Many, many people get this one wrong: its a common misconception that the STC system is a good way of measuring the "soundproofing" or isolation of a studio. You even see that on some web sites for manufacturers, distributors and re-sellers of acosutic isolation products.
This is a myth! STC is not a good measure of studio isolation ("soundproofing")
Forget STC. STC is no use for telling you how well your studio will be isolated. 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 belief 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 for "musical instruments" nor for "sound systems". Because studio sound systems and musical instruments put out lots of energy at frequencies that STC does not even take into account!
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 looking 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 for contemporary music. 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 when designing your own studio, 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 a 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.
- Stuart -
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Sometimes it can help to see things: in the Document Library, IR761 has approximately 20 pages of introduction and then 350 pages of results of different tests. Look at the STC rating (in the table on the left of each page) and how that compares with the transmission loss at different frequencies.
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