John Sayers' Recording Studio Design Forum Archive

Hi Everyone, thanks in advance for reading, i have read lots on here over the past few days, and i have to say my mind is a little blown! so i may need things simplified somewhat, lol.

I am building a room in my back garden, we don't have space in the house.

The primary purpose for the room is to teach guitar and practice guitar/vocals.
I would also be using the space to write/record my own music.
In an ideal world i would love to be able to play acoustic drums in there. But from what i've read, am unsure if this will be possible with my structure.

Budget, as little as poss but can push the £2/3000 if it is going to really help.

The garden is on a slope, so we built the external structure on 6 concrete pads,

the floor is
dpc on the concrete blocks
Moisture barrier attached to
osb3 11mm underneath
2x6 joists spaced every 40 cm, with 2 noggins in each gap - joists are across the width, attached to the long pieces with joist hangers.
the space is filled with 2 lots of 75mm rockwool insulation
then i have glued and screwed a 22mm caberdek T&G flooring to the joists.

The walls are made from 2x4, spaced every 40 cm, around 2.20/30 in height, again with 2 noggins in each cavity
on the outside i glued and screwed 11mm osb3,
wrapped in breathable membrane,
25mmx50mm batten
then log lap cladding (22mm at thickest point)

inside the walls have been filled with Rockwool, i think a mixture of 75mm and 100 mm

the roof is slightly higher at the front, same construction as the stud wall, except no noggins, and osb on top is screwed not glued. again filled with knauf (ran out of rockwool) 100mm acoustic insulation.

The inner dimension are 402 x 244

i should point out that i have a upvc door and window in the front wall.

I played music inside the building at 75/80db
outside a meter away the reading was 55/60

I built a box out of left over osb, its airtight(ish)
with the speaker in the box and balancing on tennis balls
inside its 63/67
outside its 40/45

this was my very unscientific attempt at simulating a room within a room,
but is this accurate as to what i could hope to achieve?

that was with an app so may not be the most accurate. am going to purchase a proper reader today.

ideally i would like for that music to be unheard outside.

Neighbours are close, 5-10 meters away on all sides.

I was going to build another room inside this one, with as small an airgap as poss, filled with rockwool,resilient channel then 2 layers of plasterbord on the inside with maybe green glue or MLV (i already have some of this i picked up for next to nothing, but would need more) complete with second door/window.


how would this perform? would it be classed as a triple leaf?
Would my batten/cladding be classed as a leaf?
if so what problems would this cause?

Would another option be to just add as much mass as poss to the existing structure, eg 4 layers of plasterboard separated by greenglue/mlv, or would that not achieve the results of a room within a room.



What sort of performance would i get from res channels onto the existing frame then double plasterboard instead of building a separate structure?

with regards to the floor, i really have no idea what to do at the moment.

Thats probably too many stupid questions so my main question is -
With the knowledge of my external structure, what method would you suggest to best soundproof the room?

Thanks, Joe

Hi Joe, and welcome to the forum!

The garden is on a slope, so we built the external structure on 6 concrete pads,

How big is it? Dimensions are very important! And so are photos: please post a few photos of what you have there already, both inside and out.

Also, how are those concrete pads anchored? How deep do they go? How big are they?

floor is: osb3 11mm underneath ... 2x6 joists spaced every 40 cm, with 2 noggins in each gap - joists are across the width, attached to the long pieces with joist hangers. ... the space is filled with 2 lots of 75mm rockwool insulation then i have glued and screwed a 22mm caberdek T&G flooring to the joists

So just to check my understanding: You laid 2x6 joists 400mm OC on top of yoru concrete pads, with a single layer of 11mm OSB nailed to the bottom of those joists, and a single layer of 22mm flooring nailed to the top, and mineral wool in the cavity? IS that correct? If so, then you have a resonant box for your floor...

Also, how deep is the crawl-space under the floor? How did you manage to get in there to nail the 11mm OSB to the bottom of the joists?

Your floor is tuned to a resonant frequency of about 74 Hz, so it does not isolate at all until about 104 Hz, and only isolates decently above about 222 Hz. So playing drums in there is out of the question: the kick, toms and snare will be really loud outside, and on the cymbals and hi-hat will be reduced in level. In fact, chances are that the floor resonance will amplify the kick, since they are tuned to roughly the same frequencies. Bass guitar would also be very problematic: I doubt you'll get much more than about 25 dB of isolation, max.

