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PostPosted: Tue Dec 13, 2016 11:14 am 
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Location: Naples (Italy)
Hello,
I am about to finish my studio, but I have am kinda lost when it comes to baffling my HVAC system.
I have a room within a rom with the first leaf made of 2 layers of 5/8" drywall with Green Glue, 12,5cm air gap filled with 7,5cm fiberglass in metal joist, and the second leaf made of 15cm hollow concrete blocks with 1,5cm plaster on both sides. The leaves are decoupled.

My unit is a 9000 BTU Daikin Ururu Sarara. It provides fresh air intake, but I still need a separate exhaust duct.
The unit air flow is 7.5 m³/min in cooling mode and 8.6 m³/min in heating mode. My room is 50 m³ and I am the only person in the room 98% of the time, doing mixing work.
The unit comes with a 9,5mm gas tube, a 6,4mm liquid tube, a 16mm drainage tube and a ~38mm air supply tube (I believe internal diameter is about 20mm, the rest is insulation).
I'm assuming, of the four, the only tube I'm supposed to baffle is the air supply one and leave the other three alone.

Here you can see where I would like to place the return duct (which would be passive and I also need to be baffled) on the left side of the room and the supply duct feeding the unit on the right side. They are depicted in red. Here's the room:
Attachment:
HVAC baffler.jpg


I was planning on putting the one for the supply duct on the outer leaf, and the one for the return duct on the inner leaf. My plans are to build it pretty much like the internal leaf of my room: double layer of drywall with Green Glue and fiberglass.

I have a few questions...
How can I calculate the size, shape, and amount of obstacles of my baffles, both for the supply and the return duct?

I have seen a lot of baffle designs, but I don't understand whether they refer to different systems or simply larger rooms. For instance, what confuses me is should cut the tubes going into the baffles or should they just flex through the baffles?. Logic tells me I'm supposed to cut them, otherwise sound wouldn't be blocked if it stays in the tube and curves with it, but these designs showing baffles filled with fiberglass and foam confuse me. Do people just put fiberglass and foam into their baffles, unfiltered, and have them floating into their room?
Here are a few designs I've seen. Do they apply in any way to the kind of room I'm building?
foam
scary baffle
scarier RockWool baffle
flex duct
Rod's baffle
fiberless baffle

The last question I have is, how would you suggest insulating the tubes going through the leaves considering I need to keep them decoupled?

I hope I was clear and that someone will have to the patience to deal with my baffled mind. :shot:


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PostPosted: Fri Dec 16, 2016 6:40 am 
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Location: Santiago, Chile
Quote:
I am about to finish my studio, but I have am kinda lost when it comes to baffling my HVAC system.
:shock: :!: If you are bout to FINISH your room, and you still don't have a design for your HVAC, then you might be more than just a little late! :)

HVAC design should be done at the same time as the isolation design: They go together.

Quote:
My unit is a 9000 BTU Daikin Ururu Sarara. It provides fresh air intake, but I still need a separate exhaust duct.
.... and a REAL fresh air intake, too! :) The small amount of air that thing brings in is not enough for a typical studio. Yes, skin divers with snorkels can breathe using a tube of about the same diameter, but it's rather uncomfortable... !

ASHRAE specs call for a number of "room changes per hour", based on several factors, but generally the number is around 5 to 10. Let's call it "6", to be reasonable. That means that you need to replace the entire volume of air in your room, 6 times every hour. Once every ten minutes. You say that the total volume of air in your room is about 50 cubic meters. 50 x 6 = 300 cubic meters. Trying to push over 300 m3 of air per hour through a tube that is not much bigger than your finger, is one hell of a task! :) Even if you prefer to go only with the minimum necessary "make up" air volume, instead of the full spec, you are still looking at pushing many cubic meters of air through that tiny little straw...

Forget that your unit even has that capability, and build proper supply and exhaust ducts.

Quote:
The unit air flow is 7.5 m³/min in cooling mode and 8.6 m³/min in heating mode.
Ummmm... yes, but that's how much room air it moves through itself, then back into the room! That's not how much air it moves through the fresh-air drinking straw...

Quote:
I'm assuming, of the four, the only tube I'm supposed to baffle is the air supply one and leave the other three alone.
Definitely do not touch the copper pipes! :ahh: Those things are highly pressurized with refrigerant. Don't ever try to remove those...

Quote:
I have a few questions...
How can I calculate the size, shape, and amount of obstacles of my baffles, both for the supply and the return duct?
Start with two basic parameters: The flow rate that you need for your room, and the flow velocity that you need. I'll do this in imperial units, since I'm more familiar with those, but you can convert them. Assuming six room changes per hour, you need to move a volume of air equivalent to about 180 cubic feet per minute (CFM). That's the flow rate that you will need to attain in order to move 6 times the room volume, every hour.
The maximum speed that you can allow the air to come out of the registers is 300 feet per minute. That's the air SPEED or VELOCITY, not the flow rate that we just saw above. Two different things.
With those two numbers it is simple math to figure out what the cross sectional area of your ducts needs to be, in order to allow 180 CFM to flow at 300 fpm. Do the math.

