John Sayers' Recording Studio Design Forum Archive

I don't understand your reasoning for wanting to build like that, when it is so much easier to build the normal way. Maybe you could explain WHY you want to beef up between the joists, instead of just putting the drywall on top.


- Stuart -

Soundman2020 wrote:I don't understand your reasoning for wanting to build like that, when it is so much easier to build the normal way. Maybe you could explain WHY you want to beef up between the joists, instead of just putting the drywall on top.


- Stuart -

I'm sorry for not explaining this very well Stuart, let's see if I can clear up what I mean...

I agree it is much easier, just as it would be much easier to screw the drywall directly to the OSB or outer most layer of any wall that you want to beef up rather than use cleats, and yet that is not recommended. The reason being, as far as I can remember is that it is better to not have a rigid connection between the layers. There's quite a few threads where you mention this is a big no no.

I am just wondering why it is okay for a ceiling but not a wall. If you build modules, then all of the layers will be connected together rigidly to the module frame. So why is that not a big no no?

Remember, I am building my inner leaf first, and my inner leaf is also inside out, which means my inner ceiling is also my temporary roof at the moment. This means my outer most OSB layer is already in situ, with felt on top of it, all supported by the rafters, ridge and walls. I cannot put drywall layers on top of that, but I can still build modules in the normal way and insert them between my rafters. Or I can use cleats if it is better.

Paul

I'm still not understanding your reasoning, and I think you still are not understanding the way an inside-out ceiling is supposed to be built, normally.

You start with the inner-leaf walls:

Then you put up the backbone and nothing else....


Then you build the module down on the ground, starting with just a 2x4 frame:

Then you put the layers of sheathing ON TOP OF the frame: ALL of the layers:
(In this case, there's a layer of OSB first, then two layers of drywall)

Each layer is nailed into the frame exactly the same as for a normal ceiling, or a normal wall: The nails go through into the joists, around the perimeter, but NOT in the field. Nobody is saying that you have to nail or screw the drywall into the OSB, in the field between the framing! You ONLY nail (or screw) the drywall around the edges, exactly as you would for any wall, or for a normal ceiling.

Then you raise the modules up into place, between the backbone members:
The above is shown without the rim boards in place, for clarity. Here's how it will actually appear with the rim boards:



And that's it!

There is no need to beef-up from below, which is what you are showing in your diagram!

If your modules are already in place, then take them down, put the drywall on top, and raise them again. You can set them at whatever height you need within the backbone such that you get enough clearance on top, between this leaf and the outer-leaf.

If you build modules, then all of the layers will be connected together rigidly to the module frame

No they will not, if you build them correctly! They will be built exactly the same as any other ceiling, with the layers of drywall all nailed into the FRAME, and NOT nailed of screwed in the field between framing members.

This means my outer most OSB layer is already in situ,

Right! So after you get your outer-leaf on, then you will take down those inner-leaf modules, rework them with drywall on top, then raise them up again.

but I can still build modules in the normal way and insert them between my rafters.

Fine! Then do that! Take down your temporary "ceiling/roof" once you get the final outer-leaf roof in place, then build your modules in the normal way (see above), and raise them into place.

Building a studio is already complicated enough: there's no need to complicate it even more unnecessarily, by beefing up modules from below when it would be so much easier to just take them down and rebuild them properly.


- Stuart -

Soundman2020 wrote:I'm still not understanding your reasoning, and I think you still are not understanding the way an inside-out ceiling is supposed to be built, normally.

You start with the inner-leaf walls:

Inside-out-ceiling-sequence--001.jpg

Then you put up the backbone and nothing else....

Inside-out-ceiling-sequence--002.jpg

Then you build the module down on the ground, starting with just a 2x4 frame:

Inside-out-ceiling-sequence--005.jpg

Then you put the layers of sheathing ON TOP OF the frame: ALL of the layers:

Inside-out-ceiling-sequence--007.jpg

(In this case, there's a layer of OSB first, then two layers of drywall)

Each layer is nailed into the frame exactly the same as for a normal ceiling, or a normal wall: The nails go through into the joists, around the perimeter, but NOT in the field. Nobody is saying that you have to nail or screw the drywall into the OSB, in the field between the framing! You ONLY nail (or screw) the drywall around the edges, exactly as you would for any wall, or for a normal ceiling.

