John Sayers' Recording Studio Design Forum Archive

Hi - I'm sorry to be a nuisance, but does anyone have any advice?

Hi--

I'm sure I'm not the guy you want to hear from (probably you would rather hear from Stuart), and I am far from an expert, but when I read your post it reminded me of my own situation, so I thought I'd chime in.

My build is a long, narrow garage with a control room and live room. I have two silencer boxes for each room, one for intake and one for exhaust. If you're curious about the details, you can read my thread here:
http://johnlsayers.com/phpBB2/viewtopic.php?f=2&t=15430
To be brief, 3 out of the 4 silencer boxes I have are in a situation similar to the one you describe. Given my small amount of space, two of my silencers are outside of the outer leaf and one of them is inside the inner leaf. In all three cases, I had to figure out a way to get from the inner leaf, through the air gap, and then through the outer leaf with enough mass to preserve good TL while at the same time avoiding flanking paths from the duct itself. My solution (really Stuart's solution, but my execution) is as follows:

1. I made my "duct" out of wooden 2x material, not metal duct. The reason being that it has mass. I lined it with duct liner.
2. The duct was constructed in two sections, which were rabbited out to accommodate 1/2" of neoprene that wrapped around them, effectively joining them into one duct that avoided creating a flanking path between the leaves. The two sections were roughly 3/16" apart--NOT touching each other. This gap was located between the leaves.
3. Once constructed, I slid the entire duct assembly through holes I cut in both leaves, and then caulked thoroughly.
4. This approach enabled me to maintain the mass that I had built into the walls while avoiding a flanking path that would have occurred if I had built the duct out of one continuous section. The neoprene has enough mass to approximate the mass of the wood duct.

For better clarity, you might want to read my thread toward the bottom of the first page and into the second page. This is where I pose my initial question about my problem and Stuart makes his suggestion for a solution. Later on in the second page, you will see some photos of the duct I built which should make the process clear.

I am far from finished with my build, but I am able to say that this approach is a very effective solution. With a full rock band playing in the live room, I could only hear them faintly with my ear right up to the silencer opening.

So anyway, that's my two cents; I hope it helps....

Mark

Ok, so it seems like the dead vent design is overkill based on everything you are saying.

Can this type of silencer box work completely outside of the room, outside of the outer leaf?

Yes it can, depending on how much isolation you need. As Mark mentioned, he did an isolated duct system that bypasses and isolates both leaves, and works great for what he needs. So you can probably do that too. If you need extreme isolation, then you have to have silencers on each penetration of each leaf. Even then, you could do both of those inside the wall cavity, or one in the cavity and one outside the room.

In running a duct directly through the inner and outer leaf, it seems like I'll be compromising my soundproofing in a significant way (and in a way that I need to deal with).

Correct, it is a compromise. In Mark's case, it worked out fine. If you happen to need more isolation than that, you do need a silencer inside the cavity, or inside the room.

that specific duct penetrating both leaves (I've outlined it in blue on Ted's drawing below) and leading to the silencer box could be thick PVC lined with pipe wrap or MLV. In other designs I've seen, it could be a 6" round duct or a square/rectangular duct lined with duct liner or other HVAC soundproofing material.

That's one way of doing it, but the method I outlined for Mark provides even more mass and also better isolation.

It seems like some other people just run a plain metal duct directly through the walls.... which seems like it would prone to have sound go through the thin metal duct and in between the leaves.

Right. Those are the people who either put silencer boxes inside the room, or they have very noisy HVAC systems with poor isolation....

4. The duct will go through the leaves and terminate in a silencer/baffle box, which will on it's own have mass and airtightness so it will somewhat function like part of the outer leaf.

Right again! That's why yu need to make it of thick, heavy wood.

5. That small silencer box should effectively stop large amounts of sound from escaping the music room, even though the box itself contains very little insulation and their are gaping holes on both ends. The baffles, complex path and impedance mismatch are what will reduce the sound transmission from one side of the box to the other.

Exactly. As long as the box is built from heavy, thick wood, then it will indeed do its job of silencing quite well.

6. Since the entire silencer box is outside of the room, I was thinking I could put the fan directly at the end of the silencer box

You could do that, yes, but the fan might cause vibrations in the box itself, which would be transmitted into the room. So I would isolate the fan form the box in some way, even it is is only a short piece of flex duct.

