Garden Drum Room Build

[Ushmurray]

Hi,

I’m a guitar player and studio hobbyist for the last 20 years and I have a little small 1000sq foot house with a small control room studio in one room.
I bought the house about 10 years ago and have always planned on building a little drum room in the garden. I’ve read Rod’s book front to back a few times and lurked this forum over the years so I have a basic knowledge but am an expert by no means.
This is not a commercial venture I just want to be able have my mates around and record real instruments and work on a few non commercial projects without bothering my neighbours.
Keeping the neighbours happy is my main goal. With the size and dimensions I don’t expect this to be the best sounding room,

Fortunately for me my brother is an architect and he has drawn plans for the external build. He’s also read Rod’s book.
I have a building contractor employed and they have started clearing the site to lay the foundation slab.
Now I understand my situation with neighbours close proximity and neighbouring structures is not ideal at all , i’m trying to do the best with what I have available to me.
I’ve future proofed the external build so if it doesn’t work out or in 10 years I want to sell the house I can easily knock out the front wall and put glass doors in and have a nice garden room.

Here are the details and drawings below

Budget:
-€30k on external build
-€10k on internal room build

Isolation Goals:
-Record drum kit, guitar amps without bothering neighbours , I would hope to achieve 65db or more reduction.

Property:
- My house is a semi detached but there are terraces of houses on all sides.
- Closest neighbours house is 10 meters away and the rest are about 12 meters.
- One neighbour has a garden room / office directly adjacent to my build. They don’t use it all the time and we can work out amenable times.
- The build will be directly adjacent to garden boundary walls on three sides. Two sides are solid concrete block and the side with neighbours office is cinder block boundary wall and cinder block office construction.

External Build:
- Cavity wall construction (100mm / 100mm / 100mm)
- Two vents, one single FD60 fire door and no windows.
- 200mm re-enforced concrete slab roof
- Airtight construction besides for vents and doorway
- Future proofed so if it doesn’t work out I can knock out the front wall and put in glass doors to convert to garden room.

Inner Room Walls and Floor:

- Inner Concrete slab isolated form external walls via 30mm acoustic insulation
- ProSound ImpactoMat 6mm
- Single 100mm stud walls built 50mm off cavity wall
- Studs filled with Knauf Earthwool RS60
- Studs isolated from floor via ProSound Isolation strip 100mm
- Stud walls isolated from external walls via neoprene isolation pads ( no product spec’d yet)
- ProSound Reducto Clips and Furring bars
- Knauf Soundshield Plus 15mm Plaster Board
- TecSound SY 100
- Knauf Soundshield Plus 15mm Plaster Board

Inner Room Roof:
- Single 100mm timber frame built 50mm off external concrete ceiling
- Frame filled with Knauf Earthwool RS60
- Frame isolated from wall studs via ProSound Isolation strip 100mm
- Frame isolated from external roof via neoprene isolation pads ( no product spec’d yet)
- ProSound Reducto Clips and Furring bars
- Knauf Soundshield Plus 15mm Plaster Board
- TecSound SY 100
- Knauf Soundshield Plus 15mm Plaster Board

Inner Room Door:
- Sealed FD60 Fire Door
- Door Frame fixed to external walls via neoprene isolation pads.

Electrics:
- All cabling comes from house via underground duct surfaces through the concrete floor within the inner room.
- All socket and lighting will come from surface mount wall ducting. Maybe a frame for the lights.

Air Handling:
- I’m a bit lost with this , there was mention on a thread here of a mini-split Daikin Ururu Sarara unit that could provide fresh air.
- Baffle boxes for vents, again i’m a bit lost with this.


My thoughts and questions:

I understand that two leaf construction is the best way to go but in my situation with directly adjacent boundary walls and a wet climate solid block or timber exterior walls are not really a viable option.
Cavity wall is the standard external wall construction here in Ireland and the only real option I have.

In Rod’s book he explains room within a room wall construction based on timber construction but I can’t see anything that pertains to room within a room with cavity wall construction.
In my build - cavity wall - air gap - stud - isolation clips - plaster board does that count as two or three leafs?

On the two sides theres the solid block garden boundary wall so that makes it - solid block boundary wall - cavity wall - air gap - stud - isolation clips - plaster board. Three or four leafs?
On one wall its cinder block office wall and cinder block garden boundary wall - cinder block - cinder block - cavity wall - air gap - stud - isolation clips - plaster board. No idea how many leafs!

