John Sayers' Recording Studio Design Forum Archive

I am in the midst of building a basement room/booth to record voiceovers in, and am in need of some advice regarding ventilation. The quick and dirty summary is as follows (more detailed information to follow, but the summary may be enough to address my question):

• It is in the basement of a very old house, and space is limited.
• The room will be decoupled, (genie clips, hat channel, 2 layers of drywall, green glue between layers, ceiling also drywall on clips+channels)
• The room's interior measurements are going to be: 6'6" x 5" x 6'1" (height)
• The ventilation will run not outside, but from and into the rest of the unoccupied and sizable basement.
• I will only run the ventilation between takes, so I am not overly concerned about it being silent. Reasonably quiet when it is running will do fine.
• I am, however, very concerned about exterior sound coming in through any holes, vents, ducts, whathaveyou even when the system is not on. I am taking great pains to float the room and do it well, and would hate to see that subverted by my ventilation system

I've tried to do some research here and elsewhere, but it seems most builds are concerned with recording while running their HVAC system. I'm not. I am happy to record with it off (will need a switch from inside the room I suppose!) and am solely worried about sound transmission from the outside into the booth coming in through whatever system I install while the thing is switched off and I am recording. With that in mind, should I still plan to build a box to baffle the sound like this (forgive my terrible sketch, I am only now learning sketchup):



Or is there a simpler solution that will serve my purposes just fine? I wonder whether building an elaborate box to reduce fan noise that I am not really worried about is entirely necessary. Since I'm not running a high volume system, what is the smallest vent hole size I can get away with? What is the optimal material for the duct? PVC pipe ok, or... ?

My budget is fairly low, I busted the bank account on green glue, clips and other materials. But better to spend another few hundred more than planned than see those other expenses be for naught.


And the promised more detail for those who want it:

• Room to be built into an old basement, into a corner formed by two foundation walls.
• Purpose is to record voiceovers, so there are no worries at all about preventing sound inside the room from getting out, and plenty of worries the other way around.
• Interior dimensions 6'6" x 5" x 6'1" (height) Why yes. That is low!
• The two wall frames that are not up against the foundation walls will have a drywall layer on the outside as well, the two that are up against the foundation walls will not (don't want to make the 3 leaf mistake!)
• Wall frames are all 2 x 6", with a double layers of Roxul Safe and Sound bats for insulation.
• Wall frames will be secured to concrete subfloor with ISO sill and ISO gaskets.
• Inside walls will be Genie clips, hat channels, 5/8 drywall sheets, green glue, and another layer of 5/8 drywall.
• Ceiling has 24" joists stuffed with a double layer of Roxul, and I will hang a 5/8" drywall ceiling on genie clip/hat channels as with the walls
• Floor will be Rug on plywood on Barrymat on crappy particleboard subfloor.
• corners, seams, butts and such will be sealed with Silent Seal acoustic caulk.
• I will probably be making corner bass traps out of the leftover Roxul (floor-to-ceiling), and will be buying some Auralex foam for interior treatment. (hoping that this will be sufficient for my needs)

Any advice at all as to how I should approach my ventilation issues would be greatly appreciated! And here are my actual questions listed instead of scattered about in my post like needles in a haystack:

Is building an elaborate box to reduce fan noise that I am not really worried about entirely necessary for my space and needs?

Since I'm not running a high volume air exchange system, what is the smallest vent hole size I can get away with?

What is the optimal material for the duct? PVC pipe ok, or... ?

Or to put it all in one question: What is the best way for me to get a small amount of airflow into and out of my room if my primary concern is to maintain the integrity of all the nice decoupling that I am trying to achieve?

Hi there Lester, and welcome!

The room will be decoupled, (genie clips, hat channel, 2 layers of drywall, green glue between layers, ceiling also drywall on clips+channels)

Great! So clearly, good isolation is a priority for you.

The room's interior measurements are going to be: 6'6" x 5" x 6'1" (height)

Ouch! Not very big, and the ceiling is VERY low. Have you considered ways to get more ceiling height? There's not even enough room there to put the acoustic treatment you'll need up there. I'd guess at least 4" of 703, or something like that. I hope you aren't very tall!!! And with such a low ceiling, you won't be able to get good acoustics in there anyway. I'd really try to rethink this, and figure out how to get the ceiling as high as possible.

The ventilation will run not outside, but from and into the rest of the unoccupied and sizable basement.

That might not be legal. Some building codes do not allow one room to exhaust air into another room. Better check that with your building code.

I will only run the ventilation between takes, so I am not overly concerned about it being silent. Reasonably quiet when it is running will do fine.