The walls are made from 2x4, spaced every 40 cm, around 2.20/30 in height, again with 2 noggins in each cavity ... on the outside i glued and screwed 11mm osb3, wrapped in breathable membrane, 25mmx50mm batten then log lap cladding (22mm at thickest point

)If I understand that correctly, you have 2x4 studs 400mm OC with a single layer of 11mm OSB on the outside, followed by 25mm thick battens (laid flat), then your log siding, all of that on the outside of the wall. Is that correct? If so, that is also a coupled 2-leaf resonant system, and it is tuned to a resonant frequency of roughly 186 Hz, so it does not isolate at all until 262 Hz, and only isolates well for frequencies above 560 Hz. In other words, it does not isolate the low frequency regions of the spectrum, nor does it isolate half of the mid range. Even vocals will be audible through that. I'd estimate isolation at mabye 20 dB, or perhaps a bit more.

I played music inside the building at 75/80db ... outside a meter away the reading was 55/60

Yep, that's about what I would expect for that type of light-weight coupled construction.

with the speaker in the box and balancing on tennis balls ... inside its 63/67 ... outside its 40/45

That's also about what I would expect, and about what theory predicts: around 20 to 25 dB of isolation.

but is this accurate as to what i could hope to achieve?

Since your measurements do agree with what theory predicts, I'd say that it is reasonably accurate.

Regarding what you can hope to achieve: If you were to build a proper fully-decoupled 2-leaf MSM isolation system, you could expect the isolation to be at least one thousand times better than what you are getting right now, and perhaps up to ten thousand times as good. In other words, a properly constructed studio should block about a thousand times more sound energy that what yours is blocking. You could expect about 50 to 60 dB of isolation from such a studio.

that was with an app so may not be the most accurate. am going to purchase a proper reader today.

Right! Smart phone apps are not, and never can be, very accurate. Get yourself a good quality sound level meter. It will cost you around US$ 100, more or less. But do NOT get one of the cheap Chinese "toys" that are appearing lately! They sell for around US$ 25 to US$ 50, but they are useless. Get a recongized quality brand, such as Extech or even Galaxy.

ideally i would like for that music to be unheard outside.

That can be achieved, but sadly not with what you have right now.

I was going to build another room inside this one, with as small an airgap as poss, filled with rockwool, resilient channel then 2 layers of plasterbord on the inside

In theory that would help, but in practice you can't do that. Your floor joists are only 2x6, and they will not be able to support all of that extra mass safely. Sorry. You'd be adding a coupe of thousand pounds of mass in dead load, plus whatever live load will be in there, and there's no way that 2x6 joists can handle that.

with maybe green glue or MLV

Forget the MLV: It is just very expensive mass. Even if you could use it (which you can't, due the structural overload I mentioned above), it won't do anything for you that you could not accomplish for much less money using other materials.

how would this perform? would it be classed as a triple leaf?

Yes it would, and the low frequency isolation would suffer as a result. However, considering that your low frequency isolation is non-existent at present, it would still give you a large improvement over what you have now. Theoretically, you would probably get up to around 40-something dB of isolation like that. If only the floor were able to handle the load...

Would my batten/cladding be classed as a leaf?

Yes. It is a large flat, rigid, solid, massive surface over an air gap, so yes, it is a leaf.

if so what problems would this cause?

Reduced low frequency isolation. In other words, it would not be as good as it could be if you were to have only two leaves in your wall.

Would another option be to just add as much mass as poss to the existing structure, eg 4 layers of plasterboard separated by greenglue/mlv, or would that not achieve the results of a room within a room.

That would not create a "room in a room", no. Because the existing leaves are fully coupled by the studs, floor, and roof. So even though it is a 2-leaf system, it is fully coupled, and therefore acts more in accordance with mass law than it does in accordance with MSM equations. Adding another 4 layers of plasterboard would increase the total mass to about 300% of what it is now, so you would get an increase of maybe 6 or 7 dB in isolation: In other words, you'd end up with a total of maybe 27 dB isolation.

What sort of performance would i get from res channels onto the existing frame then double plasterboard instead of building a separate structure?

In theory, about 15 dB better than what you have now, so maybe 35 dB or so.

with regards to the floor, i really have no idea what to do at the moment.