I found an Irish building consultant document online that states standard cavity wall construction (100/50/100) achieves 58db reduction.
I have been told by a soundproofing product supplier that this inner room construction built on a solid brick wall achieves 65db.
Does anybody have an idea what decibel reduction I might expect to get in my build?


Any thoughts on my leaf layering and construction?

Any thoughts air handling units and baffle boxes?

Is there a better way to fix/support the stud walls, inner door and timber roof frame to the external building then with neoprene isolation pads?

Is it better to have vent baffle boxes inside or outside the building or do I need them on both?


Any help or advice is much appreciated.

Thanks

Ush

[John Steel]

Hello & howdo' Ush?
I'm a fellow member who has been designing a studio for 3 years and is preparing to start building. I would add that I'm not an expert but I have learned a few things that may be helpful to you.

In my build - cavity wall - air gap - stud - isolation clips - plaster board does that count as two or three leafs?

As you have described it, I think that's four leaves. This will probably end up being less effective and certainly a lot more costly than a two leaf design.

I found an Irish building consultant document online that states standard cavity wall construction (100/50/100) achieves 58db reduction . . .does anybody have an idea what decibel reduction I might expect to get in my build?

This is dependant on something known as "mass law". It can be calculated easily and there are many useful tools to help you do this in this forum.
http://www.johnlsayers.com/phpBB2/viewt ... =1&t=21770

Any thoughts on my leaf layering and construction?

Absolutely loads - but first you need to measure how much level you are likely to generate in your studio (ie how much isolation you need) as well as how much outside noise you need to keep out.

Any thoughts air handling units and baffle boxes?

Again, lots of thoughts. This is (in my experience) by far the most complicated aspect of designing a studio, but my advice to you would be to read some of the many design & build threads carefully and get a feel for the method of studio design. For example, the diagrams you posted seem to imply that you are planning a 'floating' floor. For a project studio this is rarely a good idea (search the forum for advice on this - don't take my word). Search the forum; the answers are nearly always there!

http://www.johnlsayers.com/phpBB2/viewt ... =1&t=21057
http://www.johnlsayers.com/phpBB2/viewt ... 10&t=21269
http://www.johnlsayers.com/phpBB2/viewt ... =2&t=21612
(there's lots more).

Any help or advice is much appreciated.

One last word - download Sketchup (the free version linked on this site) and draw your design with it. This will take a fairly large chunk of your time and patience but has the advantage of making you virtually 'build' the studio before you have spent anything and is far more useful than two dimensional diagrams. Oh - and remember, this is a marathon, not a sprint!

Good luck and I look forward to following your project.

[Ushmurray]

Hi John,

I just saw your thread there, that looks like a great space.

Thanks for your comments on my build , some very useful stuff in there.[

As you have described it, I think that's four leaves. This will probably end up being less effective and certainly a lot more costly than a two leaf design.

Yes I think your correct with four leaves.
Because I have to build adjacent to 6' tall garden boundary walls on three sides it kinda screws me over from the start.
I can't use the boundary walls in my build and the only real option is cavity wall construction.
2 of boundary walls are solid block and then my cavity wall, three leaves to start right?
3rd boundary wall is cinder block (neighbours office wall), cinder block (boundary wall) and then my cavity wall. Oh man! How many leaves is that.

I know two leaves is the best but no matter what way I slice it I've got more leaves then I could shake a stick at. And that's even before I start and inner room build.
Maybe I don't need an inner room?

Are all leafs equal ? As in how would these two hypothetical walls compare.
1. A four leaf plaster board construction.
2. A two leaf plasterboard and two leaf concrete block construction.
Surely wall 2 would perform better but still amplify some frequencies.
I've got too many leaves to start but I'm hoping the mass of the concrete will have some benefits.

This is dependant on something known as "mass law". It can be calculated easily and there are many useful tools to help you do this in this forum.
http://www.johnlsayers.com/phpBB2/viewt ... =1&t=21770

Thanks for this , I'm going to download it and try to figure it out tonight.

Absolutely loads - but first you need to measure how much level you are likely to generate in your studio (ie how much isolation you need) as well as how much outside noise you need to keep out.