That's not a good idea, even though it sounds like it: you should have a constant flow of air in a studio, and especially in such a tiny space: There's only a few minutes worth of air in there: it will get warm and stuffy and unpleasant very fast, the O2 levels will drop and the CO2 levels will rise. Not a good environment for doing a good job! Better to build a system that can be on all the time, providing a stead flow of air.

I am, however, very concerned about exterior sound coming in through any holes, vents, ducts, whathaveyou even when the system is not on.

Even more reason to leave the fan on all the time! If the ducting is correctly designed and correctly isolated, then the fan will not be audible at all, and neither will the air flow.

I am taking great pains to float the room

Did you say "float" the room? How are you planning to do that? Are you aware of the issues with trying to float a room correctly? You should probably read this thread: http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173

it seems most builds are concerned with recording while running their HVAC system. I'm not.

Yes you are, if you are smart!

With that in mind, should I still plan to build a box to baffle the sound like this

Most definitely! Absolutely! And certainly! The simple fact of having a hole through the wall or the ventilation duct destroys the isolation system, regardless of whether or not the fan is on. Building a proper silencer box restores that broken isolation, also regardless of whether or not the fan is on. So you need that anyway.


Here are some examples of how people have built their silencer boxes:

http://www.johnlsayers.com/phpBB2/viewt ... 0&start=45
http://www.johnlsayers.com/phpBB2/viewt ... 9&start=74
http://www.johnlsayers.com/phpBB2/viewt ... 25&start=2
http://www.johnlsayers.com/phpBB2/viewt ... 42&start=5
http://www.johnlsayers.com/phpBB2/viewt ... 61&start=0
http://www.johnlsayers.com/phpBB2/viewt ... 5&start=98
http://www.johnlsayers.com/phpBB2/viewt ... &start=157
http://www.johnlsayers.com/phpBB2/viewt ... =2&t=13821
http://www.johnlsayers.com/phpBB2/viewt ... 8&start=44
http://www.johnlsayers.com/phpBB2/viewt ... 2&start=16


Those are mostly bigger than what you will need, but the principle is the same.

Or is there a simpler solution that will serve my purposes just fine?

Nope. That's about as simple as it gets.

I wonder whether building an elaborate box to reduce fan noise that I am not really worried about is entirely necessary.

You seem to have the wrong impression about the purpose of the silencer box: it is not there to stop the fan noise getting in. It is there to stop ALL noise getting in, and also getting out. It restores the isolation that you broke by making a hole in the wall for the ventilation duct, so it blocks all noise transmission: The fact that it also blocks the sound of the fan is just a bonus, but that is not the main function.

so there are no worries at all about preventing sound inside the room from getting out, and plenty of worries the other way around.

It's a two.way street: if sound can get out, then it can also get in.

The two wall frames that are not up against the foundation walls will have a drywall layer on the outside as well, the two that are up against the foundation walls will not (don't want to make the 3 leaf mistake!)

How will you connect the "outer leaf" on those two walls to the foundation walls, while still avoiding flanking?

Ceiling has 24" joists stuffed with a double layer of Roxul,

24" joists??? Are you SURE that's what you wanted to say? I've seen some pretty big joists in my time, but twenty four inch joists are rather unusual, especially on a span of only 5 feet!

I will hang a 5/8" drywall ceiling on genie clip/hat channels as with the walls

so the ceiling will also be two layers of 5/8" plus green glue? Also, if this is a basement, then I assume there's a floor right above the booth: Why not use that as the outer-leaf, and put the clips/channel/drywall on that? Gain yourself several inches of headroom like that, which you will need for the acoustic treatment...

Floor will be Rug on plywood on Barrymat on crappy particleboard subfloor.

Why? This is a basement, so I'm assuming that there is a concrete floor down there: why add that unnecessary thickness of all those layers, when the concrete will do just fine as it is? You are wasting headroom like that, and you have previous little to waste!

I will probably be making corner bass traps out of the leftover Roxul (floor-to-ceiling), and will be buying some Auralex foam for interior treatment. (hoping that this will be sufficient for my needs)

Save your money, and just buy plain old OC-703 for the treatment.

Is building an elaborate box to reduce fan noise that I am not really worried about entirely necessary for my space and needs?

Yes, totally.

Since I'm not running a high volume air exchange system, what is the smallest vent hole size I can get away with?

You need to calculate that, not guess it. It's a rather complex procedure, but since you life depends on getting it right (very literally: you wont stay alive too long of you don't pump in enough oxygen!), it is well worthwhile going through it!