It needs to be beefed up drastically, but in terms of structural members and also in terms of mass. I would think about using something like 2x10s for your joists (but do not take my word for it! Call in a structural engineer to look at what you have, look at what you need to do, then he will tell you the correct size joists), then put down two or three layers of very thick OSB on top for the sub-floor. I would then put down 4" of OC-703, put a couple of layers of plywood over that, then whatever finish flooring you'd like on top. (I'd suggest laminate flooring on a suitable underlay.)

For the underside of the floor, I would just leave it open. Fill it with insulation, yes, but just use strapping or cleats of some type, or maybe battens to keep it in place. And of course plastic to keep the water out. If that is not allowed by code, then I'd put RC on the bottom of the joists and a layer of thick OSB.

With the knowledge of my external structure, what method would you suggest to best soundproof the room?

Beef up the existing second leaf from the inside, between the studs, by cutting pieces of thick plasterboard (16mm) to shape and pressing them in place between the studs, up against the second leaf, then caulking around the edges with acoustic caulk, and holding it in place with cleats. Do two layers like that (the cleats go on the second layer, not the first layer).

After you do that, and after you beef up the floor, once you have the new subfloor in place but before you put the OC-703 and plywood in for the inner-leaf floor, build your new inner-leaf wall framing about 2 cm away from the existing framing, fill the cavity with acoustically suitable insulation, then put two layers of 16mm plasterboard on that new frame, with GG in between. Then put your new floor down, inside of that area (the OC-703 plus OSB, as described above).

Then build your new inner-leaf ceiling frame on top of those new inner-leaf walls, in the same way as for the walls, using suitable dimensions for your joists, and with two layers of 16mm plasterboard, and also with suitable insulation in the cavity.

Make sure that both leaves are sealed absolutely airtight.

That would give you around 50 dB of isolation, if you build it very carefully. That's about as good as you can expect. I'm not sure that it would be good enough for drums played at 3:00 Am, but it would certainly be fine for typical rehearsal / practice / tracking / mixing during the day. That's about what most home-studios can achieve, when designed and built correctly.

- Stuart -

Soundman2020 wrote:How big is it? Dimensions are very important! And so are photos: please post a few photos of what you have there already, both inside and out.

Also, how are those concrete pads anchored? How deep do they go? How big are they?

it is 4.3m wide x 2.50 high
depth is 2.7m x 2.50 high sloping back to 2.4

Photos are comming just need to convert them

Pads are concrete building blocks, led down on the big side (opposite to how you would build a wall)
there are 2 in each column, at the back they are about 6 high, at the front they are 2 high.

the floor joists run across the short length, 2.7m, and are supported in the corners, and front and back middle section, by the concrete blocks.

Not sure why the pic has come out the wrong angle!

I have been told that this would be fine with the added weight, but do you think differently?

Soundman2020 wrote:Also, how deep is the crawl-space under the floor? How did you manage to get in there to nail the 11mm OSB to the bottom of the joists?

we put it on before lowering the floor, it was upright against the fence and we put it on and slowly lowered it into place.
my reason for having it was to stop anything getting in there to live, bugs, small mammals etc

Soundman2020 wrote:If so, that is also a coupled 2-leaf resonant system, and it is tuned to a resonant frequency of roughly 186 Hz, so it does not isolate at all until 262 Hz, and only isolates well for frequencies above 560 Hz. In other words, it does not isolate the low frequency regions of the spectrum, nor does it isolate half of the mid range. Even vocals will be audible through that. I'd estimate isolation at mabye 20 dB, or perhaps a bit more.

this is because i finished the building by putting loglap cladding onto battens?
the reason for this was to allow air behind the loglap to dry out.
Is there a different way of finishing the building? maybe putting the loglap direct onto the osb?

Soundman2020 wrote:I played music inside the building at 75/80db ... outside a meter away the reading was 55/60
Yep, that's about what I would expect for that type of light-weight coupled construction.

Quote:
with the speaker in the box and balancing on tennis balls ... inside its 63/67 ... outside its 40/45
That's also about what I would expect, and about what theory predicts: around 20 to 25 dB of isolation.

the speaker was producing around 80db, putting it in the box on tennis balls and going outside, the measurement was 40/45, thats a reduction of 30/35 db, not 20/25 is it not?


how would one go about creating a timber floor like this without the problems mine will have?

Thanks again

theres alot to think about here so i want to get it right before carrying on with the build.