7. If this design is actually ok, I was thinking I could essentially create another other of these ducts/boxes for the fresh air supply, but instead just reverse the direction of the fan at the end of the silencer box to push fresh air through the silencer box and into the room. Would that concept work?

Yes it would, but you might be able to skip the fan on this second box: Since the room is sealed air-tight, the first fan (on the outlet) will suck air out of the room, and that reduced pressure will cause the room to "suck" air in from the supply duct. So you might be able to get away with only one fan, if you dimension it correctly. Figure the static pressure imposed by the entire system (both ducts / silencers, and the room itself) then check that against the model of fan you are considering. If the fan can handle that much static pressure and still move enough air volume, then you are OK.

- Stuart -

Mark and Stuart - Thank you so much for the information. It's extremely helpful. The neoprene connector makes a ton of sense and it makes me think I should do that for my small PVC cable pass through.

A few questions

1. What type of vent cover did you use inside the room? Just curious...

2. Did you use a fan for both silencers boxes (supply and exhaust), per room? Or we're you able to get away with just an exhaust fan, for example?

3. For the silencing to work, I understand that there needs to be change in surface area to create an inpedance mismatch (?). Does the different directions of the airflow in the supply and exhaust boxes mean that the vent openings inside the room need to be different sizes? Do the holes in the silencer boxes need to be different in size depending on the direction of the airflow? Or does it just matter that the sound escaping from the room leaves a smaller duct and that area doubles when it hits the silencer box path? I'm wondering if I can build both boxes, ducts and vents identically... and maybe just skip the fan on the supply box (assuming the single fan is capable of handling that, which I wneed to calculate)

Thanks again!

Does the different directions of the airflow in the supply and exhaust boxes mean that the vent openings inside the room need to be different sizes?

Not really. It doesn't matter if the air flow moves from a larger to smaller, or smaller to larger section. It is the change itself that causes the impedance mismatch. Every time the air flow moves from one size duct to another, there is an impedance mismatch. Even at the register itself, where the duct suddenly widens out to the entire room, there is a mismatch. And likewise at the exhaust register, there's another large jump in cross section, from the entire room down to duct size.

In other words, you can make everything identical for both the supply and return systems. Some HVAC people suggest that the return duct should be larger than the supply ducts, but for a small project like this, that's not really necessary. Make them all the same.

- Stuart -

Great - thanks Stuart. I'm going to work on a design and post the idea here very soon.

Mark was kind enough to send me a detailed design of his own silencer boxes. Extremely helpful.

In his design (attached), the hole that leads to the live room is on the 1' (short) side of silencer box.

In my situation, I'm wondering if I could shift that hole around to the 2' 1 1/4" side of the box. Doing that would allow me to have the box sit up against the wall outside of my room and it would only protrude 1' out from the wall. If I build it like Mark did, it would protrude 2' 1 1/4" out from the wall. I'd greatly prefer for it to stick out 1' (because I am very cramped for space outside of the room).

Is it possible for me to move the hole to where I'm showing it in red below? If you say yes I will be thrilled.

My apologies--
Once I built the boxes, I ended up putting the hole where your red dot is, not where it's shown in the design, for the same reason you want to put it there. Also, I believe the red dot position might actually be preferable, as it effectively creates one more "corner" for the air to travel through. But again, I'm no expert.....

I ended up putting the hole where your red dot is, not where it's shown in the design,

Yup! No problem with that. It does, indeed, add one extra corner.


- Stuart -

Excellent.

Some more questions, of course.

1. What product should I use for duct liner? Is it special acoustical stuff? Do you know anywhere I can find such a small amount if this material? Could I use aurelex foam panels instead (I have a bunch layer around).

2. Can I use drywall instead of OSB for the silencer box? What are the advantages of OSB?

3. Should I use two layers of drywall/OSB with green glue in between for the silencer?

4. Can the baffles inside be just one layer thick?

5. How thick of OSB should I use (if I don't use drywall).

6. Why am I horrified at the idea of cutting large square holes in my beautiful rock solid new double walls?

1. What product should I use for duct liner? Is it special acoustical stuff? Do you know anywhere I can find such a small amount if this material? Could I use aurelex foam panels instead (I have a bunch layer around).

Go to an HVAC supply store--not a box store. They will have duct liner that's designed for that purpose. You should be able to buy just the quantity you need. I believe I used the 1" thickness. My understanding is that the thicker the liner, the better the attenuation.

