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Discussion Starter · #1 ·
Currently, I have flex duct coming right off of my supplies (buried in the joists), through a 5" hole and into the room. I didn't do any dead venting on the outside of the room, so I basically have a big 5" hole a bit of insulation, and then the the subfloor of the main floor.




I have planned on building a dead vent type of thing on the inside of the room. I will have a soffit, so hiding it shouldn't be hard. Basically, I was thinking I would take sheets of MDF, cut them into 5" wide strips, build something that looks like a coffin a few feet long, and have the flex duct enter it at the ceiling and exit it (and the soffit) at a register mounted to the bottom of the soffit. At least then, I don't have a hole ddirectly out of the room. I could possibly even build something a bit wider, and snake the flexduct through a couple of baffles.


Does anyone know what the best way to accomplish this is to prevent sound leakage/flanking through the hole?
 

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I did basically the same however slightly different. My 6" feed into the room is flex duct but I made sure that my run from the room to the main truck had an S bend in it.


Inside the theater, I built a muffler out of 1" rigid duct board. That muffler has a 6" opening to connect to the flex duct and inside the muffler there are sections of 1" duct board that cause the air flow to weave it's way around in a maze like fashion before exiting through another 6" hole. I have the muffler in a soffit that is constructed of 3/4" mdf. It seems to work well. The important part is to size the muffler so that you are not limiting the flow of air - just changing the path. Others on the forum have done this as well.


I'm going to do the same thing with another, separate 8" duct that will pull air from an unfinished area of my basement.



Best of luck.
 

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Ted White has a great diagram depicting how to reduce sound transmission through ductwork.


You could try searcing for posts by him, I'm sure you'll find it.


Wait long enough, I'm sure he'll pop into this thread too



Tim
 

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His article is here:

http://www.soundproofingcompany.com/.../the_dead_vent


I have read it more than once, but I always need a bit more concentration when reading about HVAC issues.


EDIT - Not quite fast enough.


EDIT 2 - If Ted chimes in, one thing I have alway tripped over is this part:
Quote:
If built below grade (basements, lower levels, etc) consider simply exchanging the theater room air with the rest of the lower level. This avoids tying the sound controlled into the main HVAC.

Is the thought that the exchange room is finished space or unfinished space, or does it matter?
 

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Discussion Starter · #6 ·
I've looked very closely at Ted's dead vent diagram numerous times in the past.


While I normally find his resources very clear and helpful, that article stumps me. For some reason I can't picture what it looks like in 3d/real life.


Regardless, I don't have enough room for one of those inside the room without it looking like a big, well, dead vent. So I was hoping to incorporate some of the principles into something simpler and smaller that fits in a soffit.


My flex duct connects to the ducting via a 4' long silencer, so I think I am less concerned about flanking through the ducts than I am about minimizing the sound escaping directly up to the next floor through the big hole.
 

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I have flex duct coming right off of my supplies and buried in the my joists also. I then stuffed insulation around the flex duct and covered my soffets with partical wood and 5/8 drywall with GG.


I can't hear a thing at the end of the duct where it goes into the supply.












 

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Quote:
Originally Posted by dc_pilgrim /forum/post/15571210


EDIT 2 - If Ted chimes in, one thing I have alway tripped over is this part:



Is the thought that the exchange room is finished space or unfinished space, or does it matter?

Not to hijack, but I think what it means is you run your hvac into an ajoining space, not into the theater. Then you exchange the [stale] air in the theater for the conditioned air in the adjoining room.


Basically, you are leaking sound into the adjoining room rather than into the hvac system.


Tim
 

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Discussion Starter · #9 ·
That looks good Woolly....very similar to what I am considering. I think I want a "tighter" box (leave it alone boys...) around the flex duct to minimize the volume used by the HVAC in the soffit. I still need to get can lights, electrical, low voltage and speaker runs through my soffit.