Also just wanted to say thanks for your response. thought id written it at the top but didn't, Thanks

Hi, could anyone tell me their thought on the following please?

i need to beef up my existing external wall from the inside by cutting plaster board and fitting it in the stud cavities.
i was going to put two layers of 15mm, however it works out cheaper to put three layers of 12.5 in,
it would also add more weight, per each 240x120 area, the triple layer would be 12kg heavier, and overall would cost £30 less.

i've opted away from the acoustic plasterboard as it seems to cost a lot more.

any advice would be great as i am buying it tomorrow.

Thanks, Joe

I would go with 2 layers of 15mm. The 12mm stuff is thinner, more flexible, and has higher self-resonance frequency, plus the extra mass you get isn't huge: maybe 20%, and the small amount of money you save on drywall you will certainly lose on labor, time, caulk and possibly Green Glue (assuming you are doing that).

2 layers of 15mm is a tried and tested method, well documented, and know to work well. Three layers of thinner stuff.... not so much!

But I certainly do agree that you don't need the special "acoustic" stuff: Lots of extra money for very little demonstrable benefit.

However!!! Did you check with a structural engineer to make sure your existing structure can handle the extra load? You'll be adding many hundreds of KG of mass to that flimsy floor... About the equivalent of parking a car on it... (and that's just for beefing up the existing outer leaf, without even considering the inner leaf that you need to build too...)

Not a good idea to do that unless you have signed clearance to do so, from a certified engineer...


- Stuart -

Pads are concrete building blocks, led down on the big side (opposite to how you would build a wall)
there are 2 in each column, at the back they are about 6 high, at the front they are 2 high.

OK, but how are they ANCHORED? Into the ground, I mean. You need foundations for what you are trying to do: you can't just sit a massive building on top of a couple of cement blocks that are laid flat on the ground! That might be fine for a shed, but NOT fine for a studio. Probably not even legal...

the floor joists run across the short length, 2.7m, and are supported in the corners, and front and back middle section

What dimensions are those, and what distance are longer ones spanning, unsupported?

I have been told that this would be fine with the added weight, but do you think differently?

Who told you that? Was he/she a qualified and locally certified structural engineer? Did he do the calculations? Is he aware of the huge amount of mass you will be adding to that frame? Is he aware that there are no foundations or other anchors? Did you get it in writing? I'm not sure what type of home insurance you have, or how things work legally where you live, but in most places in the world if you build that without getting a professional engineer to sign off on it, and then something goes wrong, not only will your insurance refuse to pay, but you're also be financially liable for all of the damages that you cause to the property of others (plus your own) and criminally liable for any injuries or deaths. The incident doesn't even have to be related to your studio: for example, maybe your house catches fire and damages the house next door too, as well as injuring some folks. Even though it had nothing to do with your studio, you can bet your bottom dollar the insurance company would point out that your entire property is not built to code, and therefore would refuse to pay a single cent.

we put it on before lowering the floor, it was upright against the fence and we put it on and slowly lowered it into place.

So you are saying that your joists are not actually resting on the concrete pads? There is OSB between the joists and the pad? So the entire weight of the building is actually crushing the OSB? Is that even legal where you are? Did you check the compressive strength of the type of OSB you used, and is it able to handle that? Did a structural engineer check it and approve it?

this is because i finished the building by putting loglap cladding onto battens?

Correct. That's what makes it a third leaf, and that's why your low frequency isolation will suffer.

the speaker was producing around 80db, putting it in the box on tennis balls and going outside, the measurement was 40/45,

That doesn't make any sense: first you say the level was "inside its 63/67" but now you say that inside the level was 80 dB? So which was it? Were you measuring with "A" weighting or with "C" weighting? Slow or fast response? How did you hold the meter while you were measuring? All of that is critically important.

A measurement outside of 40 dBC also seems unlikely in a residential area....

thats a reduction of 30/35 db, not 20/25 is it not?

I'd find it rather hard to believe that you are getting 35 dB of isolation from that building in it's current state...

Besides, ig you did the testing at 80 dBC, 1m, then that's way too low. Since you are talking about levels of between 10 and 1000 times that (you did say "acoustic guitars, vocals and drums"), that's the levels you should do the tests at. There are very likely resonant modes that would never be triggered by such a low level of 80 dB, and certainly would be triggered by 110 dB. I'd suggest that you read up on how to conduct a measurement correctly with a sound level meter, then repeat the test at much higher levels.

how would one go about creating a timber floor like this without the problems mine will have?