First, you need to figure out how many room changes per hour you need, based on occupancy and exertion. I'd guess maybe 6 or 8 for something like that. Now multiply that by the volume of the room, to find out how many cubic feet per hour of air you have to move. Now dived that by sixty to get the number of cubic feet per minute that you need to move. Now figure out the duct size that will allow you to move that amount of air at a speed of no grater than about 200 feet per second (it's a vocal booth, so the speed has to be even lower than for a typical live room): Now allow for reduction in duct size around corners, as well as the thickness of duct liner, etc. That gives you the final inside diameter or area of the duct. The outside diameter will be greater.

Now go back to step 3, where you figured out how many CFM you need (Cubic Feet per Minute). You need a fan that can move much MORE than that, because the amount that the fan will actually move depends on the static pressure of your duct system, so you have to figure that out. There are tables and equations to help you do that. For example, let's say that you need to move 100 CFM. You can't just buy any old fan that says its capacity is 100 CFM, because that rating is only true for free air (no duct) as soon as you put that fan on the end of a duct, it now has to fight the "resistance" of the air moving along all that duct, so it can no longer provide its full rated capacity. A fan rated at 100 CFM might only be able to do 60 or 70 CFM if the static pressure is high. So you need to know the static pressure, and choose a fan that can supply the right amount of air (CFM) when it has to work against that amount of static pressure.

It's a bit more complicated than just punching a hole in the wall and sticking a fan in it!

Also, I'd connect the fan to a door-operated switch, so that the fan comes on automatically whenever the door is closed, and turns off when it is open. You always want the fan running when the door is closed, even if you are not in there, to keep the temperature and humidity constant. (Ever smelled the inside of a fridge, closet, car or other small space that has been left closed up for a few days without air?) If you don't want the fan to run, just open the door.

What is the optimal material for the duct? PVC pipe ok, or... ?

Nope, PVC pipe is not OK. Metal round duct is OK. Sold very cheap in Home Depot and any HVAC supply store. The silencer boxes can be made of plywood, and lined with 1" duct liner. The final interior cross section of the silencer box should be at least twice the cross section of the duct itself.

What is the best way for me to get a small amount of airflow into and out of my room if my primary concern is to maintain the integrity of all the nice decoupling that I am trying to achieve?

With a properly designed duct system that includes a single fan, silencer boxes, and ducts that have been dimensioned correctly to do the job right.


- Stuart -

Wow! This is fantastic and detailed. Thank you Stuart for going through the trouble. Here is the inevitable follow up....

Soundman2020 wrote:Hi there Lester, and welcome!

Lester Peabody wrote:The room's interior measurements are going to be: 6'6" x 5" x 6'1" (height)

Ouch! Not very big, and the ceiling is VERY low. Have you considered ways to get more ceiling height? There's not even enough room there to put the acoustic treatment you'll need up there. I'd guess at least 4" of 703, or something like that. I hope you aren't very tall!!! And with such a low ceiling, you won't be able to get good acoustics in there anyway. I'd really try to rethink this, and figure out how to get the ceiling as high as possible.

I wish I could. It's an old house, and my budget is limited. I've resigned myself to a low ceiling inside. Better to have a flawed space than no space at all. I'm glad I record from a sitting position though!

The ventilation will run not outside, but from and into the rest of the unoccupied and sizable basement.

That might not be legal. Some building codes do not allow one room to exhaust air into another room. Better check that with your building code.

I will indeed, thanks.

UPDATED! Just got off the phone with the town's chief building inspector. He said I am OK to exhaust into and draw from the rest of the basement. Phew! Big headache and expense saved! Yay!

I will only run the ventilation between takes, so I am not overly concerned about it being silent. Reasonably quiet when it is running will do fine.

That's not a good idea, even though it sounds like it: you should have a constant flow of air in a studio, and especially in such a tiny space: There's only a few minutes worth of air in there: it will get warm and stuffy and unpleasant very fast, the O2 levels will drop and the CO2 levels will rise. Not a good environment for doing a good job! Better to build a system that can be on all the time, providing a stead flow of air.

Ahhh. I was afraid that would be necessary. But better to know that now than after I've installed something suboptimal and dangerous even!

I am taking great pains to float the room

Did you say "float" the room? How are you planning to do that? Are you aware of the issues with trying to float a room correctly? You should probably read this thread: http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173

Sorry. My ignorance of the appropriate terminology is shining through. I meant to say isolate the room or similar. I was distracted by the fact that the wall and ceiling drywall layers will be on the genie clips and hat channels (which, if I understand the term correctly, "floats" the drywall by isolating it from the rest of the structure. Is that right?) and extrapolated that a bit too far in saying the entire room will be floated.

it seems most builds are concerned with recording while running their HVAC system. I'm not.

Yes you are, if you are smart!

