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.

 

[Sparky4598]

Would it be most likely for the noise I am hearing to be from the centrifugal blower basically stalling and not being able to produce enough pressure differential for the flow required or would it be more likely to be just from the turbulence of the air entering the inlet of the blower wheel?

The blower is a ECM variable speed blower that spins whatever it needs to to achieve the desired 1575cfm of flow. It is a 10"x10" forward facing centrifugal blower with about 0.72inH2O pressure differential at 1575cfm. Attached is a spectral plot of the sound of that helps any.

 

[btrueblood]

I wouldn't call that white noise, exactly? There is a lot of energy down in the low (100-300 Hz) frequencies, so a lot of bass tone "rumble"...

You say the ECM varies blower speed - what feedback is being measured, pressure differential? It might need to slow down a bit, if the blower is stalling - that stall produces a lot of low rumble. See if there is a way to limit the speed and play with that setting.

One last thing - my wife complained about the furnace making noise. I used my phone mic to trace the noise levels at all the outlets, and finally back to both return air ducts. One return duct was noisier, and when I pressed the grate it flexed and the noise disappeared. Re-tightened the mounting screw that was allowing the grate to rattle - problem fixed.

 

[GregLocock]

Yes that’s pink noise, typical collapse of vortex turbulent flow noise.

 

[Sparky4598]

I wouldn't call that white noise, exactly? There is a lot of energy down in the low (100-300 Hz) frequencies, so a lot of bass tone "rumble"...

You say the ECM varies blower speed - what feedback is being measured, pressure differential? It might need to slow down a bit, if the blower is stalling - that stall produces a lot of low rumble. See if there is a way to limit the speed and play with that setting.

One last thing - my wife complained about the furnace making noise. I used my phone mic to trace the noise levels at all the outlets, and finally back to both return air ducts. One return duct was noisier, and when I pressed the grate it flexed and the noise disappeared. Re-tightened the mounting screw that was allowing the grate to rattle - problem fixed.

I'm honestly not sure what it measures, I would assume pressure differential yes. I just know the control board has a LCD display that shows the CFM flow.

I can reduce the speed by setting the dehumidify logic high and it reduces the flow by the percent and it is noticably quieter. It is also a two stage unit and it is substantially quieter on low stage. 66dBa at the return grille on low versus 75dBa on high stage cool.

 

[Sparky4598]

Yes that’s pink noise, typical collapse of vortex turbulent flow noise.

I do question what the phone microphone noise is in a quiet environment though, I'm sure it's not a flat response but I'm not sure of a good way to test that. I don't have a sound proof box to put my phone in lol

 

[Sparky4598]

Yes that’s pink noise, typical collapse of vortex turbulent flow noise.

So the quietest area I could find measured 32dBa according to my Amazon special sound meter. Attached is the spectral plot from that. I suppose it looks fairly flat actually.

 

[Sparky4598]

Yes that’s pink noise, typical collapse of vortex turbulent flow noise.

So just to ask for clarification, would this "collapse of vortex turbulent flow" be a sign of the blower stalling or just the flow trajectories of that much air going into a 10" diameter hole?

I'm wondering if a different blower wheel design that can produce higher pressure would improve the noise any. I've heard backwards inclined fans produce higher pressure but are typically noiser than forward inclined blades. Seems counterintuitive to me since they produce more pressure differential and are supposedly more efficient though. But I'm wondering if that could end up being quieter given the conditions.

 

[GregLocock]

Vortex collapse is thee main mechanism that generates the turbulent flow noise. It might be possible to find a less noisy blower, but typically we concentrate on flow straightening prior to the blower.

 

[Sparky4598]

Vortex collapse is thee main mechanism that generates the turbulent flow noise. It might be possible to find a less noisy blower, but typically we concentrate on flow straightening prior to the blower.

I see. I'm not sure the flow could be much straighter and more laminar than it currently is to be honest though. The return grille is only 2ft above the blower inlet and it's a slight offset rectangle to square transition piece of rigid metal ductwork.