Ducting for Home Theaters

**jpk** · **07-08-09, 05:30 PM**

Quote:

bpape wrote:

If you have a drop ceiling, the FIRST thing I'd do is make sure that over your head and around the perimeter are stuffed to the hard surface above with insulation before you do anything eles.

Bryan

Bryan,

Believe me I have considered stuffing the joist with fluffy insulation, covering with decorative cloth and forgetting about the dropped ceiling. It could happen!

Right now the ringing seems to be getting worse, maybe something is coming loose? This helps convince me the major problem is the duct work.

Joe

**bpape** · **07-08-09, 05:38 PM**

Well, I wouldn't just use cloth over the insulation. That'd be way too dead all in one dimension.

Metal ducting is the devil's work in a listening room or home theater.

Bryan

**blekenbleu** · **07-12-09, 09:07 AM**

Quote:

bpape wrote:

Well, I wouldn't just use cloth over the insulation. That'd be way too dead all in one dimension.

I have not found fiberglass insulation effective for damping bass resonances.

Quote:

Metal ducting is the devil's work in a listening room or home theater.

Agreed. I replaced metal ductwork in my nearly 50-year old house with fiberglass duct board
both to kill resonances and reduce airflow velocities. It is not really that hard to install
and effectively removed HVAC noise from my home theater. Along with triple-pane windows
and black-out shades, this was among the most useful improvements after speaker relocations.

To isolate resonances, feeding an audio sine wave signal generator into the system is expedient.
Slowly crank the frequency at moderate listening levels, dialing in on buzzes and rattles.

Cancelling bass resonances by IIR (or overlaying poles with zeroes, as they say)
is problematic because acoustic resonances vary over temperature, humidity, air pressure and time.
Somewhat offset zeros and poles can be more objectionable than uncorrected poles.

**bpape** · **07-12-09, 10:37 AM**

In sealed cavities, fiberglass will do an excellent job of damping. To damp ducting, not so much.

Bryan

**cavchameleon** · **07-12-09, 05:58 PM**

blekenbleu,

Can you give me a link to the ducting you used. Also, is there any chance of slough-off of material from this ducting?

Bryan,
What types of ducting do you recomend for HVAC?

Thanks,
Ray

**bpape** · **07-12-09, 06:23 PM**

There are 2 types that work well acoustically, don't ring, and don't transmit sound nearly as easily to the rest of the house:

Flex ducting

Duct board

Either, would need to be inside MDF enclosures to maintain isolation.

Bryan

**blekenbleu** · **07-12-09, 06:40 PM**

Quote:

cavchameleon wrote:

Can you give me a link to the ducting you used.

I got CertainTeed inch-thick years ago from the local Ferguson Enterprises outlet.
http://www.certainteed.com/products/...hanical/317386

Quote:

Also, is there any chance of slough-off of material from this ducting?

I suppose, particularly right after installation.
Keep in mind that nearly all ducting accumulates dust and can benefit from cleaning
every few years. I have not seen evidence of shedding in the last ten years,
but since all our ductwork is in the floor and air velocities are low,
it would be fighting gravity.

**cavchameleon** · **07-12-09, 08:24 PM**

Quote:

bpape wrote:

There are 2 types that work well acoustically, don't ring, and don't transmit sound nearly as easily to the rest of the house:

Flex ducting

Duct board

Either, would need to be inside MDF enclosures to maintain isolation.

Bryan

Hi Bryan,

So you're saying that the total length of the Flex ducting needs to be inclosed in MDF or the part just above the HT. Also, Isn't it best to lenghten flex ducting and make a lot of bends? I also once saw (need to find it again) 'flex ducting wrapped with insulation. Is that form better?

I need to look at duct board, have not used it before!

Thanks!
Ray

**cavchameleon** · **07-12-09, 08:30 PM**

Quote:

blekenbleu wrote:

I got CertainTeed inch-thick years ago from the local Ferguson Enterprises outlet.
http://www.certainteed.com/products/...hanical/317386

I suppose, particularly right after installation.
Keep in mind that nearly all ducting accumulates dust and can benefit from cleaning
every few years. I have not seen evidence of shedding in the last ten years,
but since all our ductwork is in the floor and air velocities are low,
it would be fighting gravity.

Thanks for the link!!! I do worry about fiberglass entering the breathing space. It does look like the contact area is coated, so as long as the seam is sealed well, it should be fine. I've been thinking of replacing some of our ducting to decrease some of the sound from the furnace. Our current house was built in the 70's, so non-insulated galvanized steel (open in the attic) was used for all the ducting - yes, even the furnace creates a resonance when it's running.

Thanks!
Ray

**bpape** · **07-12-09, 10:11 PM**

The problem is that while the flex or duct board is great at not passing vibrations and absorbing mids and highs, it has almost no mass to it so it's effectively a hole for bass to pass through.

