Hi there Steve, and Welcome to the Forum!
You are looking at IR-761, which is great, but don't worry too much about the STC rating. Rather, look at the actual TL graphs and tables.
STC is a single-number rating system that is absolutely meaningless for studios. Here's why: STC was never meant to measure such things! Here's an excerpt from the actual ASTM test procedure (E413) that explains the use of STC.
“These single-number ratings correlate in a general way with subjective impressions of sound transmission for speech, radio, television and similar sources of noise in offices and buildings. This classification method
is not appropriate for sound sources with spectra significantly different from those sources listed above. Such sources include machinery, industrial processes, bowling alleys, power transformers,
musical instruments, many music systems and transportation noises such as motor vehicles, aircraft and trains. For these sources, accurate assessment of sound transmission requires a detailed analysis in frequency bands.”
It's a common misconception that you can use STC ratings to decide if a particular wall, window, door, or building material will be of any use in a studio. As you can see above, in the statement from the people who designed the STC rating system and the method for calculating it, STC is simply not applicable.
Here's how it works:
To determine the STC rating for a wall, door, window, or whatever, you start by measuring the actual transmission loss at 16 specific frequency bands between 125 Hz and 4kHz. You do not measure anything above or below that range, and you do not measure anything in between those 16 points. Just those 16 small bands, and nothing else. Then you plot those 16 points on a graph, and do some fudging and nudging with the numbers and the curve, until it fits in below one of the standard STC curves. Then you read off the number of that specific curve, and that number is your STC rating. That's it. There is no true relationship to real-world decibels: it is just the index number of the reference curve that is closest to your curve. To clarify: the STC number is NOT how much isolation you will get: it is just the number that somebody once assigned to a curve on a graph. So for the STC-70 curve, they could have called it "STC-GGFQRT" or "STC-Delta-RED" or "STC-Elephant-seven" or anything else, and it would tell you just as much about isolation as "STC-70" does: ie, nothing. It's a REFERENCE number, not an actual isolation number. For speech conditions, yes, STC-70 might actually be close to 70 dB of isolation, but not for music.
When you measure the isolation of a studio wall, you want to be sure that it is isolating ALL frequencies, across the entire spectrum from 20 Hz up to 20,000 Hz, not just 16 specific points that somebody chose 50 years ago, because he thought they were a good representation of human speech. STC does not take into account the bottom two and a half octaves of the musical spectrum (nothing below 125Hz), nor does it take into account the top two and a quarter octaves (nothing above 4k). Of the ten octaves that our hearing range covers, STC ignores five of them (or nearly five). So STC tells you nothing useful about how well a wall, door or window will work in a studio. The ONLY way to determine that, is by look at the Transmission Loss curve for it, or by estimating with a sound level meter set to "C" weighting (or even "Z"), and slow response, then measuring the levels on each side. That will give you a true indication of the number of decibels that the wall/door/window is blocking, across the full audible range.
Consider this: It is quite possible to have a door rated at STC-30 that does not provide even 20 decibels of actual isolation, and I can build you a wall rated at STC-20 that provides much better than 30 dB of isolation. There simply is no direct relationship between STC rating and the ability of a barrier to stop full-spectrum sound, such as music. STC was never designed for that, and cannot be used for that. It was meant for describing isolation of speech, not music. It's reasonably useful for what it was designed for, but not very useful for music.
Then there's the issue of installation. You can buy a door that really does provide 40 dB of isolation, but unless you install it correctly, it will not provide that level! If you install it in a wall that provides only 20 dB, then the total isolation of that "wall+door" combination is about 20 dB: isolation is only as good as the worst part. Even if you put a door rated at 90 dB in that wall, it would STILL only give you 20 dB. The total is only as good as the weakest part of the system.
So forget STC as a useful indicator, and just use the actual TL graphs to judge if a wall, door, window, floor, roof, or whatever will meet your needs.
This implies that the very first thing you need to do, is to define how much isolation you need. If you don't know that, then you can't design your isolation system to provide it! So you need to get out your sound level meter, and do some testing with it set to "C" and "slow", to answer tow questions: "How loud are you?" (measure the level in a typical worst-case session), and "How quiet do I need to be?" (check your local noise regulations to find out the legal limits, and also do practical tests in and around the building, to find out what the ambient noise levels are at the quietest times of day). Subtract the second number from the first number: that's how much isolation you need.
So, the purpose of that rambling rant is to show you why it's not a good idea to evaluate possible construction methods for your studio, using STC. IR-761 does give STC ratings, yes, but the far more useful parts are the tables on the left edge of each page, and the actual TL graphs. Those are where you should concentrate your attention.
and the first consideration is being able to record at rock band SPL without bothering anyone.
That's a tall order in residential area. It is possible, but not easy.
