STC Examples

[rgorke]

Is there a video or audio that shows the difference between STC ratings. For example, "here's what the drums or guitar amp or ??? sound like standing in front of it....here's what you can expect to hear with an STC of 40, with an STC of 45, 50, 55, 60 etc?"

Just wanting to get a general frame of reference to what degree different STC's control/isolate sound. I understand the concepts but wanted some examples of several levels.

Does that make sense?

Thanks.

[Soundman2020]

Is there a video or audio that shows the difference between STC ratings. For example, "here's what the drums or guitar amp or ??? sound like standing in front of it....here's what you can expect to hear with an STC of 40, with an STC of 45, 50, 55, 60 etc?"

Just wanting to get a general frame of reference to what degree different STC's control/isolate sound. I understand the concepts but wanted some examples of several levels.

Does that make sense?

Forget STC. It is no use at all 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 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 frequencies 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, 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. There is no relationship to real-world decibels: it is just the index number of the reference curve that is closest to your curve.

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 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.

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 is 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 decibels and TL graphs to judge if a wall, door, window, floor, roof, or whatever will meet your needs.

In addition, a video that attempted to demonstrate different isolation levels would not be much use, because the way you perceived that would depend on where you had the volume set as you watched the video! It would also depend on the speakers you used as you watched the video, and also the room you were in as you watched it. There would be no point to such a video, as it would be entirely subjective.

OK, having said all that, here's a rough idea of what different sound level and isolation levels are like.

Most people would consider a sound pressure level of about 30 dBC to be "silent". That's about the level you would hear if you are inside a bedroom far from any sound sources, late at night, with the windows and doors closed, and everything in the entire house turned off. No radio, TV, computer, fridge, microwave, fan, etc. NO water running, no people or animals around. No traffic outside, no wind, rain, aircraft, etc. Just still calmness all around. That's roughly 25 to 30 dBC. So 30 dBC is pretty much the lowest level you are likely to be exposed to in normal life, unless you happen to have an acoustic laboratory with an anechoic chamber handy!

At the other extreme, a Grateful Dead rock concert might hit you with 125 dBC or so, which is plenty loud enough to cause irreversible hearing damage within a few minutes. Most rock concerts come in at about 110 to 120 dBC.

A single drum kit played hard can easily put out 115 dBC, or more. Add a bass guitar with a big cab, and the total level can push 120 dBC. Typical levels in a tracking room session for a rock band are around 100-110, peaking occasionally at 120.

Normal conversation between two people a couple of feet apart (while chatting over coffee, for example), is about 65 to 70 dB.

Studio and cinema calibration reference level is 86 dBC, and most recording engineers listen at about 70 to 80 dB while mixing, occasionally pushing 100 or so to "check the mix".

So, for example, if you are planning to build a studio where the level in the tracking room is 110 to 110 dB, the level in the control room is 80 to 100 dB, and you have a mandate to make that "silent" outside, you will need to design an isolation system that provides 80 dB of transmission loss. Because 110 - 30 = 80.

But that doesn't tell the whole story, because the human ear is more sensitive to some frequencies than others, and low frequencies are much harder to isolate than high frequencies.

No matter, 80 dB of isolation is a tall order. A VERY tall order. A typical house wall provides roughly 30 dB of isolation, so in order to provide 80 dB of isolation you would need a wall that blocks one hundred thousand times more energy than a house wall. Yes, literally 100,000. The decibel scale is logarithmic, and each time you go up by 10 decibels, that means there is ten times more intensity. From 30 to 80 is five steps of ten dB each, so the total difference is a one followed by five zeros: one hundred thousand.

It is possible to build such a wall, yes, but it is not easy. Most home studio builders are happy to get 50 or 60 dB of isolation, and 70 dB is about the practical limit for most home studios, unless you have a very large budget and a lot of space.

So, in the real world, if you have a guy playing drums at 110 dB in your studio, and you managed to build it with 60 dB of isolation, you would hear the drums outside at a level of about 50 dB, which is a bit quieter than normal conversation between two people.

But there's another factor to take into account: distance. Each time you double the distance away from a sound source, the sound level drops by about 3 to 6 dB. Let's call it roughly 5. So if you are hearing a level of 50 dB as you stand 5 feet away from the outside wall of your studio, and you move back to 10 feet away (double the distance), the level you hear will go down to about 45 dB. Move to 20 feet away (double again) and the level will be 40 dB. Move to 40 feet away, and it will be 35 dB. Move to 80 feet away, and it will be 30 dB, which I mentioned at the start is what most people consider "silent". So as long as your studio is far from your neighbors, you can get away with less isolation than you'd need if they are just a few feet away.


I'm not sure if that answers your basic question, but it does give you a lot more info on how isolation works, and a rough idea of what to expect for different isolation levels.

- Stuart -