You'd need a lot more mass on top (maybe three layers of 22mm plywood or MDF), and without the third leaf below. Use wire mesh instead of OSB to keep the critters out. Then build a "drum riser" type floor within the inner-leaf, but not touching it, also with at least two layers of thick MDF.

Also just wanted to say thanks for your response. thought id written it at the top but didn't, Thanks

That's what we are here for!

... i am buying it tomorrow.

I really wouldn't do that!!! It seems you are far from ready to be building anything, until you sort out the existing issues. I'd put the money on fixed deposit, while you first get that in order, and actually design the place correctly.


- Stuart -

Soundman2020 wrote:OK, but how are they ANCHORED? Into the ground, I mean. You need foundations for what you are trying to do: you can't just sit a massive building on top of a couple of cement blocks that are laid flat on the ground! That might be fine for a shed, but NOT fine for a studio. Probably not even legal...

each pad has its own foundation, its 'anochored to the ground' using cement.

Soundman2020 wrote:What dimensions are those, and what distance are longer ones spanning, unsupported?

they are 2x6. the longest unsupported distance is 2.7m

Soundman2020 wrote:Who told you that? Was he/she a qualified and locally certified structural engineer? Did he do the calculations? Is he aware of the huge amount of mass you will be adding to that frame? Is he aware that there are no foundations or other anchors? Did you get it in writing? I'm not sure what type of home insurance you have, or how things work legally where you live, but in most places in the world if you build that without getting a professional engineer to sign off on it, and then something goes wrong, not only will your insurance refuse to pay, but you're also be financially liable for all of the damages that you cause to the property of others (plus your own) and criminally liable for any injuries or deaths. The incident doesn't even have to be related to your studio: for example, maybe your house catches fire and damages the house next door too, as well as injuring some folks. Even though it had nothing to do with your studio, you can bet your bottom dollar the insurance company would point out that your entire property is not built to code, and therefore would refuse to pay a single cent.

My father told me it would be fine if we put a lintel underneath supporting the middle of the floor.
he has an engineering background. yes he's well aware of the added weight, he worked out how much extra weight it would be and did the necessary calculations.

Building regs aren't needed for out buildings like this as its not a 'liveable' space, ie it doesn't have sleeping quarters.

Soundman2020 wrote:That doesn't make any sense: first you say the level was "inside its 63/67" but now you say that inside the level was 80 dB? So which was it? Were you measuring with "A" weighting or with "C" weighting? Slow or fast response? How did you hold the meter while you were measuring? All of that is critically important.

A measurement outside of 40 dBC also seems unlikely in a residential area....

it does make sense, inside the room, with the speaker playing music at 80db,
in the garden the measurment is around 60db - so 20db less

back in the room with my box over the speaker, the measurment in the room is down by roughly 20db

then when you go outside, the music is reduced by a further 10db - which is 30db less than the origional 80db

so thats 80 down to 50db.

to be clear , the box i'm talking about is my simulation of a room within a room, it fits over the speaker, and is built with the same material i would use for the second room.

so yes the main shell i have built gives about 20db attenuation, however but with the box 'room within a room'
it gives 30db total when standing outside.

hope that makes sense.

Soundman2020 wrote:'d find it rather hard to believe that you are getting 35 dB of isolation from that building in it's current state...

Besides, ig you did the testing at 80 dBC, 1m, then that's way too low. Since you are talking about levels of between 10 and 1000 times that (you did say "acoustic guitars, vocals and drums"), that's the levels you should do the tests at. There are very likely resonant modes that would never be triggered by such a low level of 80 dB, and certainly would be triggered by 110 dB. I'd suggest that you read up on how to conduct a measurement correctly with a sound level meter, then repeat the test at much higher levels.

i have tested up to 100db mark, playing different music, as well as vocals.

all the results give roughly the same attenuation,


You'd need a lot more mass on top (maybe three layers of 22mm plywood or MDF), and without the third leaf below. Use wire mesh instead of OSB to keep the critters out. Then build a "drum riser" type floor within the inner-leaf, but not touching it, also with at least two layers of thick MDF.

currently the floor only has 2 leafs not three, from inside out, layer of T&G flooring, cavity filled with insulation, osb, thats 2 layers of mass therefore 2 leafs right?

