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Old 06-16-2010, 04:03 PM - Thread Starter
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I'd love to see what everyone has done for their Dead Vents and HVAC. Post some pics for us all to see.
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Old 06-16-2010, 08:28 PM
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I may be in the minority here, but I think this would make an excellent informative show me thread.

This is one area of home theater construction that I have so much more to learn.

Gathering ideas for Version 2.0 Home Theater
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Old 06-17-2010, 06:04 AM
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Here is my duct muffler. 6" inlet and outlet. size to fit in my soffit and it's connected to 6" flexible duct snaked in a S shape. I get good air flow without the sound of air moving and i don't notice any sound getting out of the theater through the duct.

There are 3 angled baffles inside that cause the air to snake it's way through. I calculated the area of the 6" inlet and made sure that I kept that amount of sq. area throughout the muffler so that I am not causing any backpressure other than what you would normally have with duct board.

It's not as large and doesn't use as much absorption as the experts willl tell you is needed, but it fits in the space I had available and appears to work really well.


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Old 06-17-2010, 10:36 AM
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Quote:
Originally Posted by jon_b View Post

i may be in the minority here, but i think this would make an excellent informative show me thread.

This is one area of home theater construction that i have so much more to learn.



+1
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Old 06-21-2010, 02:20 PM
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budk, do you have any more details on how you built the duct muffler and materials used?

Thanks
JB
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Old 06-22-2010, 06:06 AM
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1" duct board. Picture a maze.... the air comes in one side, snakes around each of the 3 baffles and then out the other side. Everything is duct board. Each baffle is sized so that they don't limit the amount of area for the air to flow but they are also spaced so the air (and sound) can not flow in a straight line. I don't have a drawing.

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Old 06-22-2010, 06:18 AM
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Would youmind cutting it open for us to see?
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Old 07-12-2010, 10:39 AM
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Old 07-12-2010, 12:07 PM
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Yep, I would mind cutting it open since it would entail removing the fabric from my soffit to get at it... not to mention that I have no intention of cutting it in half!

It's really not rocket science..... you just have to make the air move through an insulated box in a path that is not straight while making sure that any opening are big enough so as to not impair your airflow. Mine was designed to fit the space I had.... everyone's could be different so there is no one set way to accomplish the same desired result.

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Old 07-12-2010, 12:12 PM
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The biggest issue I see with some designs is a real lack of absorption. Fluffy R13 is great. Ductboard (while neat and tidy as can be) doesn't have the optimal surface area / density for high level absorption.
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Old 07-12-2010, 12:35 PM
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Ted,
I have been thinking about doing something like that with my dedicated room, but the vents would be for fresh air from outside (in the front), and exhausting room air back outside (in the back). Would this be the best dead vent/muffler to use? They'll have to be built inside the theater with just vents outside. I don't have much space, and there is no central HVAC. I'm going to use a mini-split heat pump for AC and heat (if needed).

The finished room size will be ~ 11x18x9. I guess I'll need to pull out my calculator to figure out proper fan sizes.
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Old 07-12-2010, 12:38 PM
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You can use a dead vent (short and squat) or a soffit muffler (long and thin) or as many have successfully done... a combo of both.
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Old 07-12-2010, 08:10 PM
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so this takes air out of the room. what is being done to bring air into the room for the air exchange. . is it just pulling from the supply vents, or is there a second deadvent, pushing air into the room?
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Old 07-12-2010, 09:36 PM
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Quote:
Originally Posted by Ted White View Post


Is there any reason this can't be done horizontally instead of vertically? Also, should the room side opening to the dead vent be at the end of a long (say 25') wavy run of flex duct, or is it right there on the other side of the wall?
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Old 07-13-2010, 04:57 AM
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I thought I had this figured out - why the 12" to 6"?

If my memory serves me correctly, velocity increases as the pipe narrows, but pressure decreases, isn't t he aim to get velocity at the vent lower?

In the above illustration is the vent going from narrow to wider which decreases velocity? just that the vent isn't that large so its not as obvious?.....I still dont get it

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Old 07-13-2010, 05:25 AM
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These are from Ultrafonic, so NOT my work or suggestion, but I'll be building something similar for ours though:




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Old 07-13-2010, 07:38 AM
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Imagine that the sound will not stay within the confines of the flex duct. The duct has nearly no mass, so while the airflow will stay within the flex, the sound waves (except for very high frequency) will not. All those turns are largely unnecessary. The sound waves will leave the flex and fill the soffit / muffler / dead vent. So the design shown could use a lot more absorption. Less space dedicated to the "S" turns, more space dedicated to absorptive material.

Sound waves will take the direct path diagonally across that unit, attenuating less than it could / should.
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Old 07-13-2010, 08:23 AM
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Ted, I hope you dont mind that I modified one of your drawings to better illustrate my question...

For a single stud wall the cavity where the PVC/Flex penetrates is basically contained by the framing.