Basically a drum kit running at 115db i'd like to reduce to 60db or more from two meters outside the build.

Again, lots of thoughts. This is (in my experience) by far the most complicated aspect of designing a studio, but my advice to you would be to read some of the many design & build threads carefully and get a feel for the method of studio design. For example, the diagrams you posted seem to imply that you are planning a 'floating' floor. For a project studio this is rarely a good idea (search the forum for advice on this - don't take my word). Search the forum; the answers are nearly always there!

http://www.johnlsayers.com/phpBB2/viewt ... =1&t=21057
http://www.johnlsayers.com/phpBB2/viewt ... 10&t=21269
http://www.johnlsayers.com/phpBB2/viewt ... =2&t=21612
(there's lots more).

Thanks for the link, I'll give them a read.
I've read a some threads on here but I think there's so much information that It's quite overwhelming for my little brain

I'm not building a floating floor, the floor slab is poured onto standard building floor insulation material. They frame the slab so there is 30mm gap between the slab and cavity wall to protect against water getting in. It would run down the wall to below the slab. I was going to fill this gap with acoustic insulation just because it might help a little bit.

Hahahah ,, Sketchup is what i used to do the inner room drawings with, it took me hours to do them
I'm just getting to grips with that program but i'll put together some 3d drawings.

Thanks John, your help is much appreciated.

Ush

[mc_deli]

On the neoprene pads and Reducto clips/furring channel, a few things that have been mentioned to me:

If you already have a double leaf wall then why clips/channel to decouple plaster board from the inner leaf stud (same for the ceiling)?

I note that the use of Reducto clips and Tecsound board is straight from the Sound Proofing Store designs. For your build, you should probably check if there are more mass/cash efficient materials you could use i.e. can you lose the special materials and get the same or better with an extra plasterboard everywhere (including floor rated plasterboard).

With the floor, check out the "drum riser" method. There are lots of options here with layers of load bearing rigid insulation, floor rated plasterboard, MLV etc. For your new build, maybe concrete on Styrox or similar is actually a relatively easy and mass/cost efficient option for you.

What neoprene pads, what are the rated for, why do need them at all as you have a full room-within-a-room with ceiling joists?

If you need "sway braces" or "wall ties" then I found AMC and Mason. I've talked to reps from both companies and they are incredibly helpful. My understanding is, if you need them, both manufacturers give guidance on use (spacing) but in my case I have to run the numbers with a professional.

On the 4 leaf thing, if the exterior wall is some kind of hollow block with filled cavity similar to this then I think you can treat that as a single leaf (for our purposes you have no choice but to treat it as a single leaf if I understand correctly;)).

On baffle boxes, lord it's baffling! For your build you should have 4, one each for in and out, attached to inner and outer leaves. Check the size of these before you commit to your air gap because you might find the best place for these is in the cavity between your leaves. Greg gave me a lot of info and it's quite challenging to get through. The TL;DR is to have a large enough cross sectional area throughout the path in the box to account for insertion loss. Rule of thumb seems to be something like 1,5 times the CSR of your duct. Obviously you can attack this much more scientifically;) And then compromise e.g. if you want to squeeze decoupled duct boxes between your leaves.

The wall ties and baffle boxes are also the items that I have ended up spending the most time on

And from a fellow guitar player and long time studio monkey, good luck and I am well envious of your budget and scope. Nice one!

PS Vapour barrier, do you need, probably you do as you have exterior walls, seems it should be on the air gap side of the inner leaf plasterboard, but your local regs may affect this...?

[Gregwor]

have a large enough cross sectional area throughout the path in the box to account for insertion loss.

The extra large cross sectional area within the silencer is to introduce an impedance mismatch which ultimately results in insertion loss. This is the goal.

Rule of thumb seems to be something like 1,5 times the CSR of your duct.

I recommend a MINIMUM of 2 times. More is better. If you can have 4 times the size then do it. Bigger is better. It's that simple.

seems it should be on the air gap side of the inner leaf plasterboard

Great way to word this. I've struggled to clearly explain the location of barrier and failed every time.

Greg

[Ushmurray]

Hi guys,

The external build is coming along, see pics below. Waiting for the concrete lintel, they should have that in and the walls completed next week ready for the roof.