So that's the minimum cross sectional area of your duct! It cannot be smaller than that. And you need to allow for the duct liner that goes on the inside of your ducts, which is 1" thick (25mm). So your duct needs to be 2" wider and 2" higher than you think it does, based on cross section alone.

Now, with that in place, you can figure out the internals of your silencers. The general rule is that the cross sectional area must change, very suddenly, by a factor of two. In other words, at the point where the duct comes into the silencer box, it needs to widen out very suddenly to twice that cross section. Then you need at least two baffles inside your box, and they must be spaced far enough apart that the cross sectional area does not get any smaller. Once again, you need to allow for the thickness of the duct liner: your silencers will be completely lined with 1" duct liner, on all internal surfaces.

The thickness of the wood that you use to make your silencer boxes must be about the same as the thickness of the leaf that the box is working on. Or rather, it's the surface density that needs to be the same, not the thickness.

Quote:
For instance, what confuses me is should cut the tubes going into the baffles or should they just flex through the baffles?. Logic tells me I'm supposed to cut them, otherwise sound wouldn't be blocked if it stays in the tube and curves with it,
Correct. And it's not a tube we are talking about: HAVC ducts are BIG! Even the smallest ones are 4" in diameter... You will probably be using 6", or maybe even 8".

Quote:
but these designs showing baffles filled with fiberglass and foam confuse me. Do people just put fiberglass and foam into their baffles, unfiltered, and have them floating into their room?
They do, but only because they are very ignorant! :)

You should NEVER line your silencer boxes with ordinary insulation. Only ever use proper duct liner. That's what it is designed for. With normal insulation the air will erode the fibers away over time. Duct liner has a bonded surface that cannot be eroded by air. The fibers are held in place, and cannot fly away.

Quote:
The last question I have is, how would you suggest insulating the tubes going through the leaves considering I need to keep them decoupled?
I NEVER pass a duct directly through a leaf. That would be rather silly! The hole and the duct would totally trash your isolation. Instead, I design a wood "sleeve" that passes through the leaf. The wood sleeve has the correct internal cross section, and the duct connects to the end of it, on the other side of the leaf. If I need very high levels of isolation, I make the hole through the leaf just a few mm larger than the size of the sleeve, and I seal that gap with acoustic caulk. If the leaf is two layers, then the silencer will also be two layers, so I stagger the hole through the leaf, as two different size holes. Yes, that requires very careful, accurate work, and carefully thinking in advance regarding the construction sequence, but you should be doing that on your entire room anyway: It is a studio, after all! :)

Of all the ones on your list, the only one that seems to be done correctly is the first one.


- Stuart -

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PostPosted: Sat Dec 17, 2016 2:02 pm 
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[I made the questions bold for an easier read]

Quote:
That's not how much air it moves through the fresh-air drinking straw...

Got it. I misled myself there.

Quote:
Definitely do not touch the copper pipes!

I wouldn't. :D What about the drainage tube (the one expelling humidity outside in the form of water)? That seems to be a large source of noise. Are you suggesting it (along with the copper pipes) should just go through the leaves untouched, simply caulking the holes?

Quote:
Start with two basic parameters: The flow rate that you need for your room, and the flow velocity that you need.

Awesome! Thanks for the crystal clear explanation. According to my calculations, my cross section needs to be 86.5 sq inch, making it about 9.3x9.3", and making the silencer 18.6x18.6 (346 sq inch) (plus the thickness of the duct liner and the two drywall layers.
For other people looking for the equation, according to Engineering Toolbox it is:
Code:
A = q / v

where
A = duct cross sectional area (m2)
q = air flow rate (m3/s)
v = air speed (m/s)


Quote:
Then you need at least two baffles inside your box, and they must be spaced far enough apart that the cross sectional area does not get any smaller.

I'm aware that it's better for the silencer to be attached to the most sensitive leaf, but that would take up quite a lot of space here (yup, I'm paying the consequences of not planning ahead) so I'm only going to do that for the exhaust. I read your comment in another topic about an alternative when using one silencer on the least-sensitive leaf and it gave me hope:
Quote:
use a massive extension of the silencer box to pass through the cavity. So for example, you would build your silencer box on the outside of the outer leaf using plywood, then create a "duct" out of the same plywood that goes completely through both leaves and terminates in the register, on the other side of both leaves. That plywood "duct" would still need to be broken in the middle (to prevent sound flanking along it) by simply sawing all the way through it, then wrapping the gap with thick rubber (EPDM, Neoprene or Sorbathane) to seal the gap and make up the missing mass. [...] But you can only do that in cases where you need the register fairly close to the point where the duct penetrates the wall.

