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Sound transmission in ducts

eleeski | Posted in Energy, Heating & Insulation on January 21, 2009 09:26am

The new ski cabin is really loud. Sound carries throughout the house through the FAU ducts. The ducting is sheet metal with bubble wrap/foil insulation on the outside of the ducts. Is there anything I can do to quiet the ducting? Note, it is not the heater noise but talking in other rooms that carries.

The heater works great and the house is well insulated so maybe I can spray foam on the inside of the duct (trade some heater efficiency for sound dampening)? Or are there sound dampening registers?

Thanks

Eric

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  1. Clewless1 | Jan 24, 2009 12:42pm | #1

    no foam on the inside of the ducts!! There are such animals as sound attenuators. Used all the time in commercial systems. I was in a LARGE AHU once on the other side of one talking to my colleague on the other side ... he couldn't hear me.

    Now ... finding one for a small scale.

    I'm a bit skeptical, though ... noise from conversation carries? You do have filters, right? What kind? 2 inch pleated might give you more filtering and more sound attenuation. You could also maybe line the duct w/ sound absorbing material ... that is also 'standard' I think ... semi rigid fiber insulation covered by metal w/ holes in it. Affectively the same as a sound attenuator. The down side is it makes the duct smaller.

    Also maybe baffles in the RA duct.

  2. DaveRicheson | Jan 24, 2009 12:51pm | #2

    >>Note, it is not the heater noise but talking in other rooms that carries.

    How is the duct run?  Overhead exposed, under floor through a crawl space, or in an attic ?

    Could the conversation noise be coming from duct penetrations through walls?

    As mentioned return air openings would be suspect also. 

    1. eleeski | Jan 25, 2009 01:19am | #3

      Sound attenuators "sound" exactly like what I need. Where can I find some or at least downscale copy them. In the bedrooms, there is little need for the central heat as each room has a supplemental gas fireplace on a thermostat. When I'm there alone I don't even turn on the central heat. So reducing the flow in these rooms is an acceptable tradeoff.

      Return air is not a problem as it is in a normally quiet hallway and ducted through a bathroom down to the heater.

      Forced air going elsewhere is in a closet  behind drywall or in a drop ceiling in the garage and not too noisy.

      The problem ducts run in the floor joists and split up/down between levels. The bedrooms share the same feed ducts (and share with an area that has a TV and lots of living space noise). Two upstairs bedrooms have the register in the floor and the bedroom and living area below have ceiling mounted registers.

      Sound dampening material added to the inside of the ducts (a couple of feet are accessable through the registers) and a sound blocking register are the best I can do without really tearing the new house apart. Any hints as to exactly what I can use (and where to find it?) would be great. I could even tolerate blocking off the bedroom ducts completely with some serious sound muffling material if there is something safe anf effective (note that the house is very large and closing off the bedrooms will just make the other rooms warmer - a good thing).

      Thanks, Eric

      1. Clewless1 | Jan 25, 2009 11:39pm | #5

        Bingo, I think. Your supply duct goes out and serves up/down at the same location? That's your noise problem. Conventional attenuation won't help. You might consider a baffle splitting the duct for like a foot or two ... not sure how far. You might experiment w/ cardboard to get an idea of effectiveness.

         

        1. eleeski | Jan 26, 2009 07:34pm | #6

          I am pretty sure that the supply duct does not end in a Tee - I can't see into the other room with the register off. But the distance is pretty short. And the steel duct carries the sound VERY well - even from remote locations.

          The more I think about it, the more I want to just block off the bedroom ducts with a good acoustic plug. Note that when the FAU kicks on, the bedrooms can get too hot with that heat added to the fireplace. The registers are always closed now. If I can get a good sound deadening way to permanently seal off those ducts, I'd be happy. Stuff a pillow down the duct?

          Thanks, Eric

          1. Clewless1 | Jan 27, 2009 05:06am | #7

            Yeah ... in concept. No cooling going on?

  3. Clewless1 | Jan 25, 2009 11:35pm | #4

    do you have a return air plenum rather than ducted? That could be an issue w/ sound.

  4. terryb | Jan 27, 2009 06:45am | #8

    I had a similar problem - not voices, but the air movement in the intake. I called several hvac people and got no decent answer. Finally I glued some foam carpet padding into the intake duct. Cut noise significantly and have had no problems in the 3 or 4 years it's been in place.

    1. Clewless1 | Jan 27, 2009 04:14pm | #9

      Sure that worked for sound ... but that kind of approach can be problematic in other ways ... e.g. air flow, catching dirt (which is OK, it's the build-up that would be an issue). There may be other issues, too.

      1. eleeski | Jan 27, 2009 06:24pm | #10

        Since the cabin is mostly a winter cabin and it never gets hot there in the summer, the house has (and needs) no air conditioning. Unless you count window screens.