Then you raise the modules up into place, between the backbone members:

Inside-out-ceiling-sequence--004.jpg

The above is shown without the rim boards in place, for clarity. Here's how it will actually appear with the rim boards:

Inside-out-ceiling-sequence--008.jpg

And that's it!

There is no need to beef-up from below, which is what you are showing in your diagram!

If your modules are already in place, then take them down, put the drywall on top, and raise them again. You can set them at whatever height you need within the backbone such that you get enough clearance on top, between this leaf and the outer-leaf.

If you build modules, then all of the layers will be connected together rigidly to the module frame

No they will not, if you build them correctly! They will be built exactly the same as any other ceiling, with the layers of drywall all nailed into the FRAME, and NOT nailed of screwed in the field between framing members.

This means my outer most OSB layer is already in situ,

Right! So after you get your outer-leaf on, then you will take down those inner-leaf modules, rework them with drywall on top, then raise them up again.

but I can still build modules in the normal way and insert them between my rafters.

Fine! Then do that! Take down your temporary "ceiling/roof" once you get the final outer-leaf roof in place, then build your modules in the normal way (see above), and raise them into place.

Building a studio is already complicated enough: there's no need to complicate it even more unnecessarily, by beefing up modules from below when it would be so much easier to just take them down and rebuild them properly.


- Stuart -

Thanks Stuart, I completely understand how the modules are supposed to be constructed, and your descriptions and images confirmed that. The question I had was about the best way to fix the layers to the frame. you answered the question by pointing out that the panels are fastened to the frame and not in the field, which I was also aware of, however, I wrongly thought that it would be undesireable due to the panels being cut down smaller and therefore stiffer, rather than in a normal wall where the panels are larger and have more flex. Clearly this doesn’t matter in this case.

Though I still have not quite managed to convey how my structure is built at present, i got the answer I needed!

Just for future clarification: I do not have any ceiling modules in place, I simply have a frame with rafters and a ridge and walls. The osb is fixed to the outside of the entire frame including the roof. That osb is my roof, which is felted.

Paul

It's about time I got round to designing my HVAC system properly.

I've been putting it off because I lack so much understanding on the subject, there's a fair amount of threads that cover this subject but either the info is conflicting or the math just goes right over my head, I am a mere simpleton when it comes to this. So this is going to be a real challenge for me and I need to ask you guys for a lot of help as well as patience.

For now, let's just concentrate on the control room ventilation until I have grasped the basics:

I'll have a mini-split AC unit and I would like to install an HRV for ventilation, which will be located in my lobby. The outer leaf of my control room will make up one of the walls to this lobby. I would then have a silencer box for the supply and return going in and out of the control room. The ducting and silencer boxes for my inner leaf are going to be inside my room, hidden within soffits or in the ceiling space. My inner leaf has a vaulted ceiling where I am going to install hangers and other treatment which will all be covered with fabric and possibly an angled cloud. So there's plenty of space up there for the ducting. The silencer boxes can go high up on my gable end within a soffit or hidden amongst the treatment there and the plan is to have the 2 boxes basically on top of each other, with neoprene in between them, supported on top of the framing of my hanger trap on the rear wall.

Now I have a few questions:

1. I've drawn up some ideas for the control room, blue being my supply of fresh air and red being my returns. Would some of these ideas work? If so, which of these locations would be best for my return ducts?

2. I would like to have 2 supply and 2 return ducts so I've designed a silencer box that splits into 2. I've drawn up what I had in mind, would something like this work well?

3. I know I need a silencer box for both the supply and return on both leaves, (4 boxes) but do I also need a box for each vent?

4. Do the silencer boxes need to have the same equivalent amount of mass as my walls?

5. I am thinking I should use flex duct in the cavity between my 2 leaves, to join between the outer and inner silencer boxes, and then could I use round PVC pipe for the ducting inside the room?

Once I've figured this out I'll need to ask how to work out what size ducting I need as well as the size of the boxes themselves. Now I'm off to learn more about HRVs and how much air needs to be supplied to my room...

Paul

Anyone?

The overall idea is fine, but there's some details that need ironing out...

1) Remove the central baffle, directly in front of the inlet duct: it is not doing you any good at all, and in fact is creating additional friction that you do not need.

2) It does not look like the cross sectional area inside the silencer remains roughly constant, and it does not seem to be at least twice the area of the ducts.

3) What happens on the two outboard ends? It seems you plan to attach duct there? Think about that... the silencer won't be doing anything at all if the air that goes into it through one duct, then comes out of it again and through other ducts that are inside the same noisy place it was before...

