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PostPosted: Sat Nov 17, 2007 3:26 am 
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Location: Newark, DE
My question is regarding my silencer box idea.

I am planning on using the area in between my 24"OC 2x6 joists as the silencer box. They will be 4 feet long with 3 layers of drywall on top and bottom. There will be a 4" round duct hole leading to the fan unit above in attic and another hole for the room vent.

Please help me finalize these plans. Once the drywall goes up there's no going back!

They are inspired by the models I'm seen on the board. They utilize Owens-Corning 703 to line the boxes and for baffles.

Please see pic below(not drawn to scale and NOT showing 703 on top and sides):

What spacing should I use between the baffles?

I need to keep the flow area at twice the area of the 4" (A = pi*D)round duct which would be about 24 square inches?

So if my boxes are 3.5" high(after 703 lining on top), then the baffles should be about 7.5" apart?

Any other comments are appreciated.

FYI:

I'm using a Fantech HRV for ventilation. It's rated at 70 cfm.

My room is approx. 9.5W x 18L x 8.5H = 1453 cubic ft.[/b]


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PostPosted: Sat Nov 17, 2007 5:34 am 
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Location: Easton, Pennsylvania, USA
studio_drums wrote:
My question is regarding my silencer box idea.

I am planning on using the area in between my 24"OC 2x6 joists as the silencer box. They will be 4 feet long with 3 layers of drywall on top and bottom. There will be a 4" round duct hole leading to the fan unit above in attic and another hole for the room vent.

Please help me finalize these plans. Once the drywall goes up there's no going back!

They are inspired by the models I'm seen on the board. They utilize Owens-Corning 703 to line the boxes and for baffles.

Please see pic below(not drawn to scale and NOT showing 703 on top and sides):

What spacing should I use between the baffles?

I need to keep the flow area at twice the area of the 4" (A = pi*D)round duct which would be about 24 square inches?

So if my boxes are 3.5" high(after 703 lining on top), then the baffles should be about 7.5" apart?

Any other comments are appreciated.

FYI:

I'm using a Fantech HRV for ventilation. It's rated at 70 cfm.

My room is approx. 9.5W x 18L x 8.5H = 1453 cubic ft.[/b]



SD -

I just built one - and you can see my pix in my post (Easton, PA Studio Build). There are questions that I too have and am looking for answers. I'm wondering if need to build one for EVERY duct that comes off the manin trunk? And are they needed for air returns? If you get any insight on these topics, please let me know as well.

Best of luck.

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PostPosted: Mon Nov 19, 2007 5:13 am 
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interesting question... i think you need to have a silencer for each supply or return you have into the room otherwise what keeps the sound from simply going up a 12" hole and into the lightwight (by comparison to the wall or ceiling mass) duct?... or can you simply build a massive MDF box around it?


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PostPosted: Tue Nov 27, 2007 12:52 am 
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Location: Newark, DE
Here's what I ended up building...

3 layers of green glued drywall are the top.

I decided to forget the baffles and just use fabric covered 703 as my "duct liner" due to lack of space.An opening will be cut out for a vent into the room per ABOVE sketchup drawing.

This leaves me with a 18" x 1 1/2" space for the air to travel,- this is a 27 inch area, which is a little more than twice the area of the 4 inch round duct.

3 layers of drywall will provide the bottom to the box(they will cover the exposed 703).

I glued white fabric to the 703 to hold the fibers in place. This will introduce some air friction, but the drywall bottom will compensate.

The vent going into the room is yet to be determined, but will probably be 4"x10".


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PostPosted: Tue Nov 27, 2007 12:32 pm 
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I like em - and might copy for my returns!

Anyone see any issues with this design?

Bump, nudge-nudge...

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PostPosted: Tue Nov 27, 2007 3:48 pm 
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BradJacob wrote:
I like em - and might copy for my returns!

Anyone see any issues with this design?

Bump, nudge-nudge...


Sorry it took me so long to get here,,

I do not like it at all........

1st off I have no knowledge of the velocity of the air you're moving through that duct - so I have no way of knowing if the 2 times the size rule (my general rule) has any real meaning in this case.......

Next - a 1 1/2" channel to push air through is going to create quiet a bit of back [ressure on it's own........

Understand - some ofthis has to go hand in glove with common sense -

Suppose you have a duct that is one square foot in area - I can double that by installing a piece of duct 4' long by 4' wide by a 1/2" clear opening in height - that should work - right?