However for a double stud wall there is a gap (say 1")between the two walls that provides a path for the sound to travel up/sideways/etc. between the framing members. Other than just adding pipe wrap to the PVC are there any other special considerations for the path of the HVAC through the walls in this situation?


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Old 07-13-2010, 09:30 AM
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I have a new drawing coming... Some improvements as well as simplifications.
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Old 07-13-2010, 05:12 PM
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Quote:
Originally Posted by Ted White View Post

I have a new drawing coming... Some improvements as well as simplifications.

Great! I had a lot of questions about the seeming multiple leafs of double drywall + green glue that one seems to show.
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Old 07-13-2010, 05:17 PM
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Hmm, ok, I so Ted's diagram is better from a soundproofing perspective, but higher velocity could be a problem from a HVAC noise perspective - I guess this is where the vent is important?

Or does the drop in pressure from a narrowing vent make it quieter, even if velocity is higher?

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Old 07-14-2010, 03:35 AM
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Peter:

That design could be improved ... all of those sharp 180 turns will create velocity and noise problems of their own.

Just talking (typing) aloud here:

I would not use flex duct (unless there's a local code issue). Basically, I'd keep the MDF baffles but make each chamber larger. Then, I'd line the entire inside of the muffler with two 1" layers of something like Johns-Manville insul-shield. Between those two layers I'd install a 3 to six mill poly sheeting. The first, fully exposed layer will latch on to the higher frequency noises. The poly sheeting turns the second layer into a diaphramatic absorber improving lower frequency performance. The whole thing is more efficient if the fan is located close to the exhaust (pulling air from the room). Pulling air is more efficient than pushing and the fan is further from the room.

Also, while you are creating an opportunity for sound leaving the room, your "real" objective is to eliminate outside sounds from getting into the room. To not affect the rest of the house, you need a 70dB reduction. For the house to not affect the theater, you need a 17dB reduction.

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Old 07-14-2010, 04:25 PM
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Quote:
Originally Posted by Dennis Erskine View Post

The whole thing is more efficient if the fan is located close to the exhaust (pulling air from the room).

Cool, thanks Dennis - DIY ducts it is, just to clarrify, you're saying put the fan on the "room side" of the dead vent? I'd have thought I'd want the fan at the other end so I dont get any potential fan noise back into the room?

Your suggestion makes more sense to me re velocity etc and I can build a duct around any vent I decide on so that is easier as well

Whilst I primarily dont want noise to get in I also really dont want noise to get out -70dB is a tough ask! see how we go I guess.....

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Old 07-15-2010, 08:33 AM
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- High level of absorption from the significant amount of insulation

- Slight "S" bends to accomodate the very high frequencies.

- Rapid increase in cross section (6" duct to 24" x 24" DV)

- Reduced air flow resistance
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Old 07-15-2010, 02:51 PM
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Is this a superior design to the old one, or simply a different way to skin a cat?

Also, is there any benefit to placing a large amount of flex duct between the intake vent in the room and the dead vent? Like the dead vent is in the back corner, but the room's intake side of it is in an adjacent corner with say 20' of duct between them?
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Old 07-15-2010, 04:14 PM
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Excellent, this thread is very informative, now I need to figure out how to incorporate Ted's last design with an exhaust line from the hush box of my PJ. Anyone have any ideas how to take 2 ducts (1 intake from HT and 1 duct from PJ) and combine them into 1 exhaust, kind of a like a Y adapter?
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Old 07-15-2010, 04:25 PM
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you can get y adaptors - not sure about fan type though (pressure issues). I had planned on having my exhaust running from the "hushbox" anyway

Duct - hushbox - duct/little dead vent - room

Kill two birds with one stone so to speak

Edit: I am not having a totally sealed hushbox i.e. no glass front over the lens

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Old 07-15-2010, 05:07 PM
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It sounds like you're attempting something similar. Could the intake of the DV also be the intake of your hushbox? Essentially pulling the air from the room, past the PJ and exhausting it through the DV? Or am I totally lost?
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Old 07-15-2010, 05:45 PM
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yep thats what I had planned - thing is I dont know how effective it'll be, becuase the hush box is partly open at the front that'll pull air. I had thought about putting a diffuser at the back of the hush box (facing the floor) or I just have a fabric cover on the entire front of the "not so hush box" and let air pull in there

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Old 07-15-2010, 07:29 PM
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Quote:
Originally Posted by Dennis Erskine View Post

The whole thing is more efficient if the fan is located close to the exhaust (pulling air from the room). Pulling air is more efficient than pushing and the fan is further from the room.

Could you explain this further? I'm just curious, one of the few things I remember from thermo is it's usually easier to push a fluid than pull because at most you can generate is ~30" Hg of vacuum (I'm going from memory on that, IIRC that's a perfect vacuum).

Like I said, just curious about the physics of the problem

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