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Discussion Starter #1
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
 

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Re: The astounding new AS-EQ1 SubEQ goes on sale!

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
 

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Re: The astounding new AS-EQ1 SubEQ goes on sale!

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.
 

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Re: The astounding new AS-EQ1 SubEQ goes on sale!

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

Bryan
 

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Re: The astounding new AS-EQ1 SubEQ goes on sale!

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
 

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Re: The astounding new AS-EQ1 SubEQ goes on sale!

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
 

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Re: The astounding new AS-EQ1 SubEQ goes on sale!

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.
 

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Re: The astounding new AS-EQ1 SubEQ goes on sale!

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
 

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Re: The astounding new AS-EQ1 SubEQ goes on sale!

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
 

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Re: The astounding new AS-EQ1 SubEQ goes on sale!

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
 

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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!!! :bigsmile:).

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
 

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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
 

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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
 

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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
 

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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?
 

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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
 

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Discussion Starter #19
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
 

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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
 
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