If you already have a double leaf wall then why clips/channel to decouple plaster board from the inner leaf stud (same for the ceiling)?

Am i correct in thinking the cavity wall counts as one leaf?
Yes I had the same design for the ceiling.

I note that the use of Reducto clips and Tecsound board is straight from the Sound Proofing Store designs. For your build, you should probably check if there are more mass/cash efficient materials you could use i.e. can you lose the special materials and get the same or better with an extra plasterboard everywhere (including floor rated plasterboard).

Yes I'm looking at buying from the Sound Proofing Store and using their system.
There are a few soundproofing companies in Ireland but they only deal with large projects and then there are a few smaller guys but they wouldn't have as much experience as the Sound Proofing Store.
If anyone knows a good guy in Ireland please let me know.

Do you think there will be no real benefits from using Reducto clips?
Replace Tecsound board with a layer of plasterboard?

With the floor, check out the "drum riser" method. There are lots of options here with layers of load bearing rigid insulation, floor rated plasterboard, MLV etc. For your new build, maybe concrete on Styrox or similar is actually a relatively easy and mass/cost efficient option for you.

The floor concrete slab is going to be poured on a high density roofing insulation which I think is very similar to Styrox. I'll check the product with the builder.
There will be a 30mm gap between this slab and the external walls and then a 6mm Prosound ImpactoMat
Are you saying I should build a "Drum Riser" on top of that?

What neoprene pads, what are the rated for, why do need them at all as you have a full room-within-a-room with ceiling joists?

If you need "sway braces" or "wall ties" then I found AMC and Mason. I've talked to reps from both companies and they are incredibly helpful. My understanding is, if you need them, both manufacturers give guidance on use (spacing) but in my case I have to run the numbers with a professional

Thank you that's exactly what I was looking for . AMC and Mason

On the 4 leaf thing, if the exterior wall is some kind of hollow block with filled cavity similar to this then I think you can treat that as a single leaf (for our purposes you have no choice but to treat it as a single leaf if I understand correctly;)).

The exterior cavity wall is 100mm solid sand and cement block , 100mm building insulation and 100mm solid sand and cement block.
There is lots of metal ties between the two blocks. There are two photos below of the doorway which shows the construction.

On baffle boxes, lord it's baffling! For your build you should have 4, one each for in and out, attached to inner and outer leaves. Check the size of these before you commit to your air gap because you might find the best place for these is in the cavity between your leaves. Greg gave me a lot of info and it's quite challenging to get through. The TL;DR is to have a large enough cross sectional area throughout the path in the box to account for insertion loss. Rule of thumb seems to be something like 1,5 times the CSR of your duct. Obviously you can attack this much more scientifically;) And then compromise e.g. if you want to squeeze decoupled duct boxes between your leaves.

I really need to read up on this. I've got a bit of time to plan as the internal build is going to me in May. Hopefully the baffles don't have to be too big as internal space is valuable.

And from a fellow guitar player and long time studio monkey, good luck and I am well envious of your budget and scope. Nice one!

Thanks man, it's bloody scary and I can't afford to make a balls of it

PS Vapour barrier, do you need, probably you do as you have exterior walls, seems it should be on the air gap side of the inner leaf plasterboard, but your local regs may affect this...?

Do you mean DPC (damp proof course) ? There is a DPC built into the cavity wall. Do I need a Vapour barrier on air gap side of the inner leaf plasterboard?

The extra large cross sectional area within the silencer is to introduce an impedance mismatch which ultimately results in insertion loss. This is the goal.

I recommend a MINIMUM of 2 times. More is better. If you can have 4 times the size then do it. Bigger is better. It's that simple.

Thanks Greg, I'm lost with this at the moment, I need to go away and read up on all of this.

Also guys in the Front View picture you can see the proximity of the neighbours garden room / office on the left. The boundary wall there is cinder block and then their garden room wall is also cinder block..........

[mc_deli]

...obviously I am not an expert, just another DIYer, hence my answers are typically non-committal!


Am i correct in thinking the cavity wall counts as one leaf?
I think the answer is: technically you could class it as two coupled leaves, but I think it's a moot point. You have to build an decouple inner room-within-a-room irrespective of how you class your cavity wall. If you are making MSM calculations, then an expert can tell you more.