So no sound appears to come in...but do you think the bass is escaping to the other floors?
 

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I just built two of these based on Ted's designs.


I'm sucking air in from the adjoining room, and venting it as well. The room is unfinished at the moment, but will be finished some day.


The key thing with not tapping into your existing HVAC system is CFM. I bought el-cheapo fans at home depot, and while they do move some air, it is very low speed. It will be an interesting question to see if actually keep the room comfortable.


I also deviated from Ted's design by not green gluing drywall to the MDF boxes I built. There simply wasn't space in my soffit to do so, so I can't comment on the boxes ability to keep bass from radiating out through the boxes into adjoining room, they certainly stop normal midbass and highs from coming through, I can't even hear the fan when I put my ear up to the vent.


As Ted explains it, think of it as a silencer for a gun. The two most important things are:

A) Dampen the chamber to keep bass from vibrating the box

B) include S bends and large volume transitions to maximize absorption of flanking noise.


I found I didn't have to go crazy with the bends, in fact my supply line doesn't have any until it exist the theater, simply routing it around the ceiling joists introduced plenty of bends.


Pics of my build (many, actually) are in my build thread. Click the sig link...
 

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Discussion Starter · #11 ·
I checked out your dead vents. Very elaborate. I might be able to get something like that up into my soffit. Or at least a simplified/shrunk version of it.


Here's a pic of my setup. There is a mirror image duct on the opposite wall. I slapped up some green tape where I think the dead vent will run inside the soffit. My door frames (I have three doors into my room. An unfortunate necessity) are only 9" from the ceiling, so I am much more depth limited, than width limited on my soffits.






Is that enough room to get a worthwhile deadvent? It's about five feet from the duct entry to the 2x6 that represents the false wall.
 

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Discussion Starter · #12 ·
Also:


1. I presume one can add the fans later, if required. Does it really matter if an in-line fan is right at the vent, or closer to the take-off of the furnace?


2. I have a lot of 1/2" MDF i was hoping to use. Is it worthwhile to double this up with GG in between?
 

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Quote:
Originally Posted by MacBuster /forum/post/15584860


Also:


1. I presume one can add the fans later, if required. Does it really matter if an in-line fan is right at the vent, or closer to the take-off of the furnace?


2. I have a lot of 1/2" MDF i was hoping to use. Is it worthwhile to double this up with GG in between?

What I chose to do was route the duct work within a few feet of the existing supply and return main trunks. I did in case I ever wanted to sell the house, as I"m sure a dead vent's not to code in my area. I also did this in case my assumption that I just need to "cool" the room is wrong. With the projector and equipment rack in the room, i assumed heating it wouldn't be an issue.


I did position my return vent right between the projector and rack, so that it sucks heat off the equipemnt. If you can do the same thing, I suggest doing so. Makes it very easy to build a vented hush box later.


To answer your questions:

1. No, I don't think it matter in terms of airflow, but it matters in terms of other reasons. My fans are at the very end. I put them there because they need to be in a serviceable location. Fans wear out over time, so I wanted to be able to get to them. Fans are also noisy. I put them at the opposite end of where my ears will be.
So my suggestion is mount them at the endw here you can reach them.


Don't forget you will need 14/2 run to them in a junction box. I also made mine a switched outlet in case I need to turn them off. Makes servicing them a bit easier too.



2. Doubling up 1/2" MDF with GG would be excellent. The GG serves as the dampening. MDF vibrates a lot on its own. My boxes need bracing and additional dampening, but there isn't room to do so. I am GG drywall to the bottom and sides of them, just can't get to the top to do it. I do have them wrapped in loose fiberglass when I'm done.


I guess it just depends what your goals are. If you are just worried about flanking noise, then 1/2" by itself might be find, but that won't stop bass vibration.
 

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Quote:
Originally Posted by MacBuster
I checked out your dead vents. Very elaborate. I might be able to get something like that up into my soffit. Or at least a simplified/shrunk version of it.


