Ducting for Home Theaters

[jpk]

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]

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]

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.

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]

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

Bryan

[cavchameleon]

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]

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]

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/insulation/hvac-mechanical/317386

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]

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]

blekenbleu wrote:

I got CertainTeed inch-thick years ago from the local Ferguson Enterprises outlet.
http://www.certainteed.com/products/insulation/hvac-mechanical/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]

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