Do you think there will be no real benefits from using Reducto clips?
My opinion is of no consequence. In my thread and a few others, the experienced mods have pointed out that, if you already have a decoupled inner leaf, then it might not be "worth" adding clips/channel. Only the numbers can tell you if it's worth it to you. What is the effect on the STL graph of the clips/channel compared to the cost, does it bring down the MSM freq significantly etc.?. Do you need it, can you afford it, and is there a cheaper alternative?

What are the alternatives - well, extra plasterboard is one, also extra air gap is another I'm not knocking the reducto system though. Looks great!

(When counting the cost, include the extra installation time etc. and consider what you could buy instead! You could also consider some knock on effects here. When I started using SketchUP I realised that I would need to attach some room treatment framing/cloud to the inner leaf. For me I realised that would mean attaching through the plasterboard into the channel rather than into stud. What might this mean for you, I don't know;))

Are you saying I should build a "Drum Riser" on top of that?
No, drum riser as alternative to concrete floor. But, as said, because you have a new build, and it seems like a "standard" method for an ext build, seems like the concrete floor is a good solution for you.

Do you mean DPC (damp proof course) ? There is a DPC built into the cavity wall. Do I need a Vapour barrier on air gap side of the inner leaf plasterboard?
I wish I could answer this for you. This is where you need expert local advice based on your regs. What I have read on here (and some good GS threads) is to consider, yes, your exterior outer leaf wall has DPC, that's normal. And normally that would be it. But now you are building an airtight inner leaf. That's not "normal" for everyday builders;) Now you will have an air gap between leaves that has no air circulation and is not necessarily heated. So, do you need a vapour barrier for your inner leaf to prevent moisture/mould in the gap - and do your local climate/regs necessitate it?


This is your opportunity to be really critical and scrutinize your material choices;) Good luck.

[Ushmurray]

Hi guys,

I hope all is well with you.

It's been a few weeks now and the builders have stopped working due to the Covid 19 pandemic.
All that's left to do is pour the concrete slab roof and internal slab floor and the the external build is complete.

I've been reading through the forum trying to gather as much information about air vents and baffles etc.
I've gone through the math and I understand it quite a bit more but I still feel like i'm guessing a bit.
So far I've come up with three different designs, two of which the baffles were huge and I just don't have enough space to fit them.

I've read through Waka's studio build thread.
http://www.johnlsayers.com/phpBB2/viewt ... =1&t=21057
There's lot's of great info there and his studio is a little bit bigger then mine, his baffles weren't huge either so I thought i'd use this as a basis of my design

The plan so far is

In Vent :

150mm In Line fan - external room baffle internal dims 150mm x 250mm = 37500mm2 - 150mm flexible ducting - internal room baffle internal dims 150mm x 250mm
There is an air gap of 30mm between external room baffle and the internal room and 30mm air gap between internal baffle and external room.

Out Vent : Same as above

The math:

My internal room dims : 2.6m x 3.54m x 3.91m = 35.98m3 or 1270 CF
Based on maximum number of 5 people in the room at any time = 75 CFM
1270 CF x 6 changes per hour = 7,620 CF
6 changes per hour = 7,620 ft / hr x 60 = 127CFM divided by (cross-sectional area of the fan in SQUARE FEET) = FPM of the fan (air velocity)
Based on a 150mm fan = 75 x 75 x 3.14 = 17,662mm2 = .1901 square foot
To get the FPM of the fan divide the cubic feet per minute (127CFM) by the cross sectional area of the fan (.1901 square feet) = 669 FPM

I know I want 300 FPM or less at the register , will the doubling of cross-sectional area in the baffle manage to bring the 669 FPM fan down to 300 FPM?

Have I got the math right?

I'm basing 6 changes per hour on the size of the room 1270CFM or should I base it off the amount of the air required 5 people in the room 75 CFM.

Below is sketchup of the design so far , I have the baffles on the front and back wall but I could move them to the side walls.
I could also make the baffles longer, should I make them longer?

Cheers

[Paulus87]

Hi,

It looks like you have all 4 of you silencer boxes inside your room?...

You need 1 in and 1 out for your inner leaf and then corresponding 1 in and 1 out for your outer leaf.

The simplest way is to have your inner leaf boxes inside your room and then your outer leaf boxes on the outside of your building.