- Stuart -

3) What happens on the two outboard ends? It seems you plan to attach duct there? Think about that... the silencer won't be doing anything at all if the air that goes into it through one duct, then comes out of it again and through other ducts that are inside the same noisy place it was before...

I am exhausted right now, but I'm not understanding the issue with the Y splitter design here other than the issues your pointed out in points 1 and 2. Can someone please make this more clear for me to understand?

1. I've drawn up some ideas for the control room, blue being my supply of fresh air and red being my returns. Would some of these ideas work? If so, which of these locations would be best for my return ducts?

Probably one of the middle two images would provide the best circulation.

3. I know I need a silencer box for both the supply and return on both leaves, (4 boxes) but do I also need a box for each vent?

The silencer boxes provide insertion loss but also maintain the mass as you're penetrating your leaf. Nothing to do with the vent grilles unless you're using the silencer as an actual inlet/outlet to your room with no duct work running in your room.

4. Do the silencer boxes need to have the same equivalent amount of mass as my walls?

Yes.

5. I am thinking I should use flex duct in the cavity between my 2 leaves, to join between the outer and inner silencer boxes, and then could I use round PVC pipe for the ducting inside the room?

Flex duct works well.

PVC won't work well and will cost a fortune. First off, it's best if you use duct lined duct work. Also, you will need quite large duct work to keep your air velocity and pressure drop low. Lastly, I'm not sure how you would connect a grille to PVC.

Once I've figured this out I'll need to ask how to work out what size ducting I need as well as the size of the boxes themselves. Now I'm off to learn more about HRVs and how much air needs to be supplied to my room...

HVAC is a step learning curve dude. Especially weirdo studio HVAC! I know there are a lot of threads on the forum that should answer most of your questions about it. There are a handful that go through all of the calculations you need to do. Don't freak out too bad when you can't find answers. Please try and do your homework by checking the threads but before you get to the crying stage, just ask your questions here on your thread. I know I almost got to the crying stage as I was researching HVAC!

Greg

Soundman2020 wrote:The overall idea is fine, but there's some details that need ironing out...

1) Remove the central baffle, directly in front of the inlet duct: it is not doing you any good at all, and in fact is creating additional friction that you do not need.

2) It does not look like the cross sectional area inside the silencer remains roughly constant, and it does not seem to be at least twice the area of the ducts.

3) What happens on the two outboard ends? It seems you plan to attach duct there? Think about that... the silencer won't be doing anything at all if the air that goes into it through one duct, then comes out of it again and through other ducts that are inside the same noisy place it was before...

- Stuart -

Thank-you Stuart. Okay, so something more like this? (see attached)

So, as an example if I used a round 4" duct then the cross sectional area would be 12.56" is that correct? Then the cross-sectional area of the passage way through the silencer baffles needs to be constant (i.e. same width and height?) and at least twice that of the duct... So would I measure the width and height of the passageway to work out the area but not the length of the passageway? I am so confused

Though I am confident you are correct, I am also confused about what you mean in your last point... the outboard ends of the silencer would connect to ducts, and the end of the ducts would connect to registers, or vents, or whatever you want to call them, inside my hopefully quiet control room...

The noisy ventilator would be in a different room, then the supply and return coming directly off of it would go through their own silencers, then through the 2 leaves without rigid connection in the cavity, then into silencers again, then the control room ducts, then the vents in the room. Is this not correct?

Gregwor wrote:

3) What happens on the two outboard ends? It seems you plan to attach duct there? Think about that... the silencer won't be doing anything at all if the air that goes into it through one duct, then comes out of it again and through other ducts that are inside the same noisy place it was before...

I am exhausted right now, but I'm not understanding the issue with the Y splitter design here other than the issues your pointed out in points 1 and 2. Can someone please make this more clear for me to understand?

1. I've drawn up some ideas for the control room, blue being my supply of fresh air and red being my returns. Would some of these ideas work? If so, which of these locations would be best for my return ducts?

Probably one of the middle two images would provide the best circulation.

3. I know I need a silencer box for both the supply and return on both leaves, (4 boxes) but do I also need a box for each vent?

The silencer boxes provide insertion loss but also maintain the mass as you're penetrating your leaf. Nothing to do with the vent grilles unless you're using the silencer as an actual inlet/outlet to your room with no duct work running in your room.

