Originally, there was just a layer of plywood (around 30mm thick) for each wall,
OK, great, but what is beyond that? What's the entire sequence, from the rooms on the other side of your walls? You say that there was already plywood on there, implying that there is some structure behind that which holds the plywood up! What is that structure? Wood framing? Metal framing? Magical incantations? Assuming it is some type of framing, that also implies there is something else attached to the other side of the framing: Drywall (plasterboard)? Plywood? MDF? OSB? Stucco? The cast-off shells of gnomes, fairies, pixies and elves?
I'm assuming here, but if you have a stud framed wall with drywall on the far side and plywood on your side, then you already have a two leaf wall! Anything additional you put in front of that will make it either a three-leaf wall, with the resulting potential penalty in low frequency isolation, or even worse, a four-leaf wall, with the even larger reduction in isolation.
The plan was to build a two leaf, double wall system with layers of acoustic plasterboards (15mm) on each leaf. Mass – air – mass
Not if you already have two leaves in place! In that case, adding two MORE leaves, will result in a four-leaf system: Mass - Air - Mass - Air - Mass - Air - Mass.
In simple, visual terms, you started out with the situation on the left or second-from-left in the diagram below:
2-leaf-3-leaf-classic-walls-diagram-MSM-walls.gif
In either case, you had a typical house wall, with roughly 30 dB of isolation (Yeah, the labels are in STC numbers, not dB, unfortunately, so they are not an accurate indicator of isolation, but they still serve admirably for this purpose).
So let's say you starter out with the second situation: A stud framed wall with sheathing on each side. You then built a second stud-framed wall next to that, also with sheathing on both sides, expecting that you would get a massive boost in isolation. But since you created a four-leaf system, with the notoriously poor low-frequency isolation that it provides, all that you got was the situation in the third diagram, labeled "STC-40". In real, world terms, your isolation in creased by maybe 3 to 6 dB: You get STC-40, instead of the STC-36 that you already had. A lot of work, and money, for very little return.
If you would have built your new wall with only ONE leaf on it, instead of the two you have, then you would have gotten the situation depicted in the fourth diagram: an increase of 14 STC points, instead of just 4! That's huge. Probably around 10 to 16 dB, in real-world terms. In other words, you would have produces a three-leaf system, which is still not good, but still far better than a four-leaf. By doing that you would be leaving out one quarter of the mass, and getting an improvement of about double the isolation, subjectively. This is not intuitive at all, but it is, in fact, the way sound behaves in resonant systems.
And if you would have first taken off the plywood from the existing wall before building the new wall, then you would have the situation in the fifth diagram, labeled STC-57, which is a true two-leaf system, thus providing optimal isolation for the least use of materials, and the least cost. That one shows a 21 point jump in STC rating, with respect to the original wall, instead of the 4 you actually get with the third diagram. In real world terms, that's probably around 18 to 23 dB increase in isolation. In other words, that wall is about four times better, subjectively than a four leaf wall, even though it only has half the mass!
Not intuitive, not "logical", but a clear demonstration of the power of resonant systems. 50% reduction in mass, produces a 200% increase in isolation.
Finally, if you would have used the layers of mass that you took out of those other two leaves, to beef up the only two leaves that you need, then the result would have been yet another jump in isolation: Probably about 6 dB, give or take. For a total increase of around 25 to 30 dB, with respect to the original wall. Instead of 3 or 4.
The first part consisted in sealing all gaps and perimeters of the room with acoustic sealant.
Then to add three layers of plasterboard to each wall/ceiling. We used green glue for each layer
That sounds suspiciously like you used the Green Glue as though it were an adhesive? Is that it? Or did you also nail through the drywall into the studs, in the normal way?
We used green glue for each layer and tecsound 50 in the left and back walls.
Not sure I understand? Why would you use both, and how would you even do that? If you already used "green glue for each layer", then where would you put the Tecsound 50 membrane? And why would you even do that? Do you have accesses to some study done in a reputable acoustic laboratory that shows how such a "sandwich" performs?¡ How can two products that essentially do the same thing (constrained layer damping) work together, when neither is being used as designed? Personally, I would expect that combining a visco-elastc polymer that never hardens with a with visco-elastic membrane, would not do anything useful, and would likely degrade the performance...
For the back wall (thick brickwall that leads to the street) we left it as it was.
You DID paint it with a masonry sealant, right? Brick and mortar are very porous. Without that sealant, you lose a lot of isolation...
... built a wooden frame around the room, with an air gap of 25mm from current boundaries. That 25mm is the air gap between walls and ceiling.
No it is not: Assuming you used standard 2x4 wood framing, then your air gap is 114mm. THe depth of standard studs is 89mm, plus the distance from the face of that framing to the face of the final layer of drywall that you added to the original wall (25mm). So you have an air gap of 114 mm.
The framework was filled with rock wool 60 Kg m3
Just checking: So you have filled the entire air cavity with mineral wool, to a total thickness of 114 mm? In other words, the 89mm between the framing members, plus the 25mm up against the drywall that you put in the original wall?
We used those two doors...plus the original door! I think maybe that was a mistake.
OK, now this is a myster, for sure! How on earth can you open the middle door???? And what on earth did you attach it to???? You have one door on the original outer leaf, firmly attached to the outer-leaf framing, that door opens outwards (away form the studio). Then you have your second door on the inner-leaf, firmly attached to the inner-leaf framing, and that one opens inwards (into the studio). So how did you manage to get a third door in between those, and how can it possibly even open? I'm intrigued....