2. Can I use drywall instead of OSB for the silencer box? What are the advantages of OSB?

I would use OSB--you need the structure. Drywall would not hold together by itself. I suppose you could do plywood as well. I used GG between my layers--seems like a good idea.

4. Can the baffles inside be just one layer thick?

I hope that's ok; that's what I did....

How thick of OSB should I use (if I don't use drywall).

The idea is to match the mass of your leaf, I believe. So if your wall has two layers of 5/8" drywall with GG in between, then ideally you would use material with at least the same amount of mass for your box. I don't think 1/2" OSB has as much mass as 5/8" drywall, but that's what I used. Then again, I haven't read far enough back in your thread to see what your leaf structure is.

6. Why am I horrified at the idea of cutting large square holes in my beautiful rock solid new double walls?

Yeah, I was horrified too! But it really is cool to read about these things and then build one and then then see that it actually works!

I hope this helps, and I hope Stuart will correct me if I've said anything that's not quite right!!

Mark

Hey Mark - That is really helpful. Thanks.

Why did you use an 8 inch fan vs a 6 inch? I'm wondering if I could get away with a 6 inch. And, if so, what impact that would have on the whole system, dimensions, etc.

Any thoughts?

Oh, and I did use 5/8 drywall for my leaves. I have some left over. Maybe I'll use OSB for the first later of the silencer box and 5/8 drywall for the second layer.

Did you paint the OSB? I've heard I need to use oil based primer...

Hi--
Actually, the fan is 6"--it's a Fantech FG6, which for my room pushes plenty of air. At the other end, the square hole ended up being 6" as well.

I don't see why you couldn't use the drywall as a second layer.

I didn't paint mine, but that's probably a good idea; it might reduce the possibility of mold growth. I would think oil-based primer would be the way to go.

and I hope Stuart will correct me if I've said anything that's not quite right!!

Spot on so far!

I used the 1" thickness. My understanding is that the thicker the liner, the better the attenuation.

Yup. 1" is fine, and even though thicker is better, it also takes up a lot more volume. so if you used 2" duct liner, you'd have to male your boxes 2" bigger in all dimensions to still keep the same cross sectional area... So 1" duct liner is good, in most cases.

I would use OSB--you need the structure.

Right!

I hope that's ok; that's what I did...

Yup. Also fine. The baffles are there more to direct the air flow around the corners, than to actually stop the sound (although they do that too!).

The idea is to match the mass of your leaf, I believe.

Right. For maximum isolation, that's what you need.

But it really is cool to read about these things and then build one and then then see that it actually works!

- Stuart -

Ok so I'm working on silencer box designs and trying to figure out how large the duct needs to be that runs through both leaves.

I am working from the design of the silencer box suggested earlier in this thread and am trying to properly size the components (size and number of fans and duct) for the room. I am just confused and would appreciate any advice.

I can use the Fantech FG6. This pushes 257 CFM.

15 cubic feet of air per minute per person. I hope to fit 4 of us in the room sometimes...

The room is 653 cubic feet. So, I need to ventilate 10.84 cubic feet per minute. I think I want to be in the range of 10-12 ACH (right?). So, the fan needs to ventilate 131 CFM. The Fantech FG6 seems ok.

I think my next calculation is about the static pressure. I am just getting lost and confused. Is the method for this laid out somewhere? My room is small so I want to figure out if I can decrease the size of the duct that runs between the inner and outer leaf to 4" (cross sectional area is 16) or if I need the 6" (cross sectional area is 36).

To illustrate my question about the duct size, I'm wondering if the size inch duct below could be 4 inches (because it would be easier for me to pull off) Then, depending on the size of the duct and whether than impacts static pressure, I'm not sure if that means I'll need a fan for both silencer boxes or I can get awat with only 1 fan or if each silencer box has to have one...

And finally, I'm wondering if I can put both the fresh air supply vent and the exhaust vent on the same wall. Because of how I constructed the room, it would be ideal to put both vent/silencer boxes on my long wall that is 10' 9.5". If I put one towards the top left corner of the wall, and the other is towards the bottom right corner of the wall, would that be enough separation between the vents to get proper circulation, even though they are on the same wall? Because my room is so small I'm hoping that would work.

I really appreciate all of the help. So, so much.

Thanks