4. Do the silencer boxes need to have the same equivalent amount of mass as my walls?

Yes.

5. I am thinking I should use flex duct in the cavity between my 2 leaves, to join between the outer and inner silencer boxes, and then could I use round PVC pipe for the ducting inside the room?

Flex duct works well.

PVC won't work well and will cost a fortune. First off, it's best if you use duct lined duct work. Also, you will need quite large duct work to keep your air velocity and pressure drop low. Lastly, I'm not sure how you would connect a grille to PVC.

Once I've figured this out I'll need to ask how to work out what size ducting I need as well as the size of the boxes themselves. Now I'm off to learn more about HRVs and how much air needs to be supplied to my room...

HVAC is a step learning curve dude. Especially weirdo studio HVAC! I know there are a lot of threads on the forum that should answer most of your questions about it. There are a handful that go through all of the calculations you need to do. Don't freak out too bad when you can't find answers. Please try and do your homework by checking the threads but before you get to the crying stage, just ask your questions here on your thread. I know I almost got to the crying stage as I was researching HVAC!

Greg

Thanks a lot Greg, yeah for sure this is a steep learning curve... just like most things in studio design!

Regarding sizing the ducts and duct material, this is what I found written by Rod on GS:

"If your plans are a split system you need to think this though a bit more.

First off - how did you determine system size?

Is your space well isolated from the outside world? If the answer is "yes" then that would also mean it is well insulated - and in that case the thermal looses might be small enough that an over-sized unit will never have enough run time to dehumidify to any meaningful degree. By the same token - an undersized unit will never get the job done.

Proper sizing is key here.

You already know you want to keep velocity down - no way you want to exceed 300 fpm (feet per minute) at the inlet to the space - and preferably 100 fpm (that's always the design goal I shoot for if space permits)

But velocity outside of that is not really important - so you can decrease duct size outside of the isolation box as long as you deal with it at the box.

That minimizes duct costs.

I never (ever) design systems using flex duct except for a very short length of flex at the box to decouple one side of the box/room from the air handler. I'm pretty sure I mentioned this in the book."

So it seems that I should use rigid duct in the actual rooms, but flex duct in the cavity. What type of rigid ducting would you recommend? Galvanized metal? And would round be better than rectangular?

Also, from what Rod has said, the ducting coming out of the silencers in the control room can be small so long as the silencer brings the velocity down... so would that mean the silencer box needs to double the cross sectional area of the duct work feeding it? (from the ventilator)

If I say there would be a maximum of 10 people in the control room (more likely it'll only be 5 or 6, but you never know) then how what do I need to look for in terms of specs for either a fan or HRV? I know that it needs to provide enough fresh air for each person (15cfm) with 6 exchanges per hour, so does that mean I need a fan or HRV that is rated at providing 150cfm?

Then, how does the ducting effect it? If I run it through a ducting calculator it says my ducts should be about 8" for a round duct, but does length of duct effect that? how about if you split it into two ducts at 90 degree angles? Then, as Rod says, if you make your silencer big enough you can decrease the size... but by how much? These are the types of questions I need help with, so if you could help I'd really be grateful.

Paul

So, as an example if I used a round 4" duct then the cross sectional area would be 12.56" is that correct? Then the cross-sectional area of the passage way through the silencer baffles needs to be constant (i.e. same width and height?) and at least twice that of the duct... So would I measure the width and height of the passageway to work out the area but not the length of the passageway? I am so confused

I will bet my life savings that your room needs a way bigger duct than a 4" RD feeding your silencer. I didn't look to see how big your room is but you probably need an 8 or bigger.

Here is my answer for your cross sectional area question copy/pasted from my notes:

Cross sectional areas of common duct:

Area of a circle = π r2
Radius = half of the diameter

Duct CSA:

4” RD duct = 12.57 sq in. Double = 25.14 sq in.
5” RD duct = 19.63 sq in. Double = 39.26 sq in.
6” RD duct = 28.27 sq in. Double = 56.54 sq in.
8” RD duct = 50.27 sq in. Double = 100.53 sq in.
9” RD duct = 63.62 sq in. Double = 127.23 sq in.
10” RD duct = 78.54 sq in. Double = 157.08 sq in.
12” RD duct = 113.1 sq in. Double = 226.19 sq in.