If you have space between your inner and outer leaves then you can attempt to put them in there as long as they are not touching each other or the opposite leaf. You could also fit them in your roof/ceiling, or a mix of all these options. The bottom line is, your inner box IS part of you inner leaf, and your outer box IS part of your outer leaf, so they need to remain decoupled just like the leaves themselves.

They also need to built from materials that have the same surface density as your walls/ceiling. It looks like you have a concrete cavity wall with rigid insulation between? In this case 1. you'll struggle to match the surface density for your silencer boxes and 2. you'll probably only need 1 in and 1 out since you don't really have a decoupled two leaf system. Well, you kind of do, but the rigid foam is confusing as it isn't really mass and yet it is also not "air" like fluffy insulation would be. You also have wall ties which will short circuit your decoupling.

In either case you'll only need one fan, usually on the outlet.

Sorry if I have completely misunderstood your drawings.

Paul

[John Steel]

Hello again Ush,
Good to hear that you've made some progress!

I know I want 300 FPM or less at the register , will the doubling of cross-sectional area in the baffle manage to bring the 669 FPM fan down to 300 FPM?

Conventional wisdom has it that 300 FPM is the absolute maximum velocity at the supply register. Ideally you'd want it much lower otherwise the turbulence will be audible.

Have I got the math right?

I followed it up to

127CFM divided by (cross-sectional area of the fan in SQUARE FEET) = FPM of the fan (air velocity)

Don't base a law suit on this, but I think if you are confident in your assumption that a 150mm fan and ducting is sufficient for your room (and I have come to a similar conclusion with my design where the room is roughly 40 cubic metres) then the outer leaf silencer on the supply side needs to be at least twice the cross sectional area of the ducting (which is roughly 28in sq or 180 cm sq ) so let's say 400 cm squared. Remember this is the dimension of the air space inside the silencer, not the outside dimension. To slow the air velocity further as it reaches the inner leaf on the supply side, the inner leaf silencer should have a CSA of 600 cm sq (or more) and ideally should vent into the room via a straight duct run no less than 90cm.

I'm basing 6 changes per hour on the size of the room 1270CFM or should I base it off the amount of the air required 5 people in the room 75 CFM.

I know Rod Gervais specifies 15 CFM per person (x5 = 75CFM or 4500 CF per hour) but I would go on room volume. With a vari-speed fan you will be able to adjust to suit occupancy and room conditions.

[Gregwor]

If your room is 1270 cubic feet, then:

The amount of air required for your room would be at least 1270 cubic ft * 6 air changes per hour / 60 min = 127 CFM. I really don't ever design HVAC based on how many people are in the room.

So, assuming you're doing a forced air system, where are you putting your air handler? If you're doing forced air like this, you'll have to have a separate room outside to house your furnace/air conditioning unit. I'm struggling to see where your doing this.

So, since I don't see any plans for an air handler "shed" or anything, let's assume you're going to go the more conspicuous route and you're going to use a ductless mini split unit. If this is the case, you can have smaller silencer boxes. This is because the ventilation only has to replace stale air with fresh air. So, let's say you need ~30% fresh air.

127 CFM * 30% = 38.1 CFM

You could drive this with a 4" round duct and something similar to this Fantech FG 4 inline fan:

Fantech FG 4.png

You can see that at an estimated 0.2 in wg, you are well within the operating range of the fan!

4” RD duct = 12.57 sq in. Double = 25.14 sq in.

So, the path inside your silencer boxes need to be at LEAST 25 sq inches. Your boxes only have 2 baffles. I would recommend at least 4 for your construction. Also, I recommend at LEAST 1" MDF to build your boxes. Honestly, I would probably build them out of 1" MDF and then add a layer of ~3/4" OSB on top. It's going to be tough to match the surface density of your building.

Greg

[Gregwor]

Oh, and for air velocity, if you have ~38 CFM and a 25 sq inch opening, that means your air velocity will be ~218 feet per min which is nice and slow

Greg

[Ushmurray]

Hi guys,

For some reason I wasn't getting email notifications and I thought no one was replying

It looks like you have all 4 of you silencer boxes inside your room?...

Hi Paul
Sorry that wasn't clear from the drawing. The drawing doesn't show the inner room at all . The baffles are in the roof between the leaves.
I didn't draw the interior room yet because if i need to change the size or move the baffles i'd have to redraw the interior room.
I'm new to sketchup and these things take me ages.