But before figuring out what building materials and techniques you need, you should first calculate exactly how much isolation (transmission loss) you need, in decibels. You do that by first defining how loud you are, and how quite you need to be.
Just a drum kit by itself can easily put out 110 dBC, 115 dBC without too much extra effort, and approaching 120 dBC if you have a really crazy drummer. Add to that a bass cab that "goes up to eleven(!)", screaming keyboards, growling electric guitar, etc., and you can indeed hit 120 dBC inside the room, ir the band tries hard. That's deafeningly loud, and in fact far exceeds the permitted levels for a workplace: If you had that level of sound in an office, shop, or heavy industry factory, all workers would be require, by law, to wearing full hearing protection. The maximum allowed exposure time at 115 dB is just 3 minutes each day. Any more than that, and they would all have to wear ear protection. But rock musicians don't really care, a jamming session isn't a regulated workplace (as far as I know....
), so it's common to have those levels in a studio live room.
So that's one side of the equation: you will have levels of 115 dB, perhaps 120 dB in the studio. The other side of the equation is: "How quite does that have to be outside?" That's not so easy to answer. It depends on many factors. Firstly, there's the issue of whether you are aiming to be legal, or totally inaudible, or just "not annoy the neighbors" too much. That's three different things. The "Legal" part is simple to define: check with your local municipality website, to find the actual noise regulations, and take note of what it says. Commonly there are two levels defined as the limit for noise: one for daylight hours, and a lower limit for night time. There are also usually "zones", where each zone has a different day and night level, and the "zones" are named things like: Residential, commercial, light industrial, heavy industrial, etc. So look that up to find out what zone you are in, and what the limits are, for all times of day.
Let's assume that your regulations say something like 45 dB during the day, and 40 dB at night (often they rae much tougher than that, but just hypothetically speaking...). So, if your limit is 45 dB, and you are producing 115 dB, then you need 70 dB of isolation. That's REALLY hard to do. That's about the limit of what can be achieved with typical home studios and project studios, with very deep pockets...
However, it might not be necessary to go that far! Because distance is your friend! Most regulations specify that the testing is to be done at one of two places: 1) The property line, or 2) The location of the person complaining. If your regulations are like #2, then you are lucky! If yours are like #1, you are not so lucky, but still not as bad as you thought...
Sound expands outwards in a sphere around your building, which is good, because it means the levels drop off quite fast as distance increases. The reason is simple: the total sound power put out by your studio outside the walls, is spread around the surface area of that expanding sphere. And if you recall your high-school geometry classes, you'll know that the area of a sphere increases proportional to the SQUARE of the radius. So a small change in distance is a large change in area of the sphere.
In real terms, the sound level decreases by 6 dB each time you double the distance from the source. And the reverse is also true: sound level increases by 6 dB each time you halve the distance to the source. And this is where it gets good for you, if there's a long distance between your studio and the property line. Let's say that your regulations say you can't be any louder than 45 dB measured at the property line, and your studio is 24 feet from the line. Thus, the level can be 45 dB at 24 feet, which means it can be 51 dB at 12 feet, and 57 dB at 6 feet, and 63 dB at three feet. 3 feet is the "standard" measuring distance for small sound sources, such as instruments, not really valid for large things, like buildings, but let's assume it's OK.
Thus, as long as you have enough isolation to get your 115 dB "inside" level down to 63 db when measured 3 feet away from the wall, then you only need 52 dB of isolation (instead of the 70 we figured before). Getting 52 dB of isolation is a hell of a lot easier than getting 70! In fact, it's nearly one thousand times easier.... 52 dB isolation is definitely achievable for a home studio or project studio.
Of course, we are assuming that you have 24 feet from the studio to the property line for the above. If you have more distance, even better. If you have less distance, then you have a problem.
Adding another wrinkle: Do you just want to meet the legal limits, or do you want to actually be totally silent? The thing is, even if you are down to 45 dB at the property line (in this hypothetical situation), that might not be enough. If the ambient noise level in the area is very low, then your music could still be audible at 45 dB. You need to get it down considerably below the background ambient level in order to make it inaudible. On the other hand, if the background levels are high (eg, nearby roads, industry, airport, railway, or just a generally noisy city environment), then the ambient level itself might be in the high 50's or low 60's, fore example, in which case your 45 dB can't be heard at all. But if this is a quiet residential neighborhood where the background level is 35 dB, then your 45 db is very audible. It might be legal, but still annoying to the neighbors.
And here's the kicker: most noise regulations have some fine print hidden in there someplace, that says something like "... or any other noise that is deemed to be unpleasant, annoying, or unwanted", but without defining what those terms mean! So even if you do get below the actual numbers, they can still get you on that...
Thus, the very first thing you need to do is to figure out your "number" here: "How much isolation do you need, in decibels?".
Once you have that number clearly identified, then you can start looking at construction methods and materials that can produce that level of isolation.
- Stuart -