Area of a rectangle = width x height
Area of a oval duct = major radius x minor radius x π
This is for a true oval, not a flat oval like HVAC duct though :-S
Area of flat oval =
π (minor radius)2 + [(major diameter- minor diameter) x minor diameter)]
OR
[(π minor 2) / 4] + 3 (major - minor)

To answer your question about the path inside your silencer, it needs to at LEAST double in cross sectional area. It certainly does not have to be a square (equal height and width) path. However, the more square it is, the less pressure drop with be introduced which is very important in your design.

Addressing your comment regarding a constant size, the CSA within the silencer can change. Actually, the more changes, the more attenuation. However, realize that with each CSA change, a pressure drop is realized. Remember, the larger the CSA change, the more the attenuation. So, on your Y split type box, I would double the CSA on each half. This would essentially quadruple the CSA of the inlet. This will also really slow down your air velocity, reduce the static pressure drop and reduce the air movement noise within the box.

The noisy ventilator would be in a different room, then the supply and return coming directly off of it would go through their own silencers, then through the 2 leaves without rigid connection in the cavity, then into silencers again, then the control room ducts, then the vents in the room. Is this not correct?

So, you are using a forced air ducted system? Not a ductless mini split? Your statement seems to be correct.

So it seems that I should use rigid duct in the actual rooms, but flex duct in the cavity. What type of rigid ducting would you recommend? Galvanized metal? And would round be better than rectangular?

Once it's in your room, you can use whatever you want. RD always has the least friction coefficients. Rect allows you to maximize space. Remember, inside your room you should always use duct liner.

Also, from what Rod has said, the ducting coming out of the silencers in the control room can be small so long as the silencer brings the velocity down... so would that mean the silencer box needs to double the cross sectional area of the duct work feeding it? (from the ventilator)

You've misunderstood his point. I'll try to explain this better:
The CFM is a constant throughout a duct run.

Air velocity in ft/min = CFM Flow rate in ( ft3/min) / CSA Cross sectional area in ft2

So, if CFM is a constant, then the velocity changes depending on the CSA. So, if you have a smaller duct, your velocity will increase. If you use larger ducts, your velocity decreases.

In your room, you need to achieve a maximum of 300 feet per minute at your register/grille. So yes, you COULD use smaller duct inside your room as long as you make your CSA large enough to reduce the air velocity before it hits your register/grille. However, you will increase your pressure drop and air noise inside those ducts. Duct liner will help with the air noise.

Most importantly, you want to deal with turbulence noise introduced by any change of direction or CSA change. You could use a conical transition which would not allow you to gain the immediate CSA change (insertion loss achieved by a gross impedance mismatch). A gradual change in CSA like a cone (or wave guide) provides will prevent distortion or turbulence, the true rule of thumb is that you need a certain distance between any direction change or CSA change and the mouth of your grille/register. I've heard two values for this distance. It's either 3 or 5 times the diameter of that which is feeding the grille/register. So let's say your have a 4" round duct that goes to a required 6" round grille, you'd need a distance of between 12 and 20" of straight duct to prevent noise at the mouth.

If I say there would be a maximum of 10 people in the control room (more likely it'll only be 5 or 6, but you never know) then how what do I need to look for in terms of specs for either a fan or HRV? I know that it needs to provide enough fresh air for each person (15cfm) with 6 exchanges per hour, so does that mean I need a fan or HRV that is rated at providing 150cfm?

Purely for the required 30% fresh air circulation, you can figure it out like this:

Flow rate in CFM = ft3/min = [# of air changes per hour X cubic volume of room] / 60 min
# of air changes must be at least 6 changers per hour

Air velocity in ft/min = CFM Flow rate in ( ft3/min) / CSA Cross sectional area in ft2
This is actually expressed as v=q/A
Air velocity must be less than 300 ft/min for studios

So, if you need 30% fresh air, take your total CFM and calculate 30% of it. That would be your fresh air CFM. Size your HRV or fan based off of that.

Sizing your actual air handler unit is a different story though as you need to need to figure out your sensible and latent loads.

Then, how does the ducting effect it? If I run it through a ducting calculator it says my ducts should be about 8" for a round duct, but does length of duct effect that?

Typical duct sizing calculators assume a 0.1 friction rate. You can use these to calculate your runs from your air handler to your silencer boxes. From the boxes to your register/grilles you need to size these based on your velocity and CSA changes. Once you've sized everything this way, you need to calculate your total static pressure. If your blower will not run within your system pressure, you may have to change your duct sizing from the air handler to the silencer boxes and then ultimately, that may change everything down the line!

how about if you split it into two ducts at 90 degree angles?