Here's a pic of my setup. There is a mirror image duct on the opposite wall. I slapped up some green tape where I think the dead vent will run inside the soffit. My door frames (I have three doors into my room. An unfortunate necessity) are only 9" from the ceiling, so I am much more depth limited, than width limited on my soffits.






Is that enough room to get a worthwhile deadvent? It's about five feet from the duct entry to the 2x6 that represents the false wall.



Here's a typical design for you. PM me your e-mail address and I'll send you the file. I'd build a box 60" by 9" by 24". Those are the outside dimensions. build it in two layers of 1/2" MDF with GG, and those inner partitions can just be 1/2".


Use a router or coping saw to make 6" diamter holes to accept flex duct. I would not run that full lenght of flex duct, you're going to want some transitions. My design here has two transitions for you. The first is the flex duct into the 9" x 10" x 24" inlet. That's a massive space transition.


THe square inches of the inlet pipe pi(3")^2 = 28.2". The square area of the box is: 9x10=90". so you've basically tripled the area, which cuts the air speed down.


The middle sectino is where you run your flex duct. Try to snake it around in there. 6" duct is about 11" wide, so you cant' really double back on yourself, but you can make a decent "Z" shape there.


The last section where I show the vent has a small section of flex duct attaching the vent to the 6" hole. Stuff the space around that chamber with R-13 fiberglass to absorb any direct transmission into the chamber.


If this is too big just shrink the depth, just leave enough room to fit the inlet vent in (which is about 16-18" long, or change the design to have the vent go to the side.


You can also build it in three sectinos to make it easier to go up. THe exterior will still need framed to support drywall, the MDF won't support drywall on it's own.


Drill through the box into the framing above to mount it.


EDIT: Found I can just simply attach the file.

 

macbuster.zip 21.4833984375k . file
 

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Discussion Starter · #15 ·
You rock!


I will PM you my address immediately.


Thanks!
 

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Discussion Starter · #16 ·
The inlet segment doesn't have flex duct, right? It's just the space created by the box.


So what do you line it with so the MDF skin isn't exposed to direct heat?
 

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Quote:
Originally Posted by MacBuster /forum/post/15589017


The inlet segment doesn't have flex duct, right? It's just the space created by the box.


So what do you line it with so the MDF skin isn't exposed to direct heat?

Hmm, how hot do your vents get? I hadn't considered that aspect, since no heat goes through mine. You could line it with lincoustic duct liner, that would prevent heat issues, but that would have to be some dang hot air going in to ignite MDF.


And no, no flex duct there, unless you can locate and fit some 12" duct in there, which you can't, because of the 9" height limitation. Going to 12" duct is a superior solution, but not really space feasible in many installs.
 

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Wow. How'd I miss this thread...


A few overall points about the Dead Vent:


#1 you can use it on supplies and returns


#2 you can use it to exchange air with the rest of the basement or install them before the ducts meet up with the main HVAC


#3 The outer cabinet walls ideally would be double ply or MDF and damped (GG, etc)


#4 The air path would ideally be surrounded by insulation through the entire length of the Dead Vent. Better attenuation per foot that way.


#5 The change in air volume is a big deal. This is fundamental in most sound attenuators. Go from 6" flex to 12" for example.
 

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Discussion Starter · #19 ·
Unfortunately, #5 is out for me. Because these things are residing in a soffit, I'm restricted to an 8.5" depth for the entire box.


I was thinking of doing multiple baffles inside a 4-5' box and winding the flex duct through that in a Z pattern.


That way, there will at least be a physical barrier between the hole int he aquarium and the exit hole in the duct.
 

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Increasing the length of the run would help a lot. Also, we want to have the duct travelling through a protected soffit. That is, the soffit is covered with double rock. Otherwise the sound going through the single drywalled soffit would continue to pollute the flex duct.
 
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