Looks like I’ve been reading much that isn’t really what it should be. ... On STC, makes me look a bit foolish (to say the least)
Don't feel bad about that at all! There sure is a LOT of such incorrect "reading material" on the internet... and much of it is in the marketing hype for people wanting to sell you stuff! There are way too many snake-oil vendors, and half-honest vendors of acoustic products. Or even well-meaning but ignorant ones... There's only a few "good guy" manufacturers who are honest and really do know their stuff. There's so much bad stuff out there, that it's actually not so easy to find good, solid, valid information to guide you. But you found the forum, so you are fine now!
115 dBC is insanely loud inside a room, isn’t it?
Oh yeah. Insanely loud. In fact, if that was the normal level in your workplace, the legal maximum exposure you would be allowed is about 15 minutes per day. More than that, and you'd run the risk of suffering permanent hearing damage. OSHA (the Occupational Safety and Health administration ) sets those levels for workplaces, based on years of research, but even then some people say that the exposure is still too high, and and it should be no more than 5 minutes per day at 115 dB.
Mix engineers do not listen that loud. If they do, it's because they are already totally deaf! (Or soon will be...)
Standard calibration level for movie houses and control rooms, is 86 dBC at the listening position(s) with all speakers firing. That's the level we use to set up the system, and tune it. That's also about as loud as some mix engineers listen normally, but most use lower levels, because they still want to be mix engineers for a few more years, and it's hard to mix well if you are deaf! OSHA says that you can listen to 85 dB sound levels for 8 hours a day before you suffer permanent hearing loss... so that's already risky for mix engineers. 75 to 80 dBC is more reasonable.
1 - I need to know how the mix works if played loud. So, WHAT is my definition of loud (dB C-weighted).
As above: OSHA says 85 dB for 8 hours every day is the legal limit for a workplace, without hearing protection. Others say even that is too high, and suggest 75 or 80.
Now, having said that, there certainly is a need to push the levels every now and then to "check the mix", or "check the bass", as mix engineers euphemistically call it, and that's OK, as long as it is just a few brief periods each day, and probably not more than about 100 dB or so, even then. There's also the occasional need to knock the socks off a client, and impress the hell out of him by blowing him out of his chair with a pure sound explosion... but do keep those to a minimum! Short bursts, very infrequently (a couple of times per week, max). If you have to do it more often, wear hearing protection.
2 - If I ever need to demo my own speakers, where the question would be “How loud is “the customers loud”.
Depends on the customer! If he's a DJ in a club, then 100 is probably too quiet for him
You'd probably want to push it to 110 or so to keep him happy. But if it is a folk singer with an acoustic guitar, he's probably be happier hearing it "pushed" to 80 or so...
As with so many things in recording, there's the science that shows the objective reality, and then there's the subjective preferences that say "screw the scientists! I'll listen any way I want! (Even though I'll be deaf by the time I'm 40...)
Interestingly enough, in a study of classical musicians a few years back, it was found that symphony orchestra violinists almost all suffer from partial hearing loss in their left ears... Take a look at how close a violinists ear is to the strings and sound holes...
What can be considered a trustful source for the TL graphs?
If it is published by an independent acoustic research lab, then you are safe. Riverbank is one of the biggest and best known, but there are many others. Universities often publish reliable data too. All of them usually do add STC numbers to the graphs as well, which is unfortunate, but I think they do that to keep the vendors honest...
1 - Buy a sound level meter (I do have REW on my Mac but a handmeter may be more useful to run around with).
Are you using a pre-calibrated USB mic on your Mac? If not, then you still need a hand-held sound level meter to calibrate REW, so it knows how loud the sound really is in the room. If you don't tell it, then it has no way of knowing. You might find this article I wrote a while back, useful:
http://www.johnlsayers.com/phpBB2/viewt ... =3&t=21122 .
2 - Draw up a map of my shed and it’s environment.
Yup! Consider doing that in SkethUp, but don't use the on-line free version: it's a joke. Use the real version, called "SketchUp Make 2017". It's a full 3D modelling package that is fairly easy to use. I use it with all of my clients, to design their studios.
3 - Do measuring outside (how loud is my environment). There is a road next to our house.
Measure outside, and inside, at various times of day, and make careful notes of everything. Levels, time of day, where you measured, what else was going on around you, etc.
4 - Measure how loud that same environment is, when I measure INSIDE the shed as it is now (outside - in). That way I know the transmission loss of the actual building in its CURRENT state.
Right. Set up a full-spectrum sound system inside the location, play some bass-heavy contemporary music on it, loud (maybe 100 dBC measured three feet away), leave it looping the same song over and over, and measure everywhere you can think of in and around your property.
Oh, and wear hearing protection any time you are doing loud stuff like this: assuming you still want to be mixing music ten years from now!
5 - Measure the level I use when mixing. This would be the “normal” level that will be produced INSIDE the room.
If you do that and find that you normally mix higher than 80 dBC, consider retraining yourself to mix at a lower level... assuming you still want to be mixing music ten years from now!
6 - Define my MAX level produced by the loudspeakers. Like in your example 115 dBC.