I am, alas, quite the opposite more often than not. Fortunately, I have smart people that help me when I go far astray.

With that in mind, should I still plan to build a box to baffle the sound like this

Most definitely! Absolutely! And certainly! The simple fact of having a hole through the wall or the ventilation duct destroys the isolation system, regardless of whether or not the fan is on. Building a proper silencer box restores that broken isolation, also regardless of whether or not the fan is on. So you need that anyway.

Ahh! The crux of my question - and a moot point if I have to run the fan all the time, but still. Good to know (since I suspected that it provided a better seal even with the fan off than just acoustic caulking a duct in place, and one always enjoys having their suspicions proven correct!)

Assuming that my local building codes here in this part of Canuckistan allow me to vent into the unoccupied basement beside the room, (update: assumption correct. Yay!) will it be sufficient to attach the silencer box right up snug (and green glued as well as fastened even) to the outside of the room similar to what they use for prefab vocal booths? And if so, should I be concerned about the extra "leaves" that the box adds to my outside wall, or is it an insignificant concern over such a small area?

I wonder whether building an elaborate box to reduce fan noise that I am not really worried about is entirely necessary.

You seem to have the wrong impression about the purpose of the silencer box: it is not there to stop the fan noise getting in. It is there to stop ALL noise getting in, and also getting out. It restores the isolation that you broke by making a hole in the wall for the ventilation duct, so it blocks all noise transmission: The fact that it also blocks the sound of the fan is just a bonus, but that is not the main function.

Excellent. Then if all works out, my main concern (that putting any sort of ventilation in will compromise my isolation from external noise) will be put to rest, and what should have been an even bigger concern (that I not die from lack of oxygen) also gets taken care of.

The two wall frames that are not up against the foundation walls will have a drywall layer on the outside as well, the two that are up against the foundation walls will not (don't want to make the 3 leaf mistake!)

How will you connect the "outer leaf" on those two walls to the foundation walls, while still avoiding flanking?

I didn't know flanking would be an issue. If all goes according to plan, the only potential flanking path would be into an inch-wide (or so) gap that runs between frames well stuffed with Roxul. My drawing skills are terrible, and my sketchup chops are nonexistent, so I had to turn to the mighty microsoft paint to throw this together. Hopefully it will be clear. If not, let me know and I will dazzle you with some other sort of terrible drawing.

This is what I am planning for the walls that are close and parallel to the foundation... The West wall is an interesting case, and I am not yet 100% sure what to do. It will also be parallel to a foundation wall, but instead of being an inch away, it will be 36" away from that wall. I was told that this is far enough away that I should put drywall on the outside of that wall, but would love to know where the cut off is (that is, at what point does a wall stop being a "leaf" for your Mass-air-mass wall construction and just become that thing over there I am not concerned about. This is more or less what I have planned to that wall...

UPDATED! Bolstered by my success with the building officer, I just called the supplier for the isolation materials and asked them about the 3 foot gap. I was told it is enough to ignore in my equation and that I should put a drywall layer on the outer wall of the booth on that side. (forgot to ask what the cut-off point distance is though. Rats!)
The south wall is easy. The whole basement opens up on the far side of it and there's nothing for about 48 feet. So both my leaves will definitely be drywall on either side of the wall frame. This is also the wall that will have the ventilation running in and out of it.

Ceiling has 24" joists stuffed with a double layer of Roxul,

24" joists??? Are you SURE that's what you wanted to say? I've seen some pretty big joists in my time, but twenty four inch joists are rather unusual, especially on a span of only 5 feet!

I could be using the wrong terminology again. I mean that the ceiling joists are 24" centre-to-centre. They are 2" wide - real, honest to goodness 2", not "hey, let's use a carpentry convention that confuses laypeople like Lester to no end!" 2" (but really 1 1/2") And the third dimension is, errr, 12" I believe.

I will hang a 5/8" drywall ceiling on genie clip/hat channels as with the walls

so the ceiling will also be two layers of 5/8" plus green glue? Also, if this is a basement, then I assume there's a floor right above the booth: Why not use that as the outer-leaf, and put the clips/channel/drywall on that? Gain yourself several inches of headroom like that, which you will need for the acoustic treatment...

I was going to go with a single 5/8 layer since headroom (literal and physical headroom for my noggin, that is) clearance is at such a premium. Can I get away with a single layer? And since we're looking at my ceiling, do I need to eventually remove that vapour layer I put up to keep the rockwool dust out of the room, or can it stay there above the drywall ?

UPDATED! I also asked the supplier about the vapour barrier in the ceiling. He suggested just cutting slices between joists so that it becomes "strips" of plastic rather than a single vapour barrier. That would definitely save the hassle of removing it entirely!