Soundman2020 wrote: for example, maybe your house catches fire and damages the house next door too, as well as injuring some folks. Even though it had nothing to do with your studio, you can bet your bottom dollar the insurance company would point out that your entire property is not built to code, and therefore would refuse to pay a single cent.

thats like a car insurance company tring to squirm out of paying because the passenger in the car was on the phone.
Really dont know how this would ever happen as the building isnt attatched to the house and isn't used for sleeping in therefore doesnt require to be built to code or to be signed off.

Thanks, Joe

each pad has its own foundation, its 'anochored to the ground' using cement.

That's good to know! How deep do they go? And what is the rated load that each footing can support? My guess is that they were designed for the weight of the basic shed, and do not take into account the very much heavier weight you'll be putting on them. If you overload them, they will sink slightly, and very likely will sink at different rates, thus warping your building.

Here too you need a qualified and certified structural engineer to take a look and tell you how much extra weight you can safely put on them.

they are 2x6. the longest unsupported distance is 2.7m

I have a feeling you are going to be overloading those: Assuming you the entire building will end up at about 2000 kg total, the supported area is about 10 m2, so the load is about 40 kg/m2. 2x6 floor joists can't handle that safely. You'd need at least 2x8s for that (I'm assuming L/720, which is what you need for a studio).

My father told me it would be fine if we put a lintel underneath supporting the middle of the floor.
he has an engineering background. yes he's well aware of the added weight, he worked out how much extra weight it would be and did the necessary calculations.

So how much total weight did he come up with? That's the key: And how did he calculate that, if we don't yet know how much isolation you need (in decibels), and what frequencies you need it at?

Building regs aren't needed for out buildings like this as its not a 'liveable' space, ie it doesn't have sleeping quarters.

Are you SURE about that? Livable space, or habitable space, does not necessarily imply that beds are involved... office, schools, shops, churches are all classified as habitable space, and they don't have sleeping quarters... The international building code defines "habitable space" as: "A space in a building for living, sleeping, eating or cooking. Bathrooms, toilet rooms, closets, halls, storage or utility spaces and similar areas are not considered habitable spaces." Since your studio is most certainly a "living" room, it would be classified as "habitable space". Additional regulations make it clear that this definition includes basements, play rooms, dining rooms, lounges, TV rooms, home offices, and all similar spaces.

Also, I found this regulation which contradicts what you said about no building regulations being applicable: "Even if you do not need planning permission for your extension, because you are using permitted development rights, you must get building regulation approval. The Building Regulations set out minimum requirements for structural integrity, fire safety, energy efficiency, damp proofing, ventilation and other key aspects that ensure a building is safe." That's for London, but I expect Bristol would be the same, since most codes in the UK are uniform.

At the very least you will have to submit something called a "Building Notice" to your "local authority building control depar­­tment". The regulation then goes on to say "With a Building Notice, the building control officers can ask for proof of compliance at any stage, so it is essential to make sure they make all necessary inspec­tions and provide any structural calcula­tions when requested."

It seems to me that you are assuming an awful lot of things that are turning out to not be correct. I would highly recommend that you hire an architect or contractor to walk you through the regulations, structural issues, legal liabilities, and other things that you seem to have not considered. I found all of the above with a few minutes of Google searching, and there might well be more...

inside the room, with the speaker playing music at 80db, in the garden the measurement is around 60db - so 20db less

Then that is what you are getting right now! And that is all that matters. Period.

back in the room with my box over the speaker, the measurment in the room is down by roughly 20db ... then when you go outside, the music is reduced by a further 10db - which is 30db less than the origional 80db ... so thats 80 down to 50db. ... to be clear , the box i'm talking about is my simulation of a room within a room, it fits over the speaker, and is built with the same material i would use for the second room.

Sorry, but that is NOT a valid test at all! Not even vaguely valid. It is meaningless. Using the "same materials" is absolutely irrelevant, and does not represent anything at all about how your inner-leaf would be.

Acoustic calculation of isolation is rather complex, and includes things like the resilience of the walls, which in turn depend on their dimensions, how they are supported, what damping materials is used (in your case: none at all), and a large number of other factors. For example, one of the key factors in calculating isolation is the "cavity depth" (sometimes also called the "air gap"), which is the distance between the surface of the outer leaf and the surface of the inner leaf. In a typical wall (such as yours would be), that distance will be a few centimeters, probably around 10cm to 20cm. But with your "test" it was several METERS! That distance defines the resonant frequency of the wall system, which is the most important factor for calculating the total isolation. With a very large distance (such as you had with your "box"), the frequency will be very, very low, but in reality the frequency will be rather high when you actually build the wall, since the cavity depth will just be a small fraction of the distance you used. So your "test" actually simulated the conditions for a totally different situation, at best.