Since the supply silencer would be attached to the external leaf, and since my air gap between leaves is only 5", it sounds like a feasible solution in my case. I'd space the sawn parts of the "duct" with backer rod or some other elastic material as to avoid direct contact and wrap the gap with a few rolls of my 30mm duct liner. Should I also caulk where the two pieces meet or would that couple them? My other question is, should the "extended silencer" go through the leaf with a duct-sized, 86.5 sq inch cross section (and then inside the room attach the ø25mm of the provide pipe to go into the AC unit), should it have the 346 sq inch cross section of the silencer, or should it be the size of the pipe? The idea of drilling a 18.6x18.6" hole in the external leaf makes me feel a little dizzy... :mrgreen:
Here's a model of a 86.5 sq inch duct penetrating the two leaves and terminating with a hole for the HVAC's internal unit ø25mm pipe.
Attachment:
Schermata 2016-12-20 alle 17.59.37.jpg


Quote:
Or rather, it's the surface density that needs to be the same, not the thickness.

I'm using 2 layers of 5/8" drywall w/ Green Glue. Seems to have a slightly higher density and to be cheaper than OSB or MDF. Since caulking a box from the inside is not the easiest thing, do you suggest cutting the drywall leaving the space for the caulk at the risk of it not being perfect, or not leaving any space, avoid caulking and letting the liner fill the gaps? Density of the liner is 60kg/m3 and mine is 30mm thick. Also, suggestions on how to put together the box made of drywall? it's not the easiest material to work with as it crumbles, and I definitely wouldn't want that in the air system.

If I used 3 baffles, I'm estimating the silencer to be about about 7.5x3.3 feet. That would leave about 10" of space between the external wall (where the pipe from the external HVAC unit comes in) and the entry to the silencer. Could I get away with going straight from the external unit through the supplied ø25mm pipe into the silencer and avoid using a duct since the distance is so short or would you rather build that 10" duct?

Are there any precautions I should take for the other three pipes (water outlet, gas, liquid) or should they just run outside, parallel the silencer, untouched and with no insulation?
This is a sketch I've made of the silencer.
Attachment:
Schermata 2016-12-18 alle 02.33.48.jpg

I'll be tweaking the design since I actually have an obstacle in the way of the lower right corner (the silencer's cross section will end up being ~16.5x21.25"), but I'm wondering if I got the concept right.

The exhaust silencer's duct/pipe will penetrate the internal audio room's leaf, then after a 5" air gap it'll go straight into the outer leaf which is also the external wall of the building.
Should it end in the outside wall as a small ø25mm pipe (as to balance what's feeding the initial stage of the fresh air inlet duct), or should it be larger? I read what you wrote about creating positive pressure and I thought balancing what's coming in with what's going out would be important.

If anyone is interested, after a lot of searching, calling companies and talking to distributors, the duct liner I managed to find here in Italy was the K-Flex St Duct. They don't have a 25mm version, so I went with the 30mm. The alternative was 20mm.

I have one last, very delicate question since underestimated the silencer and planned for it way too late, I now need to face the issue of placing the silencer with my leaves already built. My silencers will end up being about 2x1x0,65cm. The exhaust one will be placed "flat" on the inner leaf, with the 2x1 panel facing the wall, whereas the silencer on the fresh air duct will be on the outside leaf, with the 2x1 panel facing the ceiling. Do you think the best way to install them would be to:
1) build my two boxes with only 5 sides and a missing 2x1 bottom panel and applying it directly onto the wall (for the inner leaf) and under the ceiling (for the outer leaf) essentially using the wall/ceiling itself as my 2x1 panel
2) cut a 2x1 hole into the drywall and screw the (whole, 6-sided) silencer box onto the joists for the inner leaf and hang the box from the ceiling with toggle bolts for the outer leaf?
3) other ideas?

I've read your post a few times to make sure I got it right. It's not simple stuff for me, but it seems doable.
Stuart, thank you so much for the incredible care into crafting your response!

Andreas


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Last edited by saemola on Wed Jan 04, 2017 4:43 am, edited 1 time in total.

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PostPosted: Thu Dec 29, 2016 10:39 pm 
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I have proceeded to cut all the necessary drywall for the silencer. Boy that was a lot, this thing is going to weigh a ton. I'll be getting some help from my drywall installer in order to assemble the box as I don't want to screw it up. Hopefully I'll be able to put it all together next week.


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PostPosted: Fri Jan 13, 2017 1:22 pm 
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Quote:
What about the drainage tube (the one expelling humidity outside in the form of water)? That seems to be a large source of noise. Are you suggesting it (along with the copper pipes) should just go through the leaves untouched, simply caulking the holes?
There won't be much flanking through that tube. It is usually soft and flexible, sometimes even corrugated. It won't flank much.

Quote:
My other question is, should the "extended silencer" go through the leaf with a duct-sized, 86.5 sq inch cross section (and then inside the room attach the ø25mm of the provide pipe to go into the AC unit), should it have the 346 sq inch cross section of the silencer, or should it be the size of the pipe?
Just to clarify: The duct does not go through the leaf: the silencer goes through the leaf. As I already mentioned, here's an wood "sleeve" on the end of the box that penetrates the leaf. That "sleeve" can have a smaller cross section than the box itself if you want, but the transition from duct to sleeve is where the impedance magic happens, so it needs to be a fairly large change in cross section.