Here is the ceiling set up as planned (Roxul already stuffed in there):

Floor will be Rug on plywood on Barrymat on crappy particleboard subfloor.

Why? This is a basement, so I'm assuming that there is a concrete floor down there: why add that unnecessary thickness of all those layers, when the concrete will do just fine as it is? You are wasting headroom like that, and you have previous little to waste!

Ahhh... here is the saddest part of my sad tale. I have had minor flooding in the basement twice in the past 7 years. The subfloor sits on some sort of dimpled rubber mat that will let the water run off should it get in again. I am loathe to tear that up and set up directly on top of the concrete. Don't want to get anything touching the floor wet and also jeapordize all my wall-to-floor seals should I get flooded again. The flooding was (thankfully!) minor enough, and I have fixed the sump pump in that end of the basement. But still, not a risk worth taking. I'll just duck in the room

I will probably be making corner bass traps out of the leftover Roxul (floor-to-ceiling), and will be buying some Auralex foam for interior treatment. (hoping that this will be sufficient for my needs)

Save your money, and just buy plain old OC-703 for the treatment.

It's too late I'm afraid. Or partially too late at any rate. I've already got some Auralex foam on order. Though considering how terribly much of my room is bound to need treatment, it probably won't be enough. So mayhaps I'll get OC703 and get cracking on some panels, and stagger it with the auralax (time to study those nice DIY treatment ideas using OC703 I saw in a couple threads the other day). I fear that since the room is so tiny I may end up having to line every exposed surface with something or another, tough I've been told that the ersatz anechoic chamber effect of vocal booths wallpapered with acoustic foam is less than ideal for voice over work.

...and thanks for all the HVAC advice - great stuff. And scary. Whoever said "that's another entire book" in that one thread (sorry, can't recall who or which thread) wasn't kidding! Looks like I need to do hella more due diligence in that department. (though I enjoyed looking at some of the silencer box builds!) Thanks again Stuart for pointing me in the right direction. And by all means, feel free to point some more! I am a font of questions, some of them occasionally relevant even!

UPDATE: Studio design is a game of Whackamole, isn't it. I just put one issue behind me and two more crop up. Since I posted this, I realised that I have potential flanking problems overhead as well. For example, there is an airspace between the top of my wall plates and the joists overhead. It seems I am coming back to the question of how big an airspace is big enough to be a concern? Here is another poor drawing (Sorry) of my west wall with the 36" corridor from the foundation wall. (I'm leaving out the insulation in the vertical walls for this drawing just to make it clearer) Should I worry about flanking into the ceiling space above my drywall inside if the joist is perpendicular to the wall, about 1" above the top plate, and the Roxul layer is recessed a further 2" - 3" or so above that? (as depicted by sloppy freehand red arrows) And if so, do I need a airtight hard surface solution (say, a sheet of drywall overhead in the "corridor" that butts up to the top of the outside wall of the room, less a gap for acoustic caulk or somesuch), or would it be sufficient just to stuff any leftover roxul into the gap between the top plate and the joists above? I will have a smaller gap at the south wall which is parallel to a joist, directly under it, but with up to an inch gap.

I have an unrelated additional question: I have some Sonopan (that Canadian fiber board stuff) that I can salvage from the previous version of my recording space. I found some previous threads that pointed out that it didn't do a great job as a mass leaf (makes sense to me) but found nothing on it's desirability for acoustic treatment. Any chance I could use some panels made of this stuff? It'd be a shame to see it go to waste.


And finally one last question, this pertaining more to forum etiquette. My questions are drifting from the original topic, even if they did flow naturally from an exchange of posts. At what point would it be preferable for me to start a new thread? Am I OK just to post each inane question about my build (sure as Sundays, more will crop up!) in here, or start a new thread for each batch of 'em?

My wall frames are up and I wouldn't mind knowing what to do about flanking in my situation: Roxul, a solid barrier, or nothing? Not to mention all those other questions nipping at my heels - what am I gonna do about a door in such a low ceilinged space? (Can I get away with no header over it, or must have a header and duck even lower when going through? Can I get away with 2 hinges, or best use 3?) Should I scrap the Sonopan I have left over or use it for ceiling treatment and/or absorption panels? Speaking of panels, how the heck am I supposed to attach anything to my fancy walls (I am worried about shorting things out - should I only screw stuff into the hat channels through the drywall when hanging traps etc?)

Oh, so many questions, so little time

Oh, so many questions, so little time

That seems to be the motto of stdio builders! And most of us add: "And not enough money" to that...