Yes - you want bends and length but you want those to occur starting where it enters your room 'aquarium' and goes for at least 15' with at least 3 90 degree bends in it before leaving the MDF box (the mass).

Think of it this way... If you cut a hole in the ceiling that you just spent a ton of time sealing up and getting sound proofed, and then just put 1/4" thick insulation in the flex or the 3/4" duct board, you effectively have a hole in your ceiling for bass to pass straight up to and through the subfloor above you.

I don't mind answering the questions at all. In all fairness to the purpose of this thread, we should probably continue it somewhere else.

Bryan

**cavchameleon** · **07-13-09, 02:07 AM**

From another thread (hope you don't mind Bryan, I copied and pasted your last message from the AS-EQ1 thread).

Quote:
The problem is that while the flex or duct board is great at not passing vibrations and absorbing mids and highs, it has almost no mass to it so it's effectively a hole for bass to pass through.

Yes - you want bends and length but you want those to occur starting where it enters your room 'aquarium' and goes for at least 15' with at least 3 90 degree bends in it before leaving the MDF box (the mass).

Think of it this way... If you cut a hole in the ceiling that you just spent a ton of time sealing up and getting sound proofed, and then just put 1/4" thick insulation in the flex or the 3/4" duct board, you effectively have a hole in your ceiling for bass to pass straight up to and through the subfloor above you.

I don't mind answering the questions at all. In all fairness to the purpose of this thread, we should probably continue it somewhere else.

Bryan
End Quote

Curious on more of this Bryan (thanks so much for all your incredible advice so far!!!

).

So, would building box at the exit of the vent (register of the HVAC into the HT), out of 3/4" MDF, let's say 3' x 3' x 3' with 1' of insulation on all sides of the interior and the ducting at 90 deg from the register take care of 'bass' transfer? Or would this create some type of transmission line at a specific freq in the lower regions?

Thanks!
Ray

**bpape** · **07-13-09, 07:09 AM**

Just make the box big enough to hold the ducting. Then do something like....

run 5', turn 90 degrees
run 5' turn 90 degrees
run 5', turn 90 degrees
exit the mdf.

Obviously, not everyone will be able to do all of that exactly that way. Sometimes, we do all the bending outside the basic area of the room. Sometimes, it can all be accomplished in soffiting. If you can do 15', a bend, 10', a bend, then a bit more length, something like that works well too.

Just also remember that every 90 degree bend helps isolation, and makes the air flow less efficient from an HVAC standpoint. An inline fan can help considerably.

Bryan

**cavchameleon** · **07-13-09, 08:37 AM**

Thanks Bryan! That's a lot of help. Those are pretty long runs depending on the attic space available. I guess it could all be done just above the HT then run to the HVAC source. Is it good to put thick insulation over all the ducting for the total run (I'm assuming bass freq will go right through the 90 deg bends).

Ray

**bpape** · **07-13-09, 09:48 AM**

The insulation after the box really won't do much. The flex and duct board are already insulation and absorptive.

The 90 degree bends IN THE BOX are what helps with the isolation. The waves hit the wall and bounce back and cancel some while the airflow keeps moving.

Bryan

**cavchameleon** · **07-13-09, 10:17 AM**

Thanks for the clarity!
Ray

**Sonnie** · **07-13-09, 10:39 AM**

All of the posts pertaining to this were moved here from the SVS SubEQ thread.

**patchesj** · **07-13-09, 04:02 PM**

Quote:

bpape wrote:

The insulation after the box really won't do much. The flex and duct board are already insulation and absorptive.

The 90 degree bends IN THE BOX are what helps with the isolation. The waves hit the wall and bounce back and cancel some while the airflow keeps moving.

Bryan

Other than being more restrictive to airflow, would 180 degree bends work? I'm thinking 8" flex into the box, down 8 feet, 180 up, 8 feet back, 180 up, 8 feet back and out of the box. 3/4 MDF all around including dividers between 8 ft runs in the box?

**bpape** · **07-13-09, 04:33 PM**

180's are really going to restrict things. Plus, as a 180, it really doesn't count as 2 90's since there's basically zero distance between bends.

Bryan

**jpk** · **07-29-09, 04:07 PM**

Thanks Bryan for sharing your knowledge and experience with us. I didn't mean for the original discussion to get sidetracked, but useful information for us!

Some of the cold air return utilizes the space between the joist, sheet metal nailed to the bottom of the joist to form the duct. My first thought was to replace the tin with masonite or ?? but I have code concerns? Next, I thought of 1 x 2 boards across the joist with rubber against the tin, maybe replacing tin with duct board would be the best option?

What’s more problematic are the main ducts running almost the width of the room, tapering down every time a supply duct goes up into the wall. Can the supply ducts in the wall be replaced w/o knocking holes in the wall? Is replacing the main and joining it to the tin supply ducts enough?