Quote:
The idea of drilling a 18.6x18.6" hole in the external leaf makes me feel a little dizzy...
:shock: That's a huge hole! Why do you need it to be so big? Your studio is only 1800 ft3, so why do you need nearly 350 in2 cross section? Assuming 1800 ft3 and six changes per hour, you need to move 180 CFM. Moving 180 CFM through 350 in2 (2.4 ft2) implies a velocity if just 75 fpm at the register. Why do you want your air to move so very slowly? I mean, there's nothing wrong with that, but there's no need for it. As I mentioned before, 300 fpm is a more reasonable speed, which you could do with a cross section of about 90in2. I don't understand the need for such a huge section. The ducts themselves could be 6" with no problem, or 8" if you really wanted to, and even then you'd have a good cross-section change at the interface.

Quote:
my cross section needs to be 86.5 sq inch, making it about 9.3x9.3", and making the silencer 18.6x18.6 (346 sq inch) (plus the thickness of the duct liner and the two drywall layers.
Huh? How did you get there from here? Assuming you start with a 6" supply duct (as I said before), the area of that is roughly 30 in2 (28.3 in2, to be more exact). You only need to double the cross sectional area where the duct connects to the sleeve, but you are increasing it more than twelve times! Why do you need an area of 346 in2, when 60 would be fine, and 90 would be fantastic? I do not understand why you want such an enormous cross section for such a small room.

Quote:
and then inside the room attach the ø25mm of the provide pipe to go into the AC unit
Why are you insisting on still using that tiny little pipe? I already showed you why it is no use, and why you should totally forget about using that, and you said you "got it", but here you re still talking about trying to use it... What I actually said was "The small amount of air that thing brings in is not enough for a typical studio. ... Forget that your unit even has that capability, and build proper supply and exhaust ducts."

Quote:
Here's a model of a 86.5 sq inch duct penetrating the two leaves and terminating with a hole for the HVAC's internal unit ø25mm pipe.
Sorry, but I do not understand what you are trying to accomplish there at all. It just makes no sense. Why do you want to connect a 1" pipe to a silencer designed for an 11" duct? And how did the 346 in2 cross section suddenly became 86.5 in2? I'm not getting the plan at all.

Quote:
I'm aware that it's better for the silencer to be attached to the most sensitive leaf,
Sorry, but there's no such thing! In an MSM wall, BOTH leaves are EQUALLY "sensitive". Together they make up a tuned system, and it is the system that provides the isolation, not the individual leaves. You seem to be misunderstanding how MSM isolation works. Neither of the leaves is more sensitive than the the other, and neither provides more isolation that the other: They work together as a system. Saying that one is more important than the other is the same as saying that the gearbox of a car is more important than the engine in making it move: they are both equally important and a failure in either one will prevent the car from moving. So to with your wall: both leaves are equally important, which is why each leaf needs its own silencer box.

Quote:
so I'm only going to do that for the exhaust.
You are missing the point: There's no point in silencing only one duct where it passes through only one leaf. You need to silence BOTH ducts and each one of them needs to be silenced where it passes through EACH leaf. In other words, a total of four silencers.

Quote:
I read your comment in another topic about an alternative when using one silencer on the least-sensitive leaf and it gave me hope
You quoted what I said, but there is nothing at all in there about me saying that one leaf is more important than the other. Nothing. I don't know how you could read that and understand that I was saying that one leaf is more sensitive than the other, or one duct is more sensitive than another. I just don't see how you got to that conclusion from reading what I wrote.

Quote:
I'm using 2 layers of 5/8" drywall w/ Green Glue.
??? :shock: WHAT!!???? You CANNOT make a silencer box out of drywall! It has no structural strength. It will be way too heavy for the screws to hold together. Did you not notice what I said about how to make it? :

Quote:
The thickness of the wood that you use to make your silencer boxes
. Wood. Not drywall: Wood.

Quote:
Since caulking a box from the inside is not the easiest thing,
To be honest, I don't see why! I've never ha a problem doing that. You caulk the wood edges as you put the box together, then caulk them again before you put the duct liner on, then you caulk around the edge of the front panel before you put that on, and caulk that from the outside once the screws are in place. I've never had a problem doing that.

Quote:
do you suggest cutting the drywall leaving the space for the caulk at the risk of it not being perfect, or not leaving any space, avoid caulking and letting the liner fill the gaps?
I'd suggest that you don't do it with drywall at all! That thing will fall apart. I would suggest that you build it properly, out of wood.

Quote:
suggestions on how to put together the box made of drywall?
Yes: DON'T!

Quote:
it's not the easiest material to work with as it crumbles, and I definitely wouldn't want that in the air system.
That's why I definitely would not ever make a silencer box of drywall.