Just got off the phone with the town's chief building inspector. He said I am OK to exhaust into and draw from the rest of the basement.

That's good news, but make sure you get it it in writing too.

will it be sufficient to attach the silencer box right up snug (and green glued as well as fastened even) to the outside of the room similar to what they use for prefab vocal booths?

Yes, but not with Green Glue compound: That's meant for something totally different, and isn't glue anyway. Green Glue Company also makes acoustic sealant, which is what you need, or you could use any good acoustic sealant, or even ordinary bathroom/kitchen caulk.

And if so, should I be concerned about the extra "leaves" that the box adds to my outside wall, or is it an insignificant concern over such a small area?

Not a problem. Nothing to worry about.

This is what I am planning for the walls that are close and parallel to the foundation...

You do not need the genie clips and hat channel there. Your wall is already decoupled by virtue of having independent framing, so you don't need to decouple it again with clips.

For the rest of your questions, it's realy hard to tell what you are trying to do from the 2D diagrams. It would be much better to do that in SketchUp in 3D, so we can see in detail what your plans are.

Also, photos of where you are right now would help.

- Stuart -

Hi Stuart,

Thanks again for another helpful reply!

Soundman2020 wrote:

will it be sufficient to attach the silencer box right up snug (and green glued as well as fastened even) to the outside of the room similar to what they use for prefab vocal booths?

Yes, but not with Green Glue compound: That's meant for something totally different, and isn't glue anyway. Green Glue Company also makes acoustic sealant, which is what you need, or you could use any good acoustic sealant, or even ordinary bathroom/kitchen caulk.

I know that it is not a glue, hence the "as well as fastened". But it should be noted that I am the living embodiment of Murphy's Law, so I am grateful that you are NOT assuming that I am avoiding the potentially disastrous mistakes. At any rate, when perusing this board earlier I saw a thread where some poor soul had built something trying to use green glue as a "glue". My favourite way to learn from mistakes is to learn from the mistakes of others so I noted it. I meant to use the green glue as a layer between the (plywood or similar) box and the drywall it will be up against. I'm not entirely sure whether I am understanding your suggestion correctly: are you saying I should make the silencer box "open faced" and use silent seal to seal the open-faced join between the box and the drywall?

This is what I am planning for the walls that are close and parallel to the foundation...

You do not need the genie clips and hat channel there. Your wall is already decoupled by virtue of having independent framing, so you don't need to decouple it again with clips.

Oh. Errr... clips and frames are already up. Please tell me it is merely overkill, and not in any way detrimental.

For the rest of your questions, it's realy hard to tell what you are trying to do from the 2D diagrams. It would be much better to do that in SketchUp in 3D, so we can see in detail what your plans are.

Also, photos of where you are right now would help.

I'll try and get up to speed on sketchup. So little time etc. once again, I'm afraid. Even worse, my photography skills are horrible. Still, I managed to snap a reasonable picture of the frames up against the ceiling which I hope shows the upper flanking path I was worried about....
Oh! And yes, there is clearance between the hat channel and the joist above. Not that it wasn't a worry. All the ceiling clips are on blocks of wood sistered to the joists so that I could create a reasonable level plane. I even had to cut into the joists at a couple of spots to make sure everything would fit without shorting out...

At any rate, when perusing this board earlier I saw a thread where some poor soul had built something trying to use green glue as a "glue". My favourite way to learn from mistakes is to learn from the mistakes of others

My philosophy too! As they say: It's far better to learn from the mistakes of others, since you'll never live long enough to make them all yourself! ...

I meant to use the green glue as a layer between the (plywood or similar) box and the drywall it will be up against.

Right, but that's not what GG is meant for. It is meant to be used for constrained layer damping between two large flat sheets of semi-flexible mass, such as drywall, plywood, OSB, MDF, etc. It damps those specific types of resonance that occur in the sheer plane (along the surface) of leaves, but that's not what you are trying to use it for, it seems. It would help to see a diagram of your plan here, but I think what you need there is just a sealant, at the point where the silencer box sleeve passes through drywall, on its way to the other side? Maybe I misunderstood: Are you talking about pressing your silencer box entirely up against the wall, with GG in between there? If so, it would be better to not have the silencer box touching the wall: leave a small air gap instead. If you have no way of avoiding that, then yes, I guess GG would be OK.

Oh. Errr... clips and frames are already up.

Please tell me it is merely overkill, and not in any way detrimental.

Yup. Overkill. Extra expense.... It's sort of like you wanted to get really wet, so you jumped in the swimming pool, but then you wanted to get even wetter, so you bought a hosepipe to take with you into the swimming pool, for "extra" wetness....