That introduced a pressure drop. These are the things you calculate later to determine your total static pressure. Using 1.5x radius bends instead of 1x can help as well. To be honest with you, I made my own spreadsheet to calculate this because I could find no good resources online. It's a chore and takes hours to calculate

Then, as Rod says, if you make your silencer big enough you can decrease the size.

Again, changing the CSA achieves insertion loss. As I mentioned before, it has negative side effects as well. Another reason to have a large CSA throughout your box is because all of those sharp corners introduce a ton of static pressure. The bigger the path, the less pressure drop!

.. but by how much? These are the types of questions I need help with, so if you could help I'd really be grateful.

If you look at this picture, you'll see that even when we double or halve the CSA, we aren't achieving great levels of insertion loss. That's why I suggest that you double each half of your Y box so that you are quadrupling your CSA! These sort of details are the ones you have to make informed decisions on while designing your studio!

Greg

Greg - amazing info, thanks so much for the detailed reply. I want to digest it all a bit more before I reply as it's a lot to take in.

Thanks again,
Paul

Gregwor wrote:

So, as an example if I used a round 4" duct then the cross sectional area would be 12.56" is that correct? Then the cross-sectional area of the passage way through the silencer baffles needs to be constant (i.e. same width and height?) and at least twice that of the duct... So would I measure the width and height of the passageway to work out the area but not the length of the passageway? I am so confused

I will bet my life savings that your room needs a way bigger duct than a 4" RD feeding your silencer. I didn't look to see how big your room is but you probably need an 8 or bigger.

Here is my answer for your cross sectional area question copy/pasted from my notes:

Cross sectional areas of common duct:

Area of a circle = π r2
Radius = half of the diameter

Duct CSA:

4” RD duct = 12.57 sq in. Double = 25.14 sq in.
5” RD duct = 19.63 sq in. Double = 39.26 sq in.
6” RD duct = 28.27 sq in. Double = 56.54 sq in.
8” RD duct = 50.27 sq in. Double = 100.53 sq in.
9” RD duct = 63.62 sq in. Double = 127.23 sq in.
10” RD duct = 78.54 sq in. Double = 157.08 sq in.
12” RD duct = 113.1 sq in. Double = 226.19 sq in.

Area of a rectangle = width x height
Area of a oval duct = major radius x minor radius x π
This is for a true oval, not a flat oval like HVAC duct though :-S
Area of flat oval =
π (minor radius)2 + [(major diameter- minor diameter) x minor diameter)]
OR
[(π minor 2) / 4] + 3 (major - minor)

To answer your question about the path inside your silencer, it needs to at LEAST double in cross sectional area. It certainly does not have to be a square (equal height and width) path. However, the more square it is, the less pressure drop with be introduced which is very important in your design.

Addressing your comment regarding a constant size, the CSA within the silencer can change. Actually, the more changes, the more attenuation. However, realize that with each CSA change, a pressure drop is realized. Remember, the larger the CSA change, the more the attenuation. So, on your Y split type box, I would double the CSA on each half. This would essentially quadruple the CSA of the inlet. This will also really slow down your air velocity, reduce the static pressure drop and reduce the air movement noise within the box.

The noisy ventilator would be in a different room, then the supply and return coming directly off of it would go through their own silencers, then through the 2 leaves without rigid connection in the cavity, then into silencers again, then the control room ducts, then the vents in the room. Is this not correct?

So, you are using a forced air ducted system? Not a ductless mini split? Your statement seems to be correct.

So it seems that I should use rigid duct in the actual rooms, but flex duct in the cavity. What type of rigid ducting would you recommend? Galvanized metal? And would round be better than rectangular?

Once it's in your room, you can use whatever you want. RD always has the least friction coefficients. Rect allows you to maximize space. Remember, inside your room you should always use duct liner.

Also, from what Rod has said, the ducting coming out of the silencers in the control room can be small so long as the silencer brings the velocity down... so would that mean the silencer box needs to double the cross sectional area of the duct work feeding it? (from the ventilator)

You've misunderstood his point. I'll try to explain this better:
The CFM is a constant throughout a duct run.

Air velocity in ft/min = CFM Flow rate in ( ft3/min) / CSA Cross sectional area in ft2

So, if CFM is a constant, then the velocity changes depending on the CSA. So, if you have a smaller duct, your velocity will increase. If you use larger ducts, your velocity decreases.