Quote:
Could I get away with going straight from the external unit through the supplied ø25mm pipe into the silencer and avoid using a duct since the distance is so short or would you rather build that 10" duct?
Neither, because I just don't understand either of those measurements. A 10" duct makes no sense for such a small room, and a 1" pipe makes no sense either. Typical duct size would be 6" for that room. You'd have to have a hurricane going through a 1" pipe to get 180 CFM out of it.

Quote:
Are there any precautions I should take for the other three pipes (water outlet, gas, liquid) or should they just run outside, parallel the silencer, untouched and with no insulation?
Do not remove the insulation from those pipes! They come from the factory fully insulated, and you should not ever remove that.


Quote:
The exhaust silencer's duct/pipe will penetrate the internal audio room's leaf, then after a 5" air gap it'll go straight into the outer leaf which is also the external wall of the building.
Ummm... Nope! You need a silencer on EACH leaf! Every point where a duct would go through a leaf, needs to have a silencer.

Quote:
Should it end in the outside wall as a small ø25mm pipe
NO!!!! BOTH paths must end in reasonably sized registers. The intake path will have some type of register or grill or hood over the end, perhaps with a filter in it, and the area of that will probably be at least the equivalent of a 6" round duct, and likely more. The area of the exhaust grill will be something similar. I have no idea why you want to use a 1" pipe. The static pressure from that would be way beyond what any reasonable duct fan could handle for 180 CFM.

Quote:
the duct liner I managed to find here in Italy was the K-Flex St Duct. They don't have a 25mm version, so I went with the 30mm. The alternative was 20mm.
Either 20mm or 30mm would be good, but those are both rather unusual for duct liner. Are you SURE you got stuff for lining the INTERIOR of HVAC duct? That normally comes in either 1" or 2" (25mm or 50mm). Sometimes you can get 1/2" (12mm) or 1-1/2" inch (38mm), but 20mm and 30mm are unusual sizes. That's more like what I would expect for EXTERNAL duct liner, that goes on the OUTSIDE of a duct. Better check carefully that the stuff you got is designed and certified for use INSIDE HVAC ducts. If not, send it back and get the right stuff.

Quote:
Do you think the best way to install them would be to:
1) build my two boxes with only 5 sides and a missing 2x1 bottom panel and applying it directly onto the wall (for the inner leaf) and under the ceiling (for the outer leaf) essentially using the wall/ceiling itself as my 2x1 panel
2) cut a 2x1 hole into the drywall and screw the (whole, 6-sided) silencer box onto the joists for the inner leaf and hang the box from the ceiling with toggle bolts for the outer leaf?
3) other ideas?
The best idea would be to take off an entire sheet of drywall, install your silencer boxes correctly, carefully cutting the holes in the leaf a little larger than the MDF sleeve, pushing the sleeve through the hole, sealing the gap with backer rod and caulk, then doing the same for the drywall sheet that you took of, replacing it carefully over that sleeve, with the gap around it, more backer rod, more caulk.

Quote:
I've read your post a few times to make sure I got it right.
It seems like you misunderstood several things in there!

Quote:
I have proceeded to cut all the necessary drywall for the silencer. Boy that was a lot, this thing is going to weigh a ton. I'll be getting some help from my drywall installer in order to assemble the box as I don't want to screw it up. Hopefully I'll be able to put it all together next week.
Throw all of that away, and go buy some wood, preferably MDF, but you could also use OSB or plywood if you prefer. You will NOT be able to make silencer boxes out of drywall, and in any case the design you are showing is seriously flawed. Even if you did make those things properly out of MDF, they still would not work. You cannot have air coming into one end of that box at twenty six thousand feet per minute through a tiny little 1" pipe, then exiting out the other at 75 feet per minute through a 350 square inch hole. I don't even know where you would find a fan capable of producing 26,000 fpm into a one inch diameter pipe... That's 295 miles per hours! Very literally, hurricane speed. You'd need a miniature engine to move air like that.

It seems like you are completely misunderstanding the concept here.

All that you need to do is to have a typical sized duct (for example, six inch diameter, which is roughly 30 in2 cross section) on the supply end, going into the first silencer box through a wooden sleeve that passes through the outer-leaf, where the internal cross section will increase to around 60in2 (or maybe 90in2 if you really wanted to), and stay at roughly that cross section passing several baffles, before exiting at the other end back into a 6" duct, from where it will go to the second silencer box, on the inner-leaf. Here it will go to a larger cross section again, probably 90 in2, and it will stay that at that section all the way through. The sleeve that takes it through the inner leave and into the room itself will also be the same section, or perhaps even larger if you really wanted to slow the air down, and the register on the end of that sleeve will have an open area of about 90 in2. The same happens in reverse on the exhaust side. And at some point you need a fan to drive the air through the system. The fan needs to have the ability to move 180 CFM of air into a 6" duct and it must be able to handle whatever static pressure you are projecting for the system, which hopefully will be below 0.5 in.wg. The fan can go on either the intake duct, or the exhaust duct.