(and yes, assuming that you did managed to do that in some magical way, it was a mistake, since it creates a three-leaf system...)
Instead we added mass to the original door
"Original door"???? What "original door"???? You didn't mention that. So now you have FOUR doors in there? Another four-leaf system? And once again: what did you attach the other two doors to, and how in heaven's name could you even open them! Pictures please! Either you are not explaining at all well what you really did, or you have worked construction magic by hanging doors in places where they cannot possibly fit, and maing them open into spaces that are much smaller than the size of the door... Maybe your studio is called the Tardis, and the doorways are "bigger on the inside"?
I didn’t check much at that moment to be honest. I didn't know much about it and I was so concerned about maximising the balance between soundproofing/isolation and space that didn’t check much.
So you built a studio without first checking if it can even work as a studio? Ummm.... You should probably read up on room modes, standing waves, SBIR, comb filtering, flutter echo, reflections, reverberation, and such-like BEFORE you start treating the room, because you WILL need to take those into account for the treatment, seeing that you did not do so for the original design!
I was thinking that, the 70 cm corner could be a big bass trap.
Yes it can be, but it cannot also be a bookcase! Either it cam be a bookcase, or it can be a bass trap, but it cannot be both. If you want it to be a bass trap, then the bookcase has to come out.
I want to prioritise the sound quality of the room and then build the rest of the room according to that
. Start by downloading a copy of the ITU document BS.1116-2, and read through that, to understand what your final goals must be for the room acoustics, in terms of layout, frequency response, modal response, decay rates, etc. Then read "Master Handbook of Acoustics" by F. Alton Everest (that's sort of the Bible for acoustics) to find out how the theory behind how to do all of that.
What I think I didn’t set correctly is the listening spot, the distance between the speakers and walls, and speakers and microphone, which I believe is crucial as that is the the exact point to be considered. I wasn’t sure on how to place the speakers and the microphone correctly, and also, where to consider the measuring point on the speakers
Speakers go on stands: Very heavy, massive stands, behind the desk, tight up against the front wall, except for a gap of 100mm, where you will need to insert a panel of OC-703. The height of the stands much be such that the acoustic axis of your speakers is 120cm above the floor (or perhaps a little higher, under certain circumstances. Check with the manufacturer of your speakers to find out where the acoustic axis is located on the front baffle. For a 2-way speaker, it will be on the imaginary line joining the center of the woofer to the center of the tweeter, and much closer to the tweeter, but NOT at the center of the tweeter.
Your speakers in your room need be positioned 67cm from the side walls (meaning they will be 110 cm apart. Your mix position (location of your ears, and therefore also of your mic) will be about 155 cm from the front wall. Set up the speakers so they are aimed at a point 30cm behind your ears, in other words 185 cm from the front wall. Adjust the angles of the speakers such that the acoustic axis of both is aimed at that point. Set up a vertical pole (eg, mic stand) at that spot, and aim both speakers at it.
That's the theoretical optimal layout, but it should be possible to improve on it, with careful adjustments.
Now set up your measurement mic so that it is in the center of where your head will be, pointing forwards (between the speakers) but tilted up at an angle of about 60°. Calibrate REW like that, at 80 dBC for each individual speaker by itself, which implies that the level will automatically be 86 dBC with both speakers on. Do three REW tests like that: One with just the Left speaker, one with just the Right speaker, and one with both. Do not adjust anything in between tests. Leave the room when you run the tests (use "Star delay".
For the speakers and microphone distances I aimed for an equilateral triangle,
That setup is still perpetuated all over the place, but is incorrect. It has the speakers aimed at your eyeballs, bit your ears.... It only works for people who have had their ears swapped with their eyes, surgically...
around 30 cm from the left and right walls and 120 cm from the floor. Which gave me 90 cm between speakers
That makes no sense! You say the room is 244 cm wide, but your numbers add up to 150 cm! LEFT WALL - 30 cm - LEFT SPEAKER - 90 cm - RIGHT SPEAKER - 30 cm - RIGHT WALL = 150 cm. So what happened to the other 94 cm???? Either the room is not actually 244 wide (in which case all of the layout figures I gave you above are no use), or there's an error in your numbers.
It measures 84 DB for both speakers when each of them measured 80 DB separately. I think it must be related to an incorrect placement of the speakers and microphone.
There's something wrong there. If you have coherent sound playing through two sound sources, then both of them together will give you exactly 86 dBC if each of them individually is giving 80 dBC. The only reasons that you'd get less than 86 is due to setup problems, speaker problems (not in phase), or signal chain problems (EQ, phase, dynamics, delay, etc. applied when they should not be).
Attachments
I just noticed that there's a pole in front of the bookcase... What is that? It will have to be removed, along with the bookcase.
Attached are the new REW file with the new measurements.
You have two "L+R" measurements in there, but they are different. What happened in between? What did you change?
There's also a very large difference between the individual L and R measurements, implying that the room is not symmetrical. That's sort of understandable for the low end, but there's also large differences in the mids and even the highs. Something very asymmetrical about that room...
- Stuart -
that would place you among the loudest sound sources on the planet! Louder even than this speaker:
I just tried to download your MDAT, but all I got is a message saying the file is not there any more. Please upload it again, and I'll take a look.
If not, then I can only conclude that a lot of "guesstimating" went on....