In your room, you need to achieve a maximum of 300 feet per minute at your register/grille. So yes, you COULD use smaller duct inside your room as long as you make your CSA large enough to reduce the air velocity before it hits your register/grille. However, you will increase your pressure drop and air noise inside those ducts. Duct liner will help with the air noise.

Most importantly, you want to deal with turbulence noise introduced by any change of direction or CSA change. You could use a conical transition which would not allow you to gain the immediate CSA change (insertion loss achieved by a gross impedance mismatch). A gradual change in CSA like a cone (or wave guide) provides will prevent distortion or turbulence, the true rule of thumb is that you need a certain distance between any direction change or CSA change and the mouth of your grille/register. I've heard two values for this distance. It's either 3 or 5 times the diameter of that which is feeding the grille/register. So let's say your have a 4" round duct that goes to a required 6" round grille, you'd need a distance of between 12 and 20" of straight duct to prevent noise at the mouth.

If I say there would be a maximum of 10 people in the control room (more likely it'll only be 5 or 6, but you never know) then how what do I need to look for in terms of specs for either a fan or HRV? I know that it needs to provide enough fresh air for each person (15cfm) with 6 exchanges per hour, so does that mean I need a fan or HRV that is rated at providing 150cfm?

Purely for the required 30% fresh air circulation, you can figure it out like this:

Flow rate in CFM = ft3/min = [# of air changes per hour X cubic volume of room] / 60 min
# of air changes must be at least 6 changers per hour

Air velocity in ft/min = CFM Flow rate in ( ft3/min) / CSA Cross sectional area in ft2
This is actually expressed as v=q/A
Air velocity must be less than 300 ft/min for studios

So, if you need 30% fresh air, take your total CFM and calculate 30% of it. That would be your fresh air CFM. Size your HRV or fan based off of that.

Sizing your actual air handler unit is a different story though as you need to need to figure out your sensible and latent loads.

Then, how does the ducting effect it? If I run it through a ducting calculator it says my ducts should be about 8" for a round duct, but does length of duct effect that?

Typical duct sizing calculators assume a 0.1 friction rate. You can use these to calculate your runs from your air handler to your silencer boxes. From the boxes to your register/grilles you need to size these based on your velocity and CSA changes. Once you've sized everything this way, you need to calculate your total static pressure. If your blower will not run within your system pressure, you may have to change your duct sizing from the air handler to the silencer boxes and then ultimately, that may change everything down the line!

how about if you split it into two ducts at 90 degree angles?

That introduced a pressure drop. These are the things you calculate later to determine your total static pressure. Using 1.5x radius bends instead of 1x can help as well. To be honest with you, I made my own spreadsheet to calculate this because I could find no good resources online. It's a chore and takes hours to calculate

Then, as Rod says, if you make your silencer big enough you can decrease the size.

Again, changing the CSA achieves insertion loss. As I mentioned before, it has negative side effects as well. Another reason to have a large CSA throughout your box is because all of those sharp corners introduce a ton of static pressure. The bigger the path, the less pressure drop!

.. but by how much? These are the types of questions I need help with, so if you could help I'd really be grateful.

If you look at this picture, you'll see that even when we double or halve the CSA, we aren't achieving great levels of insertion loss. That's why I suggest that you double each half of your Y box so that you are quadrupling your CSA!

Insertion Loss Graph from Engineering Acoustics 2009.png

These sort of details are the ones you have to make informed decisions on while designing your studio!

Greg

Okay so I've had a good read over your reply Greg, I'm starting to understand it (i think).

Just before I go on I want to check that I am measuring CSA correctly in my baffle box, is it as shown in my diagram? (in red)

Just before I go on I want to check that I am measuring CSA correctly in my baffle box, is it as shown in my diagram? (in red)

That's PART of it, yes. But that's just the area at the tip of the baffle. The air also flows up the sides of the baffle, between it and the next baffle...


- Stuart -

Soundman2020 wrote:

Just before I go on I want to check that I am measuring CSA correctly in my baffle box, is it as shown in my diagram? (in red)

That's PART of it, yes. But that's just the area at the tip of the baffle. The air also flows up the sides of the baffle, between it and the next baffle...


- Stuart -

Hmm... okay, now I'm more confused. By that logic, surely the air travels through the entire silencer box.. so do I just measure the total area of the pathway between the baffles? What defines the cross sectional area compared to the total area?