That's it. Nothing more, nothing less.

- Stuart -

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PostPosted: Sat Jan 14, 2017 8:31 am 
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Soundman2020 wrote:
There won't be much flanking through [the drainage] tube. It is usually soft and flexible, sometimes even corrugated. It won't flank much.

I wasn't concerned about flanking but rather about the fact that the tube is hollow inside, effectively making it a hole in the leaves. I assume it either isn't a concern or it simply is an inevitable, necessary evil.

Here are a few clarifications:
1)In order to calculate the duct cross section, I used the equation A = 144 q / v where A = duct cross sectional area (in2), q = air flow rate (cfm), v= air speed (fpm), which, plugging the numbers you gave me, should be: A = 144 x 180 / 300 which gives me a cross sectional area of 86.4 in2. So I assumed that was the cross sectional area of the ducts, which then needed to be doubled for the silencer, making the silencer's cross section 172.8 in2. (and here's my first insanely stupid mistake: rather than doubling the cross section, I doubled each side of the duct, effectively coming up with a 346 in2 cross section for my silencer). It is also very likely that I messed up some other math during the conversion from imperial to metric, sq. ft to sq. in., metric to imperial... (After all, NASA's Mariner exploded because of a typo, right? :oops: ) That being said, I did the math again and I still come up with a cross section 86.4 in2. I have no idea how you come up with 28.3 in2, but I'll be more than glad to simply execute what you are suggesting without questioning!

2) The majority of the misunderstanding, and the reason why I was insisting with the 1" pipe, is because I thought the whole duct and silencer path should still start from the external supply unit and end in the internal AC unit (as I mentioned, the Ururu Sarara provides a fresh air intake). Now I seem to understand that it should not be connected to either and it should simply be a completely independent path with its own separate vent. I wasn't clear on that before. The air supply tube is what provides humidification to the room and since I can't use it, I might as well switch the unit for a much cheaper one and use a separate internal humidifier.

3) I thought you wanted me to build a wooden duct with a squared section just like I would have built the silencer box, I didn't understand the duct should be purchased. Also, I thought the silencer could be made of drywall cause it simply needed to have the same surface density of the leaf. Got it! Metal duct + MDF/OSB silencer it is.

4) I'm sorry, I didn't mean to misquote you at all, I was referring to the "least sensitive side" after reading: "If you don't need a high level of isolation, then you are fine with putting a single silencer box on each duct, provided that you put it on the "correct" side. By "correct" I mean the side that actually requires the isolation, not the one that doesn't need it so much." I interpreted "correct side" as "most sensitive side". That was it.

5) As a reminder, this is a 50 m3 (1765 cubic feet) room with only one occupant.

Quote:
There's no point in silencing only one duct where it passes through only one leaf. You need to silence BOTH ducts and each one of them needs to be silenced where it passes through EACH leaf. In other words, a total of four silencers.

I definitely want to silence both the intake and exhaust ducts, I just thought I could silence 1 leaf per duct after reading what you wrote about this being a possibility "in cases where you need the register fairly close to the point where the duct penetrates the wall", which is my situation precisely since my register would end up less than 16 inches from the duct penetration. How much isolation loss am I looking at with a 1 silenced leaf only setup?

Quote:
Do not remove the insulation from those pipes! They come from the factory fully insulated, and you should not ever remove that.

Great! I'll just have them going through the leaves untouched.

Quote:
Better check carefully that the stuff you got is designed and certified for use INSIDE HVAC ducts. If not, send it back and get the right stuff.

Thanks for the suggestion! That's what I was told by the seller, but I'll call the manufacturer to double check to make sure. Do you know of duct lining products available in Europe? I had a really hard time finding them, but I have found a few of 35, 160 and 240 kg/m2.

Quote:
It seems like you misunderstood several things in there!

At least this is much simpler than I initially planned. I like to look at the bright side of things. :mrgreen:

Quote:
All that you need to do is [...]
That's it. Nothing more, nothing less.

Basically: 6" supply duct -> 60in2 sleeve through leaf -> silencer inside air gap -> 6" duct inside air gap -> 90in2 silencer inside air gap -> 90+in2 sleeve -> register.
Got it, and this sounds much easier than I originally expected. Only thing I need to change is placing the silencers on the internal walls of each leaf as I don't have space in the air gap. That would mean going duct -> silencer -> sleeve (crossing the leaf) -> duct (inside the air gap). Then the other way around into the register. Should the duct between the leaves be flexible or is the flanking of a metal duct negligible?
When you say that the register will have an open area of about 90 in2, are you suggesting the register should have no grill and simply be an open hole coming from the sleeve?
Anything I should look out for when purchasing a register?
Is it ok to have both intake and exhaust registers on the same wall, one bottom left, the other top right, about 9 feet apart?

You are amazing, as always!


