Short flex duct sections for acoustic dampening?
Something I’ve come across in researching HVAC info is the notion that a short run of flex duct can be useful for reducing the amount of vibration and noise transmitted by mechanical systems.
Is this true? If so, is it worth the airflow penalty? I am thinking something like just a few feet placed in specific sections, although where exactly I’m not sure, maybe right at the supply and return connections of each equipment piece?
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I think this idea is something that gets shown in book and websites but almost never happens in the real world.
I do like the old school idea of a canvas joint between the furnace / air handler and the ductwork but cost cutting has mostly killed it and steel ductwork.
Maybe it's a regional thing, but I see just the opposite -- guys run a steel trunk line, then run drops using flexible duct. I think it's kind of hinky but the justification is that it provides noise insulation.
That "short flex section to the vent" is to make installation easier, since you don't have critical alignment issues when you have flex duct to allow for placement flexibility. this is the common way to install HVAC ductwork in commercial buildings -- a rigid plenum, then flex duct off of that to each vent in the ceiling.
The canvas joint is all you need to isolate mechanical noise in a rigid duct system, and these are usually installed right above the furnace or air handler. I would not install a system without such a joint, and I can't see the small flex joint making any difference in airflow. Flex duct alone might help a LITTLE with sound, but unless it's a pretty long run WITH SOME TURNS in it, it isn't going to accomplish much. You'd be better off putting a few baffles in the duct and lining that section with rigid mineral wool. To keep sound from traveling down a duct, you need to do two things: make the sound (so the airflow) go around at least one corner (no straight shots between sound source and listener), and use a squishy material to line the inside of the ductwork at that corner (to stop the sound from reflecting off the otherwise rigid interior of the duct and making it around the corner that way).
You could potentially also wrap the EXTERIOR of the duct with butyl rubber (I wouldn't use this inside as it can be smelly), which I've also used on traps and fittings in drain lines sometimes to quiet down the sound of water running in the drainpipe.
I have done a lot of work on noise dampening with respect to a larger commercial application (dedicated recording studio in a commercial building) and also a pretty intense home theatre installation. On the commercial side, we special ordered spiral duct (16" diameter) with acoustic lining which over longer lengths (over 16 feet) is hard to believe until you "hear" it.
On the residential side, I've used two methods with great success, measuring acoustic performance before and after. One is to line air returns with 1" acoustic duct liner, and the other is to install duct mufflers. Both are quite easy to do on site. Longer is better. Reduction to flow is minimal.
Fantech publishes acoustic data on their duct mufflers: https://shop.fantech.net/en-CA/ld--10--silencer/p115546 (click the "Acoustic" tab)
You'll see up to 31 db drop at 1-2 kHz where our hearing is most sensitive. That's pretty amazing considering the flow difference is negligible. The 1" fire rated duct liner is not cheap, however I have a 100 foot roll on hand (about $300) that I keep around for all of these applications. The attached pics are a few of the duct mufflers I've built in my shop. I have 3 of them addressing noise transmission from the theatre, but have also made a few more for kitchen exhaust and HRV main supply.
The length of flex works surprisingly well for sound. You can even get fancy sound absorbing flex that has acoustic data. If you want ever better and can give up a bit of pressure loss run the flex as a lazy S bend.
For example, I have my ERV connections for each bedroom with about 10' of 4" flex with 2 90deg bends to a main trunk. You can yell into one register and almost no sound makes it out the one in the other room.
Most newer ECM air handler are pretty quiet, decoupling the ducting doesn't hurt but it also doesn't make much difference. A bit of duct liner in the initial sections of the supply and return trunk does help with airflow noise.
Thank you all for the excellent info!