[brj]

Hi all.

I hunted down some resonances in my room measurements to the sheet metal ductwork running through the crawlspace immediately below my listening room.  (The crawl is fully encapsulated and thus where all of my audio gear besides the speakers themselves and my BD player.)  While some of the ductwork is pre-formed, most is composed of individual pieces of sheet metal formed into the floor joists to create the HVAC supply and return paths.

As part of an HVAC upgrade 2 years ago, all of the exposed duct work joints have been sealed with mastic. I'm not really worried about HVAC noise radiating into the listening room, but rather trying to prevent the ductwork itself from resonating in response to my stereo playing.  Any frequency capable of penetrating my floor to get into the ductwork in the first place will penetrate though any of the common thicknesses of foam or fiberglass insulation typically used (1" or similar), so really the approach would seem to be mass loading the ductwork, or implementing some form of constrained layer damping.  (That said, if insulation only adds a small amount to the product cost, I'd likely pursue it for the sake of system efficiency on the supply ducts, thus perhaps lessening what HVAC self-noise remains.)

Various internet searched have turned up a variety of products, though often at prices that were rather startling for the amount of material actually delivered.  Anyone have any particular recommendations for worthy products, where to get them, or lessons learned from your own efforts?

(I have some CLD panels from a car audio supplier, but no where near enough to come remotely close to the 25% recommended surface coverage.  I'd guess that I have 250+ square feet of total duct surface area to deaden.)

Thanks!

Edit: I just remember that I do have a remnant of the high-temp Dynmat Extreme that I bought to damp the firebox of my gas fireplace.  Still a small fraction of what I need though.  Best guess is that I need something analogous to the Dynamat, but that I can buy in bulk.  Open to other ideas, though...

[mcgsxr]

Not sure it is an option, but could you stuff the crawlspace with pink insulation?  Or build some kind of bulkhead and stuff that?

I fully stuffed my basement bulkhead that boxes in the main trunk of the HVAC setup in our home when I finished the basement a few years ago.

I filled the joist bays and interior walls with Roxul Safe n Sound, but folks here recommended I could save money and just use pink for the bulkhead.  Bought 2 big bundles from HD and jammed it all in there before the drywall.

It has been great ever since, regardless of how loud the movies or music are, I have never heard it - compared to using that same space for close to 10 years unfinished, the main trunk would vibrate significantly with bass in music.

Here's a pic from back then, including some conduit I installed so I could later run more cables between display and media closet - came in handy 2x already!

[brj]

Appreciate the response, mcgsxr.

If I understand you correctly, your speakers are in the room with your ducting (now enclosed in an insulated bulkhead).  In my case, the speakers are in the room above the crawlspace that contains the ducting.  The ducting is suspended from the floor joists, with sheet metal into the gaps between joists right up to the subfloor.  Any wavelength long enough to penetrate the floor and induce the multi-foot long piece of metal to resonate is going to penetrate the typical amounts of HVAC insulation.  Thus my focus on dampening the sheet metal vs. insulation.  (Though again, I'm game to insulate the supply side, as this increases the HVAC efficiency.)

The crawlspace is roughly a 19x19' space to which I've had vapor barrier put down on top of various supporting layers, the walls insulated, etc..  (That's what I meant by 'encapsulated' - no bare earth or chance of moisture coming up, on top of which it's open to the conditioned air of the home.  Aside from the occasional spider, it looks like a clean room.)

Pics to clarify, 2 before the gear went in, and another showing out-of-date gear, but with additional ductwork above it:







(The first 2 pics were taken from the crawlspace entry, which opens to the mechanical room.  I'd damp the ductwork in the mechanical room as well, just to minimize sheet metal banging as it flexes when the system pressurizes.)

[aldcoll]

brj I have to ask why put the equipment in the crawl space?

The barrier on the floor of the crawl space is to keep the water/moisture under it ( at least here in Oregon) and water under the barrier is OKAY.  But in my simple way of thinking that is water under you equipment.  And while I didn't see any water or sewer lines in you pictures the liner is now a dry swimming pool until one of those lines has a issue.

I for one could never place my equipment in that area no matter the liner.

I have lived in a home with a basement and three sump pumps.  And one day I came home to three sump pumps running, floor was damp, joists were soaked and I had a couple hours of work.

I would look into several water alarms in your case first off.   I would also wonder about the humidity as it is a sealed space, is there condensation when the AC runs?  Would it drip?

As for dampening the AC ducting,,,,,,, what about some or the insulation for duct work or are there some rules about that and would it create a issue?

I just got to say I am still in shock with the placement of the equipment, maybe I am just a nervous kind of guy.

There REALLY is a plus in all of this.  When the burglars stand and stare at the wonderful speakers they will never find you amp

Like I said I am just the nervous kind.

Alan

[brj]

Alan, the short version is - if I ever get water in my crawlspace, my problems will be so large that audio won't even register.

At the risk of veering off topic...

