Tom Kay

How's this idea for acoustic ducting ??

I can't take full credit for this, because I just phoned a guy at a ducting supply place. He suggested that I use the plastic dryer output tube as heating ducts for my upcoming home theater. You know, the plastic 5 or 6" pipe with wire spiralling around it for the full length? Or, how about the metal dryer tube, aluminum, but like and accordian and I would assume it will help reduce sound transmission through the duct, but it may also introduce turbulence to the airflow. Perhaps slow down the airflow too much.

This all stems from my desire to find a way to inject warm or cooled air from my furnace, into the theater, without allowing sound an easy escape route. I have tried looking for acoustic ducting, or foam and cork material that might be used as a tube liner, but I'm not having the success I'd hoped for.

I also thought about taking a long metal section of my ducting, and cutting a section in the middle, then replacing this with a soft sewn material. Sort of like an airport wind sock, but not tapered. I have a bunch of artificial leather, (I used to make hockey equipment in another life) and I could sew a large tube, 3 or 4 feet long, and clamp it onto metal ducts. This would hopefully act like a sound barrier, or a "disconnect" and the vibrations wouldn't travel the full length of the duct.

So, as always, I'd like to know, are these ideas workable, or would they fail. And does anyone else have methods of moving air in/out of the movie room without ruining the acoustic containment that the wife will ultimately demand? What have you master builders done to solve the problem?

Thanks, Tom, Ottawa Canada.

TimForman

I'm not a master builder but I've read that what you need is a band-stop filter or Helmholtz Resonator on the duct.
This paper might help.

My hobby site
My commercial site

MikeWh

I'm a little confused... Does your room currently have metal ducts running through it already, like the kind commonly found in basement HTs, that people typically enclose in a soffit? Would you have to tap into these metal ducts??

Or does your house have flexiducts-- In my area of the US, flexiduct is king. It's used in virtually all new construction. Here:
http://home.earthlink.net/~sushinut/flexiduct.jpg
It's at all of our home improvement stores.... I'm actually surprised that a ducting supply place wouldn't have suggested this, unless this product just isn't used up there in Canada.... I dunno. I used this for my HVAC ducting.

To reduce sound transmission to the rest of the system (and most importantly for me, to reduce AC motor noise into my HT, since the blower motor is literally in my theater), I placed a couple extra bends or loops in the ducting. I read this somewhere on the web about 3 years ago. I can't remember where. The theory being that sound energy will be dampened as it has to circumnavigate the loops of tubing.... sorta colliding with the walls of the flexiduct with continuous attenuation, as it reflects along the inside walls.

My biggest concern was a 10 or 12" return duct which is connected directly to the return plenum on my gas-pac. I curled 2 full loops in the run. This may reduce airflow a bit, but I decided to give it a shot. It has done the trick for me... at least as good enough as I need and considering I have no "before and after" to see how truly effective it may have been.

I have WAY worse noise infiltration into the rest of the house from low frequency noise transmitted through the wood beam construction... Yeah, baby... gotta love that subwoofer. Ducting, though, does not seem to be a problem for me.

BTW.... flexiduct (aka "flex duct" or "flexible ducting") is not the same as the dryer tubing used in our area, which is not insulated.

MikeWh

Ahah!! I think I understand now--- you must have an entirely metal ducted system... and you are worried more about the sound being transmitted through the metal in the ducts, rather than the sound moving within the ducts.... is that right??

There is a rubber sleeve that HVAC installers sometimes use to connect the HVAC blower unit to the plenum.... it's intended to isolate the vibrations from the blower unit from the rest of the house (and the duct work connected to the plenum). I'll try and find a picture (and the real name for it). It sorta looks like the fanfold material in a bellows or one of those old cameras....

[edit]
I found an article in HTMag on HVAC noise:
http://www.hometheatermag.com/showarchives.cgi?140:4 It refers to a "plenum silencer", but I don't think that's the "official" term for this boot or sleeve.

Vince Bray

You can replace the last 6 feet of your duct before the register with flex duct (which is exactly like plastic dryer vent with insulation) to help with noise. There are also inline noise suppressors.

www.hvac-talk.com for hvac experts
http://www.hvacquick.com/frfnconfig.asp?fm=ld for silencers

Tom Kay

Hi Folks,

Thanks for all the great replies. I do appreciate it. I am going to the trouble (in the near future) of doing a proper job of hanging drywall from resilient channel, adding Roxul insulation in the joist cavities, filling every crack with caulking, and I figured it would be stupid to do all that work, and not treat the ducts properly. I am concerned about BOTH noise movement through the metal and into the ducts, and certainly about noise entering the end of a duct and travelling along to the rest of the house. I am fortunately at the stage where any change is possible, so now's a good time to hunt for things like flexiduct.

I have very low headroom in this old basement. It's a wonderfully large room, being 14 by 23 feet, but the ceiling is around 7 feet, with a good foot of that taken up by the large rectangular duct that feeds the whole southern half of my house. From that big feeder, come the smaller round ducts that heat 3 bedrooms, and 2 rooms downstairs, including the HT. I'll remove the 5 inch tubes that feed the HT, and seal the round holes in the main duct. Then, I'll make 2 new runs that are fastened to the end of the big rectangular feeder, and this means that the air will travel straight, then do 2 90 degree bends, until it vents into the room. I like the fact that it has to do a couple bends, because these bent ducts will help kill the sound that travels through the whole system, as mentioned in one of your replies.

This may sound confusing (a picture really would speak a thousand words) but I think it will be better. Plus, the 2 areas that did have ducts in the ceiling, will now be empty and simply jammed with insulation.

Again, I'll look up the flexiduct material, but what would really help, are more sources of that kind of duct, especially in Canada. Home Depot does not seem to have it.

Thanks again all, and I can't wait to start posting pictures of the results. This is an excellent forum, and once you get past all the hot-chick websites, THIS is what the internet is really all about ! Tom.

MikeWh

No, I think I got it. Sounds like a good plan.

I'm surprised Home Depot doesn't carry it there. It must be a code or maketing thing... maybe flexiduct just isn't used up there.

I think Hart & Cooley made the stuff I bought:
http://www.hartandcooley.com/flex/all_flex.htm

Tom Kay

Hi Vince

I looked up the website for the silencers, then phoned the company, and they look quite interesting. An in-line muffler for the heating duct. Cost seems reasonable, (until I convert it to Canadian $) but I might try closing my drywall duct-run, then see how it all sounds without the silencers, then add them if I think they're needed.

So, I am learning at a geometric rate today !

Thanks, Tom.