I’m not expecting soundproof in this retrofit, just trying to minimize some problems.

Thanks,
Joe

**bpape** · **07-29-09, 06:35 PM**

If it were me, I'd use some pipe damping on the tin. I'd also tap into another run outside the room and bring in something separate for supply and return rather than opening up the existing ones in the room.

Bryan

**jpk** · **07-29-09, 08:56 PM**

Quote:

bpape wrote:

If it were me, I'd use some pipe damping on the tin. I'd also tap into another run outside the room and bring in something separate for supply and return rather than opening up the existing ones in the room.

Bryan

What is "pipe damping"?
I thought about that stuff the car buffs use, a denser sorbothane (?) self sticky on one side, can't remember what it's called but sticks to the metal panels and damps, at least to some degree, the obnoxious, excess bass rattles in cars. Is this worth considering and is there an an affordable source? For a quantity?
Come to think of it the company that modded my CDP strategically placed pieces of it inside and above the transport.

Thanks,
Joe

**bpape** · **07-30-09, 12:44 AM**

Same kind of thing. I can get it at a somewhat reasonable price. PM me

Bryan

**blekenbleu** · **08-01-09, 05:58 PM**

Quote:

jpk wrote:

Some of the cold air return utilizes the space between the joist, sheet metal nailed to the bottom of the joist to form the duct. My first thought was to replace the tin with masonite or ?? but I have code concerns? Next, I thought of 1 x 2 boards across the joist with rubber against the tin, maybe replacing tin with duct board would be the best option?

If those returns are wood on 3 sides, I wonder whether that meets code for new construction.
Anyway, I salvaged most of that sheet metal for other purposes and replaced it with fiberglass duct board.

Quote:

What’s more problematic are the main ducts running almost the width of the room, tapering down every time a supply duct goes up into the wall.

IMO, that is good design; maintaining constant cross-section promotes constant velocity,
which improves HVAC efficiency and minimizes noise creation. Right angles in
air ducts provoke reflections which hurt efficiency and generate noise.
In my system, most noise was generated where air impacted the first turn.
Easing air around corners with curves promotes improved air flow.
As you may know, many air movers have multiple speed taps.
Improving airflow in ducts allows running slower speed, saving energy as well as reducing noise.
With our variable speed air mover, when replacing right angles
with curves, I could hear the blower slow down for the same airflow.

Quote:

Can the supply ducts in the wall be replaced w/o knocking holes in the wall? Is replacing the main and joining it to the tin supply ducts enough?

We left many original branch ducts, but installed some larger registers to drop air exit velocities
and connected supply ducts to register heads with flex. Since my home theater is at the
end of a trunk, air velocity and noise were already low, so maintained original register sizes
but connected heads to branches with flex to isolate vibrations. Similarly,
fabricated curved transitions at the main trunk replaced right angles
and connected to branch ducts using flex.

I assume that your system has vibration isolators between trunks and the air mover..

**basementjack** · **08-02-09, 12:54 AM**

A bit late to the conversation, but I have something minor to add...

When we did our basement HT, I had a heating contractor come out and help with a few new runs so the basement could be warmed/cooled as needed. He had calculations for how much each bend reduced airflow, then increased the size of the feeds by the appropriate amount to compensate.

For the AV room, the 'last leg' consisted of square tubing (maybe 10x10 or 10x12 cross section) inside the tubing was a black insulation blanket that was coated or treated on the air side to avoid fibers coming loose. Taping on the metal ducts reveals a dead thud instead of the usual ting.

It's worked out pretty well for High and Mid frequencies - I don't hear the rest of the house at all in the theater. The rest of the house doesn't hear things like gunshots.

As for Bass frequencies, I'm of the belief that it's nearly impossible to 100% contain via the methods that are being discussed here. (I'm sure there are some extreme ways of doing it, that none of us can afford..)

My advice is do what seems reasonable, and have reasonable expectations -
Getting rid of the ringing is possible for a reasonable price, keeping the bass contained is not.

I like Bryan's suggestion of MDF/Particle board around your duct work - I can see that having benefit at a low cost.

Another suggestion (if you don't already have them) are to install bass shakers in your main seating.
Bass shakers are pretty cheap (as far as this hobby goes) and give you a lot of flexibility.

**blekenbleu** · **08-02-09, 09:09 AM**

Quote:

basementjack wrote:

As for Bass frequencies, I'm of the belief that it's nearly impossible to 100% contain via the methods that are being discussed here.

Agreed.
One problem with basement home theater: concrete and cinderblocks do not absorb bass,
and basement ceilings are fairly transparent to it.

My basement is for junk; there is a brick wall between our home theater and rest of the house.
Practically no sounds leak thru our HVAC ducts; most leaks thru the door.

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