hum (single frequency) or a roar (I suspect the latter).

Yeah, it's a pretty good white noise distribution, a roar of air and definitely not a hum

 

[dvd]

Get your duct lined with duct liner - your HVAC service company should be able to help you out.

 

[GregLocock]

I’m guessing that a labyrinth of semi porous material would be the best bet,but that might create flow noise in its own right.

 

[Sparky4598]

Get your duct lined with duct liner - your HVAC service company should be able to help you out.

That sounds like a pretty simple solution. I worry with how short the distance between the unit and the return grille is though that it won't help much. There is only a single piece of ductwork between the return filter box and the furnace blower intake. It's a rectangle to square transition 30"x10" to 18"x18" about 20" tall.

 

[Sparky4598]

I’m guessing that a labyrinth of semi porous material would be the best bet,but that might create flow noise in its own right.

What do you mean by a labyrinth? Like a few "plates" half way across the duct alternating sides so it's like a maze for the air to go through? Yes I would suspect that would create noise from the turbulence of the air as well as restrict the flow a substantial amount. It's only a 30"x10" to 18"x18” transition about 20" tall between the furnace and the return filter box and it flows about 1000cfm.

 

[XR250]

Replace that section with flex duct - that will help. If you can snake it around a bit to get extra length it will help more. I had the same issue and it reduced the noise considerably.