I have an active system which makes for more gear than normal.  Having it all 6 ft below the speakers keeps the listening room much cleaner and addresses quite a few configuration challenges, not to mention hiding everything from visiting kids, pets, and - as you point out - buglers!  It also lets me spread it out and minimize cable lengths.

I live in a semi arid environment.  The main basement has a sump well that is bone dry and has been for the 11 years I've been in the house.  It's actually sealed over with plastic and has never had a drop of moisture, much less a pump.  The crawlspace had moisture coming up from the bare dirt when I first moved in (during the Spring), thus my decision to fully encapsulate it, remove the exterior passive vents, and ensure that it shares the circulating, conditioned air of the main house.  (It is thus not a sealed space, and is in fact the most year-round thermally stable place in the home, which is the same reason my home network gear is in roughly the same area.)  In addition, I've avoided all nearby surface water concerns on that side of the house by adding a French drain and separate underground pipe to tie in the nearby downspouts from the roof, thus ensuring that all surface water is carried away from the house.  There are no water lines or sewer lines in the crawl.  In short, the basement would have to flood about 5 feet deep before it could enter the crawl, and even then, all of the gear is at least 4" above the crawl 'floor' via maple butcher block, etc..

I'm comfortable that my gear is safe.

Back to how best to damp the ductwork...

[aldcoll]

Sorry to have gotten off the subject. You have covered your base's.

I did  notice that there aren't any cross breaks in the duct work.   When they usually make the duct runs they put  diagonal X's beads or crease in the runs.  A little late now.

Is the resonance form air pressure being pushed back via the vent?  Could put a pillow in the vent to check?

If it's deading the main trunk run (square stuff) what about a mastic that doesn't dry and sheets of light plywood?

Or are the hangers attached to joists also a issue.

Alan

[brj]

No worries.  I can appreciate how the idea of using the crawlspace might appear without the background I provided.

Correct, there are only a few spots that have the stiffening 'x' in the duct work.  In general, they're absent.

The resonance is caused by the stereo vibrating the ducts.  The HVAC need not be running.

You're talking about mastic holding on panels of plywood?  Basically an attempt at constrained layer damping?  Yes, that's essentially what I was expect the solution to be in the end, but had thought there might be an existing commercial product that wan't too exorbitantly priced.

(I'm guessing that I'd have to find it locally, as shipping on something that heavy would be painful in its own right.)

Thanks!

[mresseguie]

Here's an idea for you:

Several years ago, my son and I gutted his Accord and applied a bunch of Raam Mat to quiet things down. It worked pretty well. We used the "ensolite" product. It's a thin, flexible closed-cell foam sheet with adhesive on the back.

Link: http://www.raamaudio.com/ensolite-iuo-peel-and-stick-14-sq-ft-per-running-yard-3-per-pack-of-bxt-ii-recommended-absolutely-the-best-foam-on-the-market/

Michael

[jriggy]

I used a generic dynamat product from Amazon at our old house and it did the trick.

[Peter J]

I recently used Parts Express's damping products to quiet car interior. A fairly recent addition to their line, I think. I would call it a success.

I'm not an expert, but I don't think damping the ducts would take covering them entirely. All one is trying to do is move the resonant frequency and take the ringing out. Vibration isn't really removed, but rather transformed into somethng less detrimental. I've never dealt with exactly what you're describing, but have quieted HVAC systems by adding damping sheets in the middle of the unsupported "tinking"panels.

[brj]

Thanks for the responses, all!

Odd - I associate Ensolite with sleeping pads for camping!  I hadn't seen it used in an acoustic setting before.  Looking at that product, however, it appears to be an insulation only product, without any damping properties.  Unfortunately, as I mentioned previously, I suspect that the mid-bass to sub-bass frequencies involved here are going to simply ignore a 1/8" thick insulation barrier.

I agree that full coverage with a constrained layer damping product is not needed, and is likely detrimental.  (The CLD tiles that I have come with a 25% coverage recommendation, though I've also seen higher coverage recommendations.)  This would include products like Dynamat.   (The butyl rubber forms the viscoelastic layer, and the heavy foil forms the constraining layer.  You want the constraining layer to be free to move on top of the viscoelastic layer in response to vibrations set up in the underlying surface, and wrapping the product around corners, etc. limits this.)

For a simple mass-addition, via mass-loaded vinyl or similar, intended to change the resonant frequency of the sheet metal to which it's applied, then I suspect that something closer to full coverage would be beneficial.

Thanks for the thoughts - any additional specific product recommendations appreciated!

[jk@home]

Replace the rigid metal duct with flexible duct. You reduce the flow slightly, but it will quiet it down greatly.

[brj]

Aside from a few 6" diameter branches at the end, these are the primary supply and return trunks of the house.  It would take roughly an 18" diameter flexible duct to replace their 24" x 10" rectangular cross-section, which is much larger than any flexible duct I've ever seen.