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.

kas

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.

If possible, use two 135 degree bends instead of a 90. Otherwise you have done everything else humanly possible.

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.

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

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.
Casey

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

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.

kas

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

The product I had in mind was Q-Pads by EverCoat.

kas

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.

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.

kas

did anyone else try the dynamat? I used the cheaper version (fattmat I think it was) when putting a stereo system in my car years ago...was thinking it would work well of applied to the ducting.

also curious where everyone wound up putting their silencers? I thought kasek8 had said closer to the hood but it sounds like fantech said closer to the blower?

Most people won't have unlimited attic space to provide much choice of silencer location. Mine pretty much had to be closer to the hood, and my thinking is that that location removes more duct turbulence noise. I'm unclear what Fantech's thinking is re locating it closer to the roof blower, although I am also doubtful that many would notice a difference. Note that my attic is above my kitchen, but architectures where duct passes through second floor rooms will pretty much have to have the silencer closer to the roof blower.

I wouldn't support use of 90-degree bends for added duct length if a shallower angle will fit; rather 45-degree or thereabouts would provide necessary slant to the duct and lower pressure loss at the bends.

Space planning should account for the silencer clamping duct adapters (one might call them unions) as well as a damper at the hood end if not part of the hood. Also plan to hang the silencer from the rafters; it is too heavy to be supported by the rest of the duct. Laying it on the floor joists might work but I would in that case use some method of vibration isolation under it.

In any case, until you see 10-inch duct in place with a 10-inch (14-inches diameter) silencer, you might not appreciate how much attic you are giving up.

any thoughts if I am running basically a straight shot vertical from hood to attic...but venting out wall, if it would be better to put in a 90 (or 2x45s) and then mount the blower horizontal shooting out the wall....or come up the chase into a vertically mounted blower and then upon exiting the blower immediately make the turn an exit the bldg?

I am going up from my hood through 2 snaking 45s to scoot into the chase in the room above...then putting the silencer in the chase, then some pipe to get me into the attic (another 6' or so) then the blower and our the side wall once in the attic.

I was thinking, since I am framing up this chase for the silencer to go in, and since the silencer is big and heavy...maybe I would take a piece of plywood say 15x15, cut a 10.5" hole in it, mount it to the framing and then put the silencer on that. It would support the weight of the silencer and any ducting above it easily..the 10" flange would stick through the plywood to attach to the ducting below. I was also thinking I could do the same thing above the silencer (another collar) and to eliminate any chance of vibrations maybe use a dryer vibration mat to line the top side of the bottom collar and the underside of the top collar...so it would be basically sandwiched between 2 pieces of framed in plywood lined with a vibration reducing pad. (like this https://www.amazon.com/casa-pura-Anti-Vibration-Vibration-Appliances/dp/B07D4HMW82/ref=sr_1_19?keywords=vibration%2Bisolator&qid=1559785828&s=industrial&sr=1-19&th=1 )

Thoughts on this approach? I have no science to back it up but it seems like locking the silencer in like that would work well and eliminate even slight vibrations that could be traveling down the duct so there would be minimal vibrations near the hood?

(blower will be mounted in attic with vibration isolating mounts...looking to pick a few of these up tomorrow https://www.zoro.com/mason-vibration-isolator-neoprene-35-to-75-lb-2lvr9/i/G0288583/ )

My mental computational fluid dynamics (CFL) program cannot deal with any interactions between the turbulent output of a blower and the immediate duct shapes that follow it. I suspect an axial in-line blower would be best served by a few duct diameters of straight duct on the output side. When going out a wall, one could use a roof type centrifugal blower blowing down the wall, although that might lead to some staining. (Roof blowers dumping exhaust onto asphalt shingles tend to not leave much grease because of the daily UV dose a roof-top usually receives.) Or, a small (for commercial purposes) up-blast blower mounted horizontally could be used on the outside wall.

Other than turbulent interactions close to a blower, generally a pressure loss is a pressure loss no matter where it is in the overall duct system. And keep in mind MUA. The total pressure loss seen by the hood system blower is that which starts at the baffles, continues to the outside, then back via MUA to the inside, and ends at the baffles. MUA pressure loss can dominate all other losses, but in a reasonably designed system the baffle loss should dominate, not bends in the ducting.

Otherwise, relative to the question, ability to handle the motor and perhaps some day repair it should influence where it is placed and how it is mounted.