elimiinate low frequency vibration in 8" round galvanized

[snugmonster]

I installed a central boosted return duct in the 2nd floor hallway of a 100 year old house with a new attached basement, single floor addition, and a new 105K BTU/3.5 ton central air system. The 2nd floor had no functional returns, and temps were 10F higher than the first floor. I ran about 25ft of 8" galvanized including 5 elbows down through existing available cavities into a crawl space located 650CFM 8" Centrax. There's about 15ft from the Centrax to an I-joist duct cavity including 3 elbows and an angled takeoff. This cavity dumps into the air handler return manifold about 8ft away. The takeoff actually mounts to a 1/16" aluminum plate screwed to two I-joists. Some photos may help:

http://www.hardwarefreak.com/web/construction/2nd-floor-return/gallery/resized/resized.08.JPG
http://www.hardwarefreak.com/web/construction/2nd-floor-return/gallery/resized/resized.09.JPG
http://www.hardwarefreak.com/web/construction/2nd-floor-return/gallery/resized/resized.10.JPG
http://www.hardwarefreak.com/web/construction/2nd-floor-return/gallery/resized/resized.11.JPG

It has reduced the hot daytime temp delta between the two floors from 10F to 5F, and dropped 2nd floor humidity substantially. The new attic ventilator I installed reduced it another 2F, for a current 3F delta between floors. I think this isn't too bad for a 100 year old 2 story home with 30+ year old blow in insulation, and not enough of it in the attic at that--only about 4 inches.

However, both runs of the 8" piping are oscillating at somewhere around 30Hz, enough to be felt through the floor and heard in the hallway and small laundry room directly above the ducts and blower. As you can see in the photos, existing obstacles didn't allow securing the 8" duct to the floor joists with standard methods, and required installing the longest straight section at an odd angle. Note that I didn't add the plumbing strap until _after_ the duct was fully assembled. Did that for good measure. With just the motor and takeoff attachment points I could yank on the mid section and it didn't move more than 1/8". So it's structurally solid, just not sonically solid at ultra low frequencies.

What's the best way to lock down or add mass to this long 8" duct so it doesn't vibrate the living space above it like the trunk of some kid's car? It's not that severe, but enough the homeowner is complaining and wants it fixed. Does anyone sell something like 5/10 lbs two piece round mass dampers that you can wrap around the pipe and screw/clamp together? A few of something like that would likely solve this problem. I've Google's around and haven't found such a thing. Any/all ideas appreciated. Thanks.

[boilerman856]

That kind of noise will send certain customers right to the nut house huh?
I think if it were me I would isolate the fan with rubber mounts and add a foot or two of flex on both sides of the fan. That should eliminate mechanical harmonics.


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[beenthere]

Suspend the fan with masons.

[snugmonster]

That kind of noise will send certain customers right to the nut house huh?

Thankfully this is just loud enough to be annoying, not enough to send the homeowner to a padded room.


I think if it were me I would isolate the fan with rubber mounts and add a foot or two of flex on both sides of the fan. That should eliminate mechanical harmonics.

The vibration source isn't mechanical tranfer. I ran the Centrax on the bench for 24 hours and used it as a handheld terror weapon against helpless papers, the cat, and passers-by. It's almost smooth as silk. The problem is resonance. This is how we know:

The total run length of the 8" round duct is ~35-40ft. The speed of sound at sea level is 1126 ft/s. The elevation here is 900ft so it's about 1130 ft/s here.

1130 / 35 = 32.28 Hz
1130 / 40 = 28.25 Hz

So the natural resonance frequency of this entire 8" duct run is between 28-32 Hz. So my 30Hz guess was spot on. The source of the infrasonic energy causing the resonance is very likely an interaction between the natrual resonant pulse frequencies imparted into the duct cavity by both the the main blower and the booster blower. The gauge, thus mass, of the 8" tubing apparently makes it particularly susceptible to resonating at ~30 Hz. I can't change the intrinsic material properties of the tubing, and swapping either blower is pretty much out of the question as alternatives will have the same resonance behavior. So there is really only one viable solution to this problem: Change the resonant frequency of the pipe material itself. There are typically two methods to achieve this:

1. Sufficiently increase the mass of the object. Doubling the mass of an object typically decreases its resonant frequency by half. Double it again and resonant frequency is now 1/4th the starting point. If the mass of the long straight exit section is say roughly 10 lbs and we double that to 20 lbs, our resonant frequency of that section is now approximately 15 Hz. While this would eliminate audible noise, the homeowner would still feel the vibration through the floor. If we increase the mass to 40 lbs, we get down to 7 Hz, the frequency of an earthquake. But our amplitude is so low it would neither be felt nor heard by the homeowner. So if there is a way to rigidly, permanently, solidly attach 30 lbs of mass to this section of tubing, without damaging it, this would likely solve the problem. Thus my question about concentric mass damper weights.

2. The 2nd method is simplicity itself, or we'd hope. Here we simply securely, solidly attach the 8" duct at multiple points to an object with a natural resonance frequency so low it cannot be heard nor felt. This changes the resonant frequency of the duct section to that of the object to which it is attached. In this case that means attaching to the floor joists, the dirt floor, the brick center wall, or all 3. 2x4s, screws, and plastic plumbing strap are the cheapest solution. This is the path I'd like to take, but pulling the pipe wall against the edge of a 2x4 makes me uneasy, due to the probability of crushing the tubing. Because of the pipe angle, I can't get the pipe wall flat against the wood. I was hoping some fellas here may have run into this before.

What do you think of mitering a few 2x4s long ways on the table saw and screwing them into available joists, put a triangle brace on each, then screw the duct to them with plastic plumbing strap? Maybe run a long sheet of 1/2" OSB in between the pipe and 2x4s and use more strap?

How about building a 'box' around the middle 4ft, well secured to the joists, and fill it with ready mix? ;) Adding 100+ lbs to the duct would eliminate all resonance, and being screwed to the joists would significantly reduce the resonance of the floor above. This, is overkill, obviously, and a lot of extra work. And would likely cause problems for anyone working down there in the future. But it's definitely the one way to kill this with a single shot.

[boilerman856]

Just a thought, how about attaching something to the pipe to help dampen the natural resonance? Maybe armaflex sheets or bubble wrap. I would think If either were tightly glued to the surface of the pipe it would have an effect on the sound generating tendencies and at least muffle some of it.

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[Gib's Son]

Flexible connection at inlet and outlet of fan and vibration isolation (neoprene) in-shear hanger isolators. Done.

[elvi]

You can do this 2 ways.
1. dogleg the long run to interrupt the harmonic; if the LENGTH is the driving issue.
2. Suspend or dampen from one side attach point, to approximately 1\3 of the length with a flexible material (Float one third of the total length of runner). This splits the length and will shift the resonance up; and split the frequency

Look at Grainger for material, fans or duct for this. I have in the past used round fans designed for flush or embedded mounting in non HVAC cabinets for duct boosters.
I wrap them in rummer based material like a burrito, and then attach them to solid round duct. Thy are almost silent, and use roller bearings. I have not had to service on yet. BUT; its not a bolt in deal.