Hi Bivis, and Welcome to the forum!
It's great that you have a place you can set up as your studio! Hopefully, here on the forum you'll find all the information and help that you need to make your dream int reality.
solid build, concrete & steel,
That's a good start, for acoustic isolation. High mass and high density are always good for that.
ceiling 3,5m high concrete
Excellent. A high ceiling is also good, acoustically.
Overlooking a not so busy street, but natural sunlight i dont want to giveaway
That's fine. You can certainly have glass in studios and still have good isolation.
other three walls - 2xPB _light steel framing with fluffy insulation_2xPB (together 13cm).
That's the weakest part of your structure, acoustically. That is going to need some work, to get good isolation. Fortunately, it isn't difficult to do what you wold need to do there.
normal glass 4mm-16mm-4mm.
I assume you mean they are "double-glazed" ("IGU") panels, with two glass panes of 4mm thickness, separated by an air gap of 16mm? That's not so good, and might need replacing if you need high isolation.
the main goal is to mute the sounds of scooters passing by, and ambulance from the main street which is about 100 meters away
It would be a good idea to measure the sound levels of those sounds, at their loudest levels. Have your meter ready, and each time you hear an ambulance coming close, open the window and measure the level. Note down a few of those, so you have a record of how loud it can get. The same for the traffic sounds, and any other external sounds (eg, planes or helicopters flying over occasionally, wind, rain, thunder, hail, loud music from car radios/ neighbors, dogs barking, etc.) Any type of occasional, intermittent sound should be on your list, so you have a good idea of what is "out there". Also note what the predominant frequency is, for each of those readings.
the noise floor with windows closed is about 40db (c weighed) and want to drop down to 33db.
Did you take that measurement at the time the "loud things" were happening outside, or at a very quiet time? You say "noise floor", so I'm assuming it was very quiet, and that's the level you are aiming to have inside your studio when it is finished?
Low frequencies are not so problematic, since we will mostly record VO
So you will be applying a high-pass-filter (also called a "low cut") to your mics? If so, what frequency will you set that for? That will give you an idea of what frequency range you should be concerned about for your isolation. You should aim to isolate well starting from a frequency that is about half of the lowest frequency that you expect to have in your recordings. So, for example, let's say that you want to set your low cut filter to roll off below 120 Hz with a steep 24 db per octave slope. That implies that at 60 Hz it would be reducing by 24 dB, and at 30 Hz it would be reducing 48 db, which is pretty quiet. So you could design your isolation system to work down to 15 Hz, or maybe 20 Hz, to ensure that there is no problem in your recordings. If there is already not much low frequency in the sounds outside that you want to isolate, then you might even be able to choose a higher frequency for your isolation limit. Eg. 30 Hz. That way, your room would isolate well from about 60 Hz upwards.
So i was thinking building a room inside the room:
- new decoupled walls (lightsteel frame with insulation, double PB on the inner side, backing rod and caulk) with one window opening, two other i would close completely.
- decoupled ceiling (no one above us but concrete&rebar roof) double PB with GG in between?
- heavy door
That is, indeed, the right plan! You can figure out the frequency and isolation of your wall design fairly easily. I wrote a brief article about that a while ago:
What is MSM? How does it work? That should help you choose the materials and cavity depth, to get the isolation you need. The only thing I would suggest changing in your plan, is to replace your single heavy door with two doors, back-to-back, one door in each leaf. Something like this:
site built door for high isolation Yours would not need to be quite so extreme, as that design was specifically for a very high isolation situation (drum studio in a residential area), but that's the general idea.
Just an idea if i am plannign in the right direction?
You seem to be on the right track!
- Stuart -
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, the noise floor with windows closed is about 40db (c weighed) and want to drop down to 33db.