They also need to built from materials that have the same surface density as your walls/ceiling. It looks like you have a concrete cavity wall with rigid insulation between? In this case 1. you'll struggle to match the surface density for your silencer boxes and 2. you'll probably only need 1 in and 1 out since you don't really have a decoupled two leaf system. Well, you kind of do, but the rigid foam is confusing as it isn't really mass and yet it is also not "air" like fluffy insulation would be. You also have wall ties which will short circuit your decoupling.

I'm aiming to match the baffles mass to the interior room mass . There is a drawing in the opening post of the full construction , cavity wall - 50mm air gap - 100mm stud filled with acoustic insulation - resiliant clips and bar - 15mm acoustic drywall - 5mm tecsound -15mm acoustic drywall.

Don't base a law suit on this, but I think if you are confident in your assumption that a 150mm fan and ducting is sufficient for your room (and I have come to a similar conclusion with my design where the room is roughly 40 cubic metres)

Hi John
I had done a drawing based off a 200mm fan and the baffles were huge so I went with 150mm fan which makes the baffle size way more manageable and to see if it can work.

then the outer leaf silencer on the supply side needs to be at least twice the cross sectional area of the ducting (which is roughly 28in sq or 180 cm sq ) so let's say 400 cm squared. Remember this is the dimension of the air space inside the silencer, not the outside dimension. To slow the air velocity further as it reaches the inner leaf on the supply side, the inner leaf silencer should have a CSA of 600 cm sq (or more) and ideally should vent into the room via a straight duct run no less than 90cm.

I'll draw up silencers with these dimensions and see how they fit. Should it be the same setup for the return vent ?

So, since I don't see any plans for an air handler "shed" or anything, let's assume you're going to go the more conspicuous route and you're going to use a ductless mini split unit. If this is the case, you can have smaller silencer boxes. This is because the ventilation only has to replace stale air with fresh air. So, let's say you need ~30% fresh air.

Hi Greg,
Yes my plan is to install a ductless mini split unit. The vents in the drawing are only for fresh air.
Once i have the silencer problem fixed and I know where they're going to live in the room then i was going to look at where to install a ductless mini split systems.
From what i gather all i'd need for that is 30mm to 50mm hole through both leaves for coolant lines and drainage pipe, all cauked and sealed but no baffle system?

127 CFM * 30% = 38.1 CFM

You could drive this with a 4" round duct and something similar to this Fantech FG 4 inline fan:

Oh wow, now you talking

So, the path inside your silencer boxes need to be at LEAST 25 sq inches. Your boxes only have 2 baffles. I would recommend at least 4 for your construction. Also, I recommend at LEAST 1" MDF to build your boxes. Honestly, I would probably build them out of 1" MDF and then add a layer of ~3/4" OSB on top. It's going to be tough to match the surface density of your building.

At the moment my silencer boxes have a CSA of 58 sq inches, in theory you're saying I could make the CSA smaller but increase the build mass and add two more baffles per silencer. To me this sound very workable .

Oh, and for air velocity, if you have ~38 CFM and a 25 sq inch opening, that means your air velocity will be ~218 feet per min which is nice and slow

Music to my ears

I'm going to put together some drawings of both ideas .

Thanks guys, I really appreciate your help with this.

O

[John Steel]

This is because the ventilation only has to replace stale air with fresh air.

And the penny (for me) has finally dropped!! I can't tell you how long I've spent trying to get my head around the basics of HVAC and reading this exchange has led me to realise that if you go the ductless route you only need airflow to be 30% of room volume. I thank you both and raise my chapeau!

[Paulus87]

I'm just wondering... are you going to need the extra isolation that the inner room shell will gain you? You have a hell of a lot of mass going on with the slab, cavity wall and the perimeter walls. If you were to get the roof on and seal the whole building up (even if it's just temporarily) you could do some tests by getting a friend to bash some drums/play bass etc inside and see how it sounds at the locations where you need it to be quiet. Use a sound meter or app on your phone if you have a specific target that you need to reach.

If you find it's enough then it would save you time and money of course but you'd also gain a lot more acoustic volume which means you'll potentially have less modal issues and more room for treatment.

That's what I would do anyway.

Paul