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PostPosted: Tue May 09, 2017 2:00 pm 
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Hey Stuart,

I had to put the construction on hold for months due to time constraints, but now I'm back into it.
The good news is all your suggestions and calculations made sense over time and they are all very clear now.

The not so good news is that no matter how I try to arrange it, I simply don't have the space for 4 silencers in my studio at this stage.
So, after reading your suggestion and reaction to this topic, I'd like to ask you: is cutting "the duct in the middle of the wall so that there is a gap between the two ends of the duct (a few mm or so), and wrap the gap with a strip of thick rubber (neoprene, sorbathane, etc.)" a viable solution in my case or will it destroy my isolation? I should mention that there is only a 5" gap between the two leaves, so the two pieces of duct will only be about 2.5" each plus the size of each leaf (7" on one side and two layers of 5/8" drywall on the other).
ImageImage

If so, do you suggest specific brands/models to use or simply absorption coefficients I should shoot for?

I would put the inlet and outlets silencers in what is marked as Adjacent room in my floor plan, penetrate the first leaf with a sleeve/duct that is cut in the middle and wrapped in neoprene and then penetrate the inner leaf of the Studio.
I also have access to this material which I'm using as duct liner if it is of any use (the 40mm version), and of course the usual glass wool.
Attachment:
sw.jpg


Also, I can put more than 2 baffles in the silencers (3 or 4 rather than 2) if necessary since I have space in the Adjacent room.
This might be a stupid question, but would sealing the gap between the two pieces of duct with acoustic sealant crate flanking? After all it does remain elastic after it dries out.

Only two things have changed from what was described in this topic and the first is that the air flow is not going to be 180cfm but rather 205. Since the machines I could find that would supply something closest to 180 were either 147 cfm or 205 I thought it was wiser to step it up. As a consequence of that the cross section is now calculated at 105 in2 rather than 90.
The other change is the fact that I'm going to use a heat exchanger and the air will be brought into the room from the right side wall seen on the picture.

I'd love your input on this.


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PostPosted: Wed Jun 07, 2017 12:50 pm 
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Quote:
I'd like to ask you: is cutting "the duct in the middle of the wall so that there is a gap between the two ends of the duct (a few mm or so), and wrap the gap with a strip of thick rubber (neoprene, sorbathane, etc.)" a viable solution in my case or will it destroy my isolation?
It is an option, and it can work, yes. It's not as good as having boxes on both sides, but it can still give you reasonably good isolation.

Quote:
I should mention that there is only a 5" gap between the two leaves, so the two pieces of duct will only be about 2.5" each plus the size of each leaf (7" on one side and two layers of 5/8" drywall on the other).
That's fine. Just make sure that the sleeve that you use to join them is flexible enough.

Quote:
If so, do you suggest specific brands/models to use or simply absorption coefficients I should shoot for?
For the duct liner? Any good quality duct liner that is specifically manufactured and marketed for lining the interior of HVAC ducts. You cannot use any old fiberglass or mineral wool insulation! It must be specifically made for air ducts.

Quote:
Also, I can put more than 2 baffles in the silencers (3 or 4 rather than 2) if necessary since I have space in the Adjacent room.
3 would be good, 4 would be better, provided that you have a large enough cross sectional area throughout the complete box, such that the flow velocity is very low.

Quote:
This might be a stupid question, but would sealing the gap between the two pieces of duct with acoustic sealant crate flanking? After all it does remain elastic after it dries out.
The gap needs to be larger than you can fill with caulk.


Quote:
the air flow is not going to be 180cfm but rather 205. Since the machines I could find that would supply something closest to 180 were either 147 cfm or 205 I thought it was wiser to step it up.
Don't forget that the nominal flow rate and speed of the fan are for "free air" conditions, not considering static pressure: in other words, no duct, filters, silencers, bends, registers. All of those produce "resistance" to the flow of air, so the actual real flow rate will be much lower than the nominal flow rate. The fan specs should also show what maximum static pressure it can handle, and what the flow rate will be for that static pressure. You should predict the static pressure for your system, to make sure that the fan will be able to handle it, and that you'll still get enough air flow.

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The other change is the fact that I'm going to use a heat exchanger
Smart move! And in this case, you might not need an additional fan! HRVs often have their own fan inside, and as long as your ducts/silencers/registers are with the static pressure capability of that fan, it might be the only one you'll need.

- Stuart -

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PostPosted: Wed Jun 07, 2017 8:54 pm 
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Quote:
Just make sure that the sleeve that you use to join [the leaves] is flexible enough.

Does it mean I shouldn't use the same wood as the silencer boxes as pictured above? What kind of flexible material are you suggesting?

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For the duct liner? Any good quality duct liner that is specifically manufactured and marketed for lining the interior of HVAC ducts.

I already have the right duct liner (you suggested it to me in another topic). I was actually referring to the neoprene/sorbotane, or whatever rubbery material I'm going to use to wrap the gap in between the two sleeves. Any specific brand I should look at? Or should I just get any generic thick rubber?