Then there's the issue of the coincidence dip, stiffness, the mass-law region, and all the other factors involved in isolation.

The "test" you did with that box means nothing at all. The only thing you succeeded in proving is that if you reduce the level by 20 dB inside the room, you only get a 10 dB reduction outside, which is a very, very suspicious result anyway! It indicates that something else was happening during the test, that you were not aware of. My guess is that you did not take the measurements correctly. But there is no point in repeating the tests, since the basic concept is totally invalid.

so yes the main shell i have built gives about 20db attenuation, however but with the box 'room within a room'
it gives 30db total when standing outside.

The conclusion is invalid, because the test was invalid, because the concept was invalid.

Also, you seem to be misunderstanding how building isolation works. There is no such thing as a "main shell" and a "second room". Rather, you have an "outer leaf wall" and an "inner leaf wall", and they act together, as a tuned system. They do not act independently. You cannot just add the isolation given by one wall to the isolation given by the other wall. You have to calculate the resonance of the wall system as a whole, and consider the four different regions of the spectrum, which are governed by four different aspects of the wall, as shown here:
As you can see, for the region below the resonant frequency of the wall, it is the stiffness of the panel that controls the amount of isolation. For frequencies near the resonant frequency, it is the resonance itself that controls how well the wall isolates. At slightly higher frequencies, mass law takes over, then comes the coincidence dip, which once again is controlled by resonance (but of a different type), then finally the region which is controlled by how shear waves move through the wall.

Acoustic isolation is very, very complex. You cannot just look at the individual parts of a wall, add them all up, and suppose that the complete wall will react like the sum of its parts. It does not. It is a tuned resonant system, and acts all together as one single unit, with multiple resonances.

i have tested up to 100db mark, playing different music, as well as vocals. ... all the results give roughly the same attenuation,

Then you did something wrong in the way you measured! There's no doubt about that. The building itself will have several resonant modes, many of which will not be excited at 60 dB, but pretty much all of them would be at 100 dB, which is ten thousand times the intensity! The fact that you did not detect this, shows that you were not testing correctly. The differences should have been very obvious. By guess is that you were not using a suitable speaker, your test sweeps did not cover the full spectrum, and your measurement technique was not correct.

currently the floor only has 2 leafs not three, from inside out, layer of T&G flooring, cavity filled with insulation, osb, thats 2 layers of mass therefore 2 leafs right?

Yes, and currently is is tuned to a resonant frequency of about 95 Hz, meaning it does not isolate until about 190 Hz, and only isolates well above about 280 Hz. Which is part of the reason you are seeing such poor isolation. Adding a third leaf to that will RAISE the resonant frequency even higher, thus REDUCING the isolation for the low frequency end of the spectrum: That's not intuitive, and most people don't expect that to happen, but it does, because that's the way acoustics works.

thats like a car insurance company tring to squirm out of paying because the passenger in the car was on the phone. Really dont know how this would ever happen as the building isnt attatched to the house and isn't used for sleeping in therefore doesnt require to be built to code or to be signed off.

You are wrong about that: as I mentioned before, "habitable space" has nothing at all to do with sleeping. It has to do with rooms which people "inhabit", meaning they occupy the room for long periods of time (more than just a few minutes at at time). Bathrooms, halls, closets and storage space are not "habitable", because people go into those for very short periods, them come out again. A studio is a place you go into, and stay for a while. It is, most certainly, habitable space.

It seems to me that you are not taking this project anywhere near as seriously as you should. There are major structural questions here, there are regulatory and legal issues, there's a lack of understanding of how sound works, there are issues of understand construction materials and techniques, and a general lack of planning. I would really suggest you should slow down a bit, do some research on what regulations actually say (not what you think they say, or hope they say), do some research to understand the basic principles of acoustics and construction, and hire the correct qualified and certified people to do what needs to be done to make sure your studio will be both safe and legal.

That would be my advice.

Sorry to be so harsh, but I'm hoping that this will give you the urge to do this right, instead of just guessing and hoping it will work out. I'd hate to read about you in the papers in a few months, where a garden shed collapsed and injured people because it suffered from structural overload! I'd much rather read about you in the newspapers because someone was impressed by the great little backyard studio you built, and how solid it was, and what a great sound they heard inside!


- Stuart -