Configuring ductwork for least noise

attofaradApril 17, 2012

I'm installing a 36x24 hood with variable speed control for a roof-mounted 600CFM blower. I will also be installing a Fantech duct silencer between the hood and the blower. Hood and blower take 10" duct, so that is what I will be using -- overkill for 600CFM, but the 10" duct silencer also has slightly better noise attenuation specs than the 8" version.

The straight line, no-bend distance between ceiling and roof could be about 5 feet, which should allow room for the 36" long silencer plus a short duct run. Obviously, this straight short run gives the best flow. However, adding 10 feet and a couple of 90 bends would decrease flow by less than 4 percent.

Would the noise be less by adding the extra length and two 90's, so that the blower is further away?

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My wild guess is that motor vibration and rumble would be reduced with the longer ducting because the silencer is primarily a higher frequency attenuator. Using lead-vinyl pads on the ducting could suppress motor induced duct vibration.

If the silencer is close to the hood, then the increased duct length's added noise from turbulence should be reduced along with the fan blade turbulence noise. Should is the operative word, and I wonder whether the added noise at the bends, even though attenuated, would end up being a net increase if the motor noise was low to begin with and added duct length wasn't very helpful. It would probably depend on the amount of fan turbulence noise relative to duct turbulence noise.

Sorry that I can't be definitive.


    Bookmark   April 17, 2012 at 9:50PM
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kaseki, do you have a link for an example of those lead-vinyl pads?

I'm expecting that the fan noise is considerable at the source. This is the Broan.

    Bookmark   April 17, 2012 at 10:32PM
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If possible, use two 135 degree bends instead of a 90. Otherwise you have done everything else humanly possible.

    Bookmark   April 18, 2012 at 10:51AM
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Supposedly the longer the run the less the noise would be and is necessary when using the silencer. I installed a roof mounted Abbaka blower. I think its 1400cfm. I have a Modernaire hood with baffle filters and the variable speed control.

My duct work is as follows:

Short run from the takeoff collar on the hood to a 90 degree fitting that protrudes through the ceiling. Then there was about a 4' to 5' run until I hit the silencer. Then another 5' until I hit another 90 to start going up to the blower. I think there may have been another adjustable 90 added to jog the pipe over a little to hit the blower.

While the Abaka has a built-in damper. I could feel the cold air dumping down the open pipe, during construction in the winter months. I know the duct work was sealed with mastic Tape so my guess is that the cold air was more a product of convection. Thus, I added another fantech butterfly damper between the take off collar on the hood and the 90 degree fitting that drops through the ceiling.

Now, pre-hood when it was turned on you had noise but it was basically air rushing noise. Add the baffles and the noise goes up even more. Silencer can't do anything for that. Its not quiet, even on my lowest setting, but way quieter then the OTR microwave ventilation I had before. Not sure there is anything more I could have done.

    Bookmark   April 18, 2012 at 12:42PM
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I found a page of interest on the Modernaire site. I'll have to see how much leeway I really have in adding a bit of duct, without interfering with HVAC access, electrical, etc.

davidro1, does anyone except you call a 45 a 135? By that logic, a straight pipe would be a 180 degree bend.

Here is a link that might be useful: Reduce Noise

    Bookmark   April 18, 2012 at 2:13PM
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I think he was referring to a product like Dynamat; it's used in cars with those competition-grade stereos to cancel road noise and sympathetic vibrations/secondary harmonics and other things that spoil the sound. Maybe there is a cheaper product for silencing ducts, because real dynamat is pretty expensive.

Here is a link that might be useful: Dynamat Architectural Solutions

    Bookmark   April 18, 2012 at 6:13PM
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Actually, I was referring to a product normally used inside repaired doors, among other places. It is sticky on one side, and about one-eighth-inch thick or so. I used it around much of my ducting in the attic. But I'm not home at the moment to check the brand name. It should be available from most auto parts stores that support the automotive repair trade, particularly the bodywork trade. I'll report the name later.


    Bookmark   April 18, 2012 at 6:41PM
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Thanks for the Dynamat link. I have a roll of something very similar I used in a small airplane 25 years ago. Looks like the Thermwell stuff from Home Depot, which is 6.5x cheaper, will do the trick.

Here is a link that might be useful: Cheaper than Dynamat

    Bookmark   April 18, 2012 at 7:42PM
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The product I had in mind was Q-Pads by EverCoat.


    Bookmark   April 18, 2012 at 10:23PM
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I got a response from Fantech, who though I should go with the longer duct (to put the fan further away) and bends. They also suggested putting the silencer closer to the fan.

I have nothing real to base it on, but I don't think that the turbulence noise from the bend in the duct closest to the hood will be nearly as much as the turbulence noise from the baffles. We are talking 10" duct for 600CFM.

Wow, kaseki, those Q-pads are heavy. I saw the Thermell stuff with the foil backing at HD and it is a very soft foam -- not the dense rubber stuff I have from 25 years ago.

    Bookmark   April 19, 2012 at 1:06PM
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Yes, I used a set of pairs of very long Ty-Wraps in series to keep them in place, not trusting their adhesive.

I have no experience with the Dynamat stuff, so I don't know if their lower weight solution is as good or better. But the vibration was significantly reduced.


    Bookmark   April 19, 2012 at 9:25PM
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