        There are a couple of registers where lining the duct might help me. And I'll try the pillow trick as well. I'm headed up skiing this weekend so I'll play with things then.

        But if I ever have to tear out drywall, I think I will add a few sections of flexible duct as sound breaks.

        Thanks, Eric

        1. Clewless1 | Jan 31, 2009 04:37am | #12

          Just say no to flex duct. Using this for your purpose MAY result in unintended consequences. You MAY get away with it, but do it with your eyes open. Using poor technology to solve problems often leads to other issues. Much like altering your airflow at the office w/ a piece of cardboard in the register ... it may (or may not) solve your issue, but it is simply the wrong thing to be doing (in general).

          Just be aware that you are using a solution that may not be as simple as you might think. It may very well work fine and dandy ... but don't be surprised if you 'learn' something new.  Seriously, I'm not trying to discourage you, just be aware.

          1. eleeski | Feb 13, 2009 08:12pm | #13

            I sleeved the inside of the duct back about 1' with neoprene foam (from a yoga mat). While we can still use the heater duct as an intercom it is quieter.

            Eric

          2. allaroundcobber | Feb 14, 2009 06:06pm | #14

            If you start closing off registers, and cutting down the air that goes through your furnace could be quite expensive. the system is designed to run x number of cfm's of air past the heat exchanger to keep it cooled. If the rooms are to hot shut off the fireplaces instead.

            As for noise not sure what to tell you.

            Moe

  5. junkhound | Jan 29, 2009 04:56am | #11

    Just for fan noise I filled one of my return ducts with 14" lengths of thin wall (as thin as you can find, like 100psi pvc) 3/4" bundles of pipe.

    If you really want it to be effective, drill holes in the sides of the tubes - lotsa work BTW

    Look up any old physics text or internet source on organ pipe or flute or clarinet frequencies and resonance to figure out how to get the attenuation you need for speech frequencies.

  6. frenchy | Feb 14, 2009 07:24pm | #15

    what are you doing keeping ducks in your house anyway? Shouldn't ducks geese and other barnyard creatures be kept outside?    Do you live in a barn?

     <grin> 

     Sorry I dislike ducts.. Infloor radiant heat solves those issues! Plus it's nicer heat besides!

    1. eleeski | Feb 15, 2009 01:42pm | #16

      I'm sure my wannabe architect neighbor who hates the modern design wishes the house was a barn.

      My concrete guy was terrible and put the radiant pex lines too deep in the slab. It took 3 days to warm the one little room we have radiant heated (1500W Ariston mini electric water heater with a circ pump). Most of the house is carpeted with really thick comfortable pad which also insulates the floor quite well. The kitchen has electric warmwire radiant under the tile which is great but it still takes overnight to warm up. As a ski cabin, we need the quick heating of a FAU to bring it up to temp when we arrive. Once warm, the insulation is excellent (green guard foam under stuco, tiger foam against the shear walls, foam panels in the roof sleepers and code standard fiberglass everywhere) so the house stays warm with little cycle times for the FAU despite the cold.

      I doubt that I will have a problem overheating the FAU. The FAU is a bit undersized for the house (90,000 btu for 3500 square feet in snow country). So far I haven't been able to close enough registers to keep the far end of the duct run toasty warm. The plasma TV (which my energy auditor friend hates) and the electric fake fireplace are needed to keep that corner of the house warm. Or I can build a wood fire in the stove and really get warm.

      My wife wouldn't let me stuff pillows in the bedroom ducts as she doesn't like the fireplaces. The thermostatic cycling of the firelight bugs her. The neoprene foam sleeves still allow plenty of air in.

      I might try a filter behind the registers to cut the sound a bit more.

      Eric

      1. User avater
        jonblakemore | Feb 16, 2009 12:26am | #17

        "My concrete guy was terrible and put the radiant pex lines too deep in the slab."

        How thick is your slab? 

        Jon Blakemore RappahannockINC.com Fredericksburg, VA

        1. eleeski | Feb 17, 2009 02:32am | #18

          The slab is about 6" thick. The plumber wanted the pex in the top 2 inches. The concrete guy ran the pex 4" deep. The room where we are going to use radiant will (hopefully) be occupied permanently (caretaker + a bit of rent?). There will be a pretty good thermal mass with the pex so deep. If the power fails, that room won't freeze for a couple of days - a good thing. Finding an optimal thermostat setting will be a challenge since the radiant is so deep and slow to respond. But the radiant worked well keeping the room warm on a very cold period. And 1500 watts is not too expensive.

          The other room pre plumbed for radiant is on our vacation side. Since most of our trips are fairly short, the deep pex (and corresponding slow response) doesn't work at all. So we carpeted, got the plasma hot energy hog TV and didn't hook up the radiant.

          Eric

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