Right, but my example was based on your comments, and it was meant to be scary! 115 dBC is LOUD! Very!
7 - Go to the town hall and ask for data on noise level and regulations. Although having said that, I’d better also talk to my wife and the 1 neighbour we have first…
Yup! You can probably find the noise regulations on the web site of your local municipality. Might save you a trip to the town hall.
8 - Figure out both situations: What is acceptable for the level I produce (inside - out) and what is acceptable for me to here on "outside noise" inside the studio (outside - in).
Yep. And also measure the levels inside, for typical exterior noises that could annoy you in your mixing sessions, such as wind, rain, traffic, aircraft, sirens, lawnmowers, ...
QUESTION: IS there a rule of thumb way to calculate the estimated reduction for the combination of the 2 walls I drew up. Must be complicated, but still, I feel currently like being in a catch 22.
Well, better than that, there's a set of equations for doing that!
I wrote this a while ago, so I'll just cut-and -paste:
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The equations for calculating total isolation of a two-leaf wall are simple:
First, for a single-leaf barrier you need the Mass Law equation:
TL = 14.5 log (M * 0.205) + 23 dB
Where: M = Surface density in kg/m2
For a two-leaf wall, you need to calculate the above for EACH leaf separately (call the results "R1" and "R2").
Then you need to know the resonant frequency of the system, using the MSM resonance equation:
f0 = C [ (m1 + m2) / (m1 x m2 x d)]^0.5
Where:
C=constant (60 if the cavity is empty, 43 if you fill it with suitable insulation)
m1=mass of first leaf (kg/m^2)
m2 mass of second leaf (kg/m^2)
d=depth of cavity (m)
Then you use the following three equations to determine the isolation that your wall will provide for each of the three frequency ranges:
R = 20log(f * (m1 + m2) ) - 47 ...[for the region where f < f0]
R = (R1 + R2)/2 + 20log(f * d) - 29 ...[for the region where f0 < f < f1]
R = R1 + R2 + 6 ...[for the region where f > f1]
Where:
m1 and m2 are the surface densities of leaf 1 and leaf 2, respectively
f0 is the resonant frequency from the MSM resonant equation,
f1 is 55/d Hz
R1 and R2 are the transmission loss numbers you calculated first, using the mass law equation
And that's it! Nothing complex. Any high school student can do that. It's just simple addition, subtraction, multiplication, division, square roots, and logarithms.
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In fact, one of the forum members, Greg, has written a spreadsheet to help you figure that out. I can't find the link right now, but I'll try to post it later.
The above works because of resonance. A wall is a resonant system, and you "tune" it by adjusting the mass on each leaf, and the size of the air gap between them, and the amount/type of insulation in that air gap.
Surely that can’t be the case.
Right! There are numerous studies out there, done by many acoustic test labs, on numerous different wall types, with published results. The best one I know of is IR-761, published by the Canadian National Research Council. It used to be here:
http://nparc.cisti-icist.nrc-cnrc.gc.ca ... 064b073e7f but seems to have moved... or maybe their website was down when I tried. Google it. They tested hundreds of different wall types in their labs, and published the TL curves and the details for every single one, along with a description of their testing procedure. Very informative! But do take one thing into account: you won't actually get as mush isolation as they did, because they built every single wall very carefully, under highly controlled conditions, in an extremely well isolated test facility. When you build, it will be in the real-world, with ordinary materials and limitations. So allow that your results will be lower than what they got. Probably by about 5 dB or so.
QUESTION: Would you agree that the “1 layer of 12mm OSB AND 2 layers of 12mm Gypsum included with rock wool” is a fair starting point for my situation
That depends on how you build the OTHER leaf! A wall is a resonant system, with two leaves: they work together to provide the isolation. It's not just the sum of the two sides, either. You can't measure leaf A that got 29 dB ad leaf B that got 31, then assume together they will give you A+B=60. Not true. Resonance comes into play, in several different forms... So you need to consider the entire system as a system, not just as a bunch of individual parts.
OFCOURSE I need to expect an additional effort in acoustic treatment inside the room once the basic room has finished.
Oh yeah! Isolating the room is just a small part of the overall design! You then have to design the interior acoustic response so that it meets the ITU BS.1116-3 spec! (Google it: chapters 7 and 8 apply to you...) That's the acoustic response that a control room MUST have, to be usable for critical listening. Here's an example of such a room:
thread about Studio Three Productions' studio Finished a few years ago, and currently in operation. Here's one that is under construction right now, nearing completion:
thread about Steve's high-end control room in New Orleans That one will also meet the spec. It's not easy to achieve that, and most home studios are too small to be able to hit it completely (laws of physics), but the goal is to get as close as you possibly can.
- Stuart -
;
MUST BE....!
Took me hours to get a 3D room and once I was kind of happy with it and wanted to add inside walls, the whole shabeng transformed into a big mess. Walls inside each other, holes in a wall. Really artistic to see, but NOT what I need. 
I need to know how the walls have to be to match the sound level reduction I came up with in my measurements, BUT I need to build them first and measure their effect. Once having it build, it could go 3 ways;
Thanks, Greg!