It won't do any harm, but it ain't gonna make ya no wetter!

Still, I managed to snap a reasonable picture of the frames up against the ceiling which I hope shows the upper flanking path I was worried about....

I don't see where yo would have any flanking path up there: you have decoupled framing, and you are extra-double-super-hyper decoupling with clips too, so I'm not sure hat flanking path you are concerned about: What other hard connections do you have in there, between inner and outer leaf? I can't see any in the photo....

I even had to cut into the joists at a couple of spots to make sure everything would fit without shorting out...

OK, but you now have to be very, very careful where you place your screws as the drywall goes up. You need to mark all locations where they could touch a joist or clip, and stay well clear of those.

- Stuart -

Soundman2020 wrote:Are you talking about pressing your silencer box entirely up against the wall, with GG in between there? If so, it would be better to not have the silencer box touching the wall: leave a small air gap instead. If you have no way of avoiding that, then yes, I guess GG would be OK.

That is indeed what I meant. But I can avoid it, I just assumed it would avoid a 3-leaf effect. Though I have come to understand (please correct me if I am wrong!) that I don't have to worry about that here since the "leaves" are neither over a sizable area nor in an enclosed space.

Oh. Errr... clips and frames are already up.

Yup. Overkill. Extra expense.... It's sort of like you wanted to get really wet, so you jumped in the swimming pool, but then you wanted to get even wetter, so you bought a hosepipe to take with you into the swimming pool, for "extra" wetness....

It won't do any harm, but it ain't gonna make ya no wetter!

Well, I'm all wet then. Or in light of the unnecessary expense, I should say I took a bath on that one!

I even had to cut into the joists at a couple of spots to make sure everything would fit without shorting out...

OK, but you now have to be very, very careful where you place your screws as the drywall goes up. You need to mark all locations where they could touch a joist or clip, and stay well clear of those.

Aye aye! I am paranoid about shorting anything out after so much effort (and expense) with the clips and channels. So I am double and triple checking everything!

I just assumed it would avoid a 3-leaf effect. Though I have come to understand (please correct me if I am wrong!) that I don't have to worry about that here since the "leaves" are neither over a sizable area nor in an enclosed space.

Well, there will be some four-leaf effect, but it wont be a big deal, and it's the privce you have to pay...

But to clarify, the system does not need to be sealed (enclosed) in order to be resonant: any time there is air between two leaves of anything, there is the potential do resonance. It is more efficient in a sealed space, yes, but it can still happen even when there is no seal around the edges, and all you have is parallel plates close to each other.

Also, even though there will be four leaves here, the reduction in isolation is minimal because it isn't the usual comparison of 2-leaf vs. 3-leaf vs. 4-leaf: Normally with those comparisons, we are talking about walls of the same overall thickness, and the same total mass, just with a different number of leaves inside. The main reason for reduces isolation in that case is because the single large air cavity (2-leaf) is broken up into 2 smaller ones (3-leaf) or 3 even thinner ones (4-leaf), and it is those thinner air gaps that raise the resonant frequency of the entire assembly. In your case, you are not adding more leaves inside the wall: you are adding them outside the wall. So the inner structure of the wall stays the same, with the same depth of cavity, and you are adding the new box next to that, with one very thin cavity (not good, but see later....), and one much larger cavity, comparable in size to the cavity inside the wall. You are also not splitting up the same mass that was on 2 leaves, into four leaves, but rather adding MORE mass with your two new leaves.

So yes, it is a 4-leaf system, but the effect is negligible, and nothing like the normal comparison of multi-leaf walls.

OK, about that small air gap between the box and the wall: that's just to prevent flanking. There might be resonances going on inside the box, or noise from air turbulence, or even fan noise coming in at the duct end, so the idea is to keep that silencer separate from the wall, to prevent transmission from A to B. And also the other way: there might be resonances happening in the wall, so it's also a good idea to keep those out of the silencer box. So I prefer to decouple the silencer as much as possible: mount it resiliently, leave an air gap, cut the hole for the sleeve that goes through the leaf a bit bigger than is necessary, then seal that gap with backer rod and abundant caulk, etc. I try to keep is as separate as possible, to minimize flanking. That includes leaving a small gap between the box and the drywall: even though that gap is potentially a 4-leaf issue, the risk is minimal, the effect is negligible, and the decoupling benefit is greater than the 4-leaf risk, to my way of thinking

This too is likely a bit of overkill, but not as far off track as the hosepipe and RSIC clips.... This is informed, intelligent overkill, with reasoning behind it, not "accidental" overkill...

Well, I'm all wet then. Or in light of the unnecessary expense, I should say I took a bath on that one!

