Noise suppression in ductwork? Minimum segment length?


I had a new Coleman 99.5% gas furnace installed.
The return duct is a 25' 16" diameter metal straight shot from the hallway to the furnace in the attic. Air flow is about 900 linear feet/minute. That's over twice the airflow for my 37 year-old Lennox.
I get lots of furnace noise in the hallway. There's "wind" noise plus what sounds like rumble from the blower. Noise level is 74dBC/60dBA at ear level under the duct. The difference in the numbers suggests that the problem is in the low frequency area. A spectrogram of the noise confirms that the peak is at 100-200Hz. area.
I've got 9 source vents in the floor. Noise from them is tolerable.
Googling suggests that there should be at least three right angle bends in the return duct to abate the noise. I couldn't find any more detailed recommendations.
There will be straight segments between each right angle bend. What's the optimal/practical length for each segment?
As the length of the segment approaches zero, it's contribution to noise abatement will be reduced toward zero. As the length of a segment increases, the marginal effect on noise abatement decreases. Somewhere in the middle should be a sweet spot for segment length. What should that segment length be??? How does that relate to the noise frequencies involved? Would different lengths for each segment be better to avoid resonances or spread the losses over the frequency range of interest?
The contractor wants to insert 25' of flex duct and "serpentine" it between the ends after cutting out a section of the straight pipe. His idea of a right angle is to cut another hole in the side of the return plenum. That's not an effective right angle at all.
How does inserting curves of flex duct compare with right-angle joints? Radius recommendations? How would one circular loop compare? That would take minimal adjustments to the existing pipe.
Discussion suggests that one of us (me and the contractor) doesn't understand the optimum duct situation.
The contractor is not willing to discuss noise level goals. He wants to modify the ducts for a fee, without making any promises about solving the noise problem.
Recommendations?
Thanks, mike
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You sound like an EE that hired the lowest bidder. sorry bout your luck.
If you want to correct the noise issues, then you need to insure that the ducts are correctly sized (Manual D duct calculation) and installed. 900fpm is a bit much for RA. Correct the ductwork issues, and the noise will go away. Flex duct is *NOT* the way to go. find a contractor that is going to do it right, not just cheap.
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Enough already! ...."SMD" and "SYD"!!! Nobody wants to hear about Steve or anyone else sodomizing anyone's poor dog!
-zero
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On Fri, 23 Oct 2009 11:21:16 -0400, "Existential Angst"

Its all the same shit. There's only about seven different furnace manufactures and most of them all use the same hodge podge of parts. There are a lot of label makers. Steve likes and sells Rheem. I don't think Rheem ia any greater than anything else and I think Rheems commercial stuff sucks. I don't endorse any one brand because they are all junk. That's why we have jobs.
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Steve wrote:

Yep, I'm an EE. But I've probably had more formal education in thermodynamics and physics than most people.
It's a long story, but I didn't have any choice of contractor. And the cost of the upgrade was ZERO, so I can't complain about getting what I paid for.
All I wanted was some engineering advice from someone in the know so I could make knowledgeable input to the process.
Bitching about each other is not helpful to me.
I need somebody smart and experienced to give me USEFUL input on the question I asked about minimum duct segment lengths for optimum noise abatement. Making the duct segments too short may have the same negative impact on the airflow, but possibly less positive impact on the noise abatement.
Thanks, mike

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I still stand by my original statement...

Its not about the lengths of the ducts, I can make an airhandler in a hall closet with a central return almost completely silent. Its about correct sizing and proper installation. Having installed systems in a TV station studio, and a recording studio, its all about correct design, sizing, and installation for proper airflow.....It also helps to use the right materials, and flex duct ain't it.

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You mean the one where all the recordings have that background <whoosh> and low freq rumble? THAT was you? Figgers.
Once again, zero help, zero information, but BIG pats on yer own back. Figgers.
'Course, the OP didn't seem too thrilled with my advice, either. But lessee, I *gave* advice (fwiww), and you gave, uh, what?? Oh yeah, your recording studio, that's right....
But sheeit, make a duct thick enough and heavy enough, and it WILL be quiet.
--
EA



its all about correct design, sizing, and
> installation for proper airflow.....It also helps to use the right
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Perhaps this site (and links there) may help http://www.engineeringtoolbox.com/noise-reduction-silencers-d_81.html
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Bennett Price wrote:

Thank you.
Your response is characteristic of what the web should be.
It appears that you: 1) Actually read the question...UNDERSTOOD the question. 2) Thought about the question before you started typing. 3) Provided information DIRECTLY RELATED TO SOLVING MY PROBLEM and with sufficient context to be believable/useful. 3) Presented in a lucid manner. 4) Refrained from making unfounded assumptions and aspersions. 5) Refrained from name-calling, bickering and other inciting statements.
If only a few of the newsgroup denizens followed your example, the web would be a lot better place.
I believe your input will lead to the solution to my problem.
Thanks, again, mike
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spamme0 wrote:

Oh Goody. Another Numbnuts that wants to dictate free speech.
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First off, you are *NOT* a interweb hall monitor, second, you were told repeatedly what your problem is, and what its going to take to correct it. You got your system for "free", and you got what you paid for.... now your whining about it because you have buyers remorse. Your being an EE doesn't mean that your a certified master technician any more than parking a pinto in your garage makes it a mercedes. BTW... my first clue??
" I had a new Coleman 99.5% gas furnace installed.
The return duct is a 25' 16" diameter metal straight shot from the hallway to the furnace in the attic. Air flow is about 900 linear feet/minute. That's over twice the airflow for my 37 year-old Lennox."
Coleman is also the cheapest piece of crap on the market, right next to Armstrong and Goodman. They don't even put Coleman in trailers anymore, because it won't hold up
Get over yourself, get the ductwork issues corrected and go away.
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wrote:

    No, but it does explain some things ....

    You're think that an EE would realize velocity <> airflow VOLUME.

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I admit that my skills in fluid dynamics are weak.
I'd been working with the assumption that I could get a reasonable first-order approximation to airflow by multiplying the average velocity across the duct times the area of the duct. Given that the duct size didn't change, the relative volumes should be the same as the relative velocities...to first order for small changes. I assumed I could ignore the small changes in air density due to the small change in pressure within the duct.
Please disclose the additional math required to determine the airflow volume with sufficient accuracy to solve the noise problem described.
Thank you for your willingness to educate. mike
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pressures are measures in .01 inches of water column. 900 fpm is twice what you should have for return air. airflow should be 400cfm per ton of air conditioning.

Your the EE..... you figure it out.

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wrote:

    Please educate yourself on HVAC duct design fundamentals. Try buying something called Manual D from acca.org or http://www.manuald.com /
I am not going to type the book into this newsgroup for you.

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    I just talked to him the other day. Watcha need ?
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    I hear she really rates.
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