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3 would be good, 4 would be better, provided that you have a large enough cross sectional area throughout the complete box, such that the flow velocity is very low.

4 it is then!

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The gap needs to be larger than you can fill with caulk.

How large would you suggest? I assumed a couple of millimeters were going to be ok.

Thanks for all your answers, as always. This is very encouraging!


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PostPosted: Mon Jun 12, 2017 11:48 am 
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Does it mean I shouldn't use the same wood as the silencer boxes as pictured above? What kind of flexible material are you suggesting?
You could do it with solid wood, yes. But plywood, OSB or MDF would be fine too, as long as the inner surface is sealed in some way. You don't want the wood to absorb moisture from the air, nor do you want engineered woods to off-gas any of the chemicals they are made with, into your air-stream. Ditto for whatever you use to seal the wood!

Quote:
I was actually referring to the neoprene/sorbotane, or whatever rubbery material I'm going to use to wrap the gap in between the two sleeves. Any specific brand I should look at? Or should I just get any generic thick rubber?
Neoprene or EPDM would be fine, but HVAC isolation sleeves are often also made from canvas and other fabrics that have been treated to make them air-tight. You can buy ready-made "duct connectors" like this, in many shapes and sizes.

Quote:
How large would you suggest? I assumed a couple of millimeters were going to be ok.
An inch or so would be good. About 25mm, give or take a few.


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PostPosted: Mon Jun 12, 2017 9:35 pm 
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Soundman2020 wrote:
You don't want the wood to absorb moisture from the air, nor do you want engineered woods to off-gas any of the chemicals they are made with, into your air-stream. Ditto for whatever you use to seal the wood!

I assumed that using duct liner and proper glue would be enough, but it looks like I'm gonna have to do some research on how to seal the wood.

Quote:
Neoprene or EPDM would be fine, but HVAC isolation sleeves are often also made from canvas and other fabrics that have been treated to make them air-tight. You can buy ready-made "duct connectors" like this, in many shapes and sizes.

I had no idea I could use pre-made sleeves in place of the Neoprene/EPDM. That sounds like a much easier way to go about this.

Thanks!


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PostPosted: Tue Jun 20, 2017 5:04 am 
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Hey Stuart,
do you have any brand or model to suggest for these pre-made sleeves you are talking about?
I can't seem to find any here, or at least none that are half-decent for sound blocking.

Do you think it would be ok to simply paint the wood to stop it from releasing gasses?

Best!


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PostPosted: Tue Jul 18, 2017 12:08 pm 
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For people in my same situation, I'm still awaiting some data, but it looks like I'm going to use either http://tinyurl.com/yd28mm5n (ranging from 30 to 50 on the durometer scale) or http://tinyurl.com/y8hg2mka for sealing the gap in between the duct split (both available on Amazon here in Europe). So far all I know is that for the first, the website reads "1 cm of Sorbothane can isolate up to 40 dB of sound", whatever that means... while the latter has a density of 130-170kg/m3.
Bostik 99 will be used for gluing the liner to the duct and baffle box. I'm also investigating on whether it can also act as a sealant to keep the wood's gasses in since it's a glue that's meant to completely cover the surface upon which it is applied.


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PostPosted: Thu Jun 28, 2018 2:37 am 
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saemola wrote:
For people in my same situation, I'm still awaiting some data, but it looks like I'm going to use either http://tinyurl.com/yd28mm5n (ranging from 30 to 50 on the durometer scale) or http://tinyurl.com/y8hg2mka for sealing the gap in between the duct split (both available on Amazon here in Europe). So far all I know is that for the first, the website reads "1 cm of Sorbothane can isolate up to 40 dB of sound", whatever that means... while the latter has a density of 130-170kg/m3.
Bostik 99 will be used for gluing the liner to the duct and baffle box. I'm also investigating on whether it can also act as a sealant to keep the wood's gasses in since it's a glue that's meant to completely cover the surface upon which it is applied.


Did you finish your silencers, please share some pics!


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PostPosted: Fri Jul 13, 2018 6:37 am 
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buttermuffin wrote:
saemola wrote:
For people in my same situation, I'm still awaiting some data, but it looks like I'm going to use either http://tinyurl.com/yd28mm5n (ranging from 30 to 50 on the durometer scale) or http://tinyurl.com/y8hg2mka for sealing the gap in between the duct split (both available on Amazon here in Europe). So far all I know is that for the first, the website reads "1 cm of Sorbothane can isolate up to 40 dB of sound", whatever that means... while the latter has a density of 130-170kg/m3.
Bostik 99 will be used for gluing the liner to the duct and baffle box. I'm also investigating on whether it can also act as a sealant to keep the wood's gasses in since it's a glue that's meant to completely cover the surface upon which it is applied.


Did you finish your silencers, please share some pics!


Unfortunately this is the only half decent picture I have of the finished silencers. I can't seem to find the others, but I'll keep looking.
Attachment:
1.jpg


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