Well, as long as your not all washed up, then things are still fine!


- Stuart -

Soundman2020 wrote: OK, about that small air gap between the box and the wall: that's just to prevent flanking. There might be resonances going on inside the box, or noise from air turbulence, or even fan noise coming in at the duct end, so the idea is to keep that silencer separate from the wall, to prevent transmission from A to B. And also the other way: there might be resonances happening in the wall, so it's also a good idea to keep those out of the silencer box. So I prefer to decouple the silencer as much as possible: mount it resiliently, leave an air gap, cut the hole for the sleeve that goes through the leaf a bit bigger than is necessary, then seal that gap with backer rod and abundant caulk, etc. I try to keep is as separate as possible, to minimize flanking. That includes leaving a small gap between the box and the drywall: even though that gap is potentially a 4-leaf issue, the risk is minimal, the effect is negligible, and the decoupling benefit is greater than the 4-leaf risk, to my way of thinking

This too is likely a bit of overkill, but not as far off track as the hosepipe and RSIC clips.... This is informed, intelligent overkill, with reasoning behind it, not "accidental" overkill...

My favourite kind of overkill! (the intelligent and reasoned kind) Thanks for all the great info, Stuart. One quick question before I go check some box build threads, try and figure out what a good size fan is, and then take a stab at a design that should work (which I will probably post in this here thread before moving forward in the hope that you or another generous soul with some experience in these things will weigh in):

Am I better off building one silencer box that is split down the middle, one side for intake the other side for exhaust with matching baflles and fans on each side (I'm guessing this is similar to most prefab vocal booth vent systems), or should I consider building 2 silencer boxes, one for an intake, and another one some distance away for exhaust?

I hope it's OK to bump - just about everything in my room but the ventilation system is finalized.

I bought two panasonic inline duct fans last week, each at 120 CFM. One for intake, one for exhaust. It's overkill again, but I'm hoping it's not an absurd degree of overkill. The lower capacity fans I was looking at were a lot noisier, so I went with these. And I am going to build some nice big silencer boxes.

My questions are about the silencer box. I read Rod's book and read a ton of threads here, but I just want to double check that I understand things correctly: Am I correct that the "channel" the air travels through inside the box needs to be a consistent size and at least the same as the intake opening, and the line coming out of the box needs to be double that? The intake from the fan-side will be 4" flexible duct. So I figure a 6" duct exiting the box will give me more than double (area of a 4" diameter circle is 12.5 sq inches, while a 6" diameter gets me 28.2 sq inches).

And I also wanted to double check what you had suggested earlier in this here thread: you said that the box doesn't need to be mounted against the outside wall of my booth. I'm guessing this is especially true with such small duct sizes (where the sound transmission through a very short section of duct between the box and wall would be less than the sound transmission of a box physically mounted to the wall).

In which case... what constitutes close-to-but-not-touching my wall? (one inch? two inches? half an inch from the wall?)

Also, I was thinking of further increasing the inside (inside the booth/room) grill openings similarly to this (I hope it is OK that I used and altered/scrawled on NativeLuv17's excellent drawing from this thread: http://www.johnlsayers.com/phpBB2/viewt ... &start=165 )
Would that be a good idea, seeing as the volume of air whooshing into (and out of) my room may well still be a bit much without it?

Finally, and continuing with my theme of overkill, is there any reason NOT to build the silencer box extra long with 5 or even 6 slats in it, provided that the channel cross-section area is at least as much as the incoming vent cross section area?

Duct liner cannot be had for love nor money within 300 miles of Upper Canuckistan! Madness!

Wow! So you might have to get it shipped in from somewhere that does have it. That sounds expensive... Bummer!

- Stuart -

Soundman2020 wrote:Wow! So you might have to get it shipped in from somewhere that does have it. That sounds expensive... Bummer!

- Stuart -

As it stands we are going to try to go with some generic dampening/insulation material. Can't quite recall what it is, but it won't off-gas or get dust or particles in my mics and gear. If it proves inadequate (and I have no tech specs for it, so I don't really know), then I will have to tear it out and indeed ship some huge sheets of duct liner (400 sq. ft. minimum rolls I think!) to my neck of the woods. Here's hoping it won't come to that!

Meanwhile, I have a temporary system with the fans and ducts on a remote on/off switch until we finish with the silencer boxes. I turn them off when recording a take, and back on when finished. It's kind of fun and disconcerting to be in the sealed room with the fans on and off like that. Wheeeee!

And telling! I discovered that my door seals were faulty by turning the intake fan on and blocking the exhaust vent with my hand so that I could hear the air whistling through some very tiny gaps in my door seal that I would not have noticed otherwise. Cool!