WRONG........ when looking at the work pictured above......

the duct configuration itself is going to cause problems because nothing will spead the air out throughout the width of the duct from that little 4" inlet I see in the corner - it will tend to bunch up - find the path of least resistance =- and then wind up with little to no decrease in velocitry 0- but lots more biffetting of the air as it moves......

Then there is this issue with back pressure created in the air supply duct that is tryin to turn a 90 degree corner at the end of a duct run in a 1 1/2" space - which will cause tremendous buffeting at that point and a very large reduction in air flow........

Nothing I even discussed with anyone even considered anything like this -

And I have no confidence that it's going to work even remotely like one would want it to.

You would have been much better off to figure out a way to do some sort of a drop soffit to encompass the duct work that would have worked in your space while allowing the duct outlet to do it's job....... I am ssure something architectually pleasing could have been worked out if you thought your way through it.........

Rod

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PostPosted: Tue Nov 27, 2007 10:34 pm 
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I like em - and might copy for my returns!

Anyone see any issues with this design?

Bump, nudge-nudge...

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www.theSecretSystemBand.com

"...over the years, 'the-blues' has raised many children..."


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PostPosted: Wed Nov 28, 2007 12:13 am 
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rod gervais wrote:

1st off I have no knowledge of the velocity of the air you're moving through that duct - so I have no way of knowing if the 2 times the size rule (my general rule) has any real meaning in this case.......


Fan unit is listed above- Fantech SH704 @70cfm


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PostPosted: Wed Nov 28, 2007 6:12 am 
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Location: Newark, DE
I'm happy to report that the silencer boxes work!

I cut 4"x10" holes for the 'vents' per the picture above.

The air flow was really good. Standing in the room below the opening I could feel the air blowing down on me. The return/exhaust had a good deal of suction- it had no trouble sucking in some drywall dust I created right below it...

I tested the unit prior to installation- the flow rate is pretty close considering the 15 feet of flex duct and the silencer design.

I think the reason why the boxes 'work' and don't impede the air flow that much is due to the fact that:

1. The depth is closer to 1 3/4"
2. The distance from 4" round duct to vent opening is only about 40 inches.
3. 35-40% of the interior surface area is smooth(drywall on bottom)

This design may not be ideal, but it works surprisingly well and suits my needs.


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PostPosted: Wed Nov 28, 2007 12:45 pm 
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studio_drums wrote:
The return/exhaust had a good deal of suction- it had no trouble sucking in some drywall dust I created right below it...



Rod -

Speaking of this:

For my air-returns, I'm using 6" duct.

I connected the duct (hard duct) to the main return trunk with pop-rivets and sealed the mating surfaces with caulk. I then connect some black-faced flex duct and routed into my ceiling.

When I held my hand over the opening, I didn't feel any real "powerful" suction.

Is that "normal". It was sucking air, but not like a vacuum. Should I reduce down to a 4" duct when it exits the wall?

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PostPosted: Wed Nov 28, 2007 2:36 pm 
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BradJacob wrote:
studio_drums wrote:
The return/exhaust had a good deal of suction- it had no trouble sucking in some drywall dust I created right below it...



Rod -

Speaking of this:

For my air-returns, I'm using 6" duct.

I connected the duct (hard duct) to the main return trunk with pop-rivets and sealed the mating surfaces with caulk. I then connect some black-faced flex duct and routed into my ceiling.

When I held my hand over the opening, I didn't feel any real "powerful" suction.

Is that "normal". It was sucking air, but not like a vacuum. Should I reduce down to a 4" duct when it exits the wall?



No - doing that would increase velocity - which would increase noise.......

In a perfect world you decrease velocity (what you feel) while maintaining volume (how much you move)

Rod

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PostPosted: Wed Nov 28, 2007 4:20 pm 
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Just to clarify, the suction at the return box isn't what I would classify as "powerful" (wouldn't want it to be), but it's very good. Same with the supply. In fact the rate of air flow seems "just right". Also notable, there is no wind or air buffeting noise.


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PostPosted: Wed Nov 28, 2007 6:37 pm 
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studio_drums wrote:
Just to clarify, the suction at the return box isn't what I would classify as "powerful" (wouldn't want it to be), but it's very good. Same with the supply. In fact the rate of air flow seems "just right". Also notable, there is no wind or air buffeting noise.


Drums,

man, if it works for you - in the end - that is all that matters......... I never would have thought for it to work well - but then again - I am always amazed at just how much more I have to learn.......

Sincerely,

Rod

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PostPosted: Thu Nov 29, 2007 1:35 am 
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Location: Newark, DE
Thanks Rod. I know you're looking out for all of our best interests- and we sure appreciate it. I know I do.


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