Hi "Twitchy". Please read the
forum rules for posting (click here). You seem to be missing a couple of things!
I have decided to use rubber as I believe it has the relevant STC.
Believing something be true, and that it really is true, are often two very different things. As is the case here....
... it has the relevant STC.
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 TL graphs to judge if a wall, door, window, floor, roof, or whatever will meet your needs.
The fact that you are starting out by believing and by using STC is an indicator that there's more research ahead of you to discover how acoustic isolation actually works. Fortunately, that's what the forum is all about!
I am using 10mm thick rubber mats made from recycled tyres.
That's not very much isolation. The mass of rubber is around 900 to 1200 kg/m3, so the surface density of your wall will be around 10 kg/m2. Mass law says that you'll get isolation of about 27 dB. That's even less than a typical house wall, which will get you around 30 B.
Yes I know rubber is expensive but I'm committed to total sound proofing.
There is no such things as "total sound proofing". Any sufficiently loud sound will get through any conceivable barrier. It is impossible to completely stop sound. Even the best isolated acoustic laboratory on the planet still has detectable sound inside.
So you can't stop sound, but you can attenuate it. That's why acousticians prefer not to use the term "soundproof", and rather go with "isolation". It is possible to achieve various levels of isolation, using various techniques and materials. The amount of isolation is refereed to as "transmission loss", and is measured in decibels. So the question is: How many decibels of isolation do you need? Drums can hit around 115 dBC, and more, so let's call that the high level that you need to reduce. Most people would consider 30 dbC to be "silent", and that's often the level set by noise laws. The difference, of course, is 85 dB, so in theory you need 85 dB of isolation. As you can see, your 27 dB rubber is not going to do much at all!
My goal is to be able to play midnight drum solos without the neighbour hearing a thing. A lofty goal.
Indeed. No doubt about that. 85 dB of isolation is, indeed, lofty. If that's what you want, then it cam be done, but the cost is very high. To get that type of isolation, we are talking six digits at least, perhaps seven. The very best isolated studio on the planet gets just a little over 100 dB isolation, and needed an investment of millions of dollars, and the best acoustic engineering available.
Perspective: A house wall gets you about 30 dB. The decibel scale is logarithmic. Each time you go up ten decibels, that implies an increase of ten times the intensity. So a wall that gets 40 dB needs to block ten times more sound than a house wall. One that gets 50 db must block 100 times the intensity. 60 db needs to block 1000 times the intensity. 70 dB is 10,000 times, and 80 dB is 100,000 times. So you need to block roughly one hundred thousand times more sound than a typical house wall. 10 mm of recycled rubber is not going to do anything at all for that.
So far I'm thinking of gluing the rubber mats onto both sides of a 16mm thick sheet of MDF. That will mean 10mm of rubber on each side, 20mm all up.
The math doesn't add up. 10mm rubber + 16mm MDF + 10mm rubber is 36 mm, not 20mm.
You have also increased the mass quite a bit like that, so let's check the math... We already know about the rubber, so we need to account for the MDF, which has a density of around 750 kg/m3. 16mm implies surface density of 12 kg/m2. +10 for the first layer of rubber, + another 10 for the second layer, total mass = 32 kg/m2. Wow! That's MUCH more! So it must get MUCH higher isolation, right? Nope. Instead of 27 dB from a single layer of rubber, you will now get 35 dB. Just a bit better than an average house wall.
The problem is a principle of physics called "Mass Law". It goes like this:
TL = 14.5 log (Ms * 0.205) + 23 dB
Where: Ms = Surface Mass in kg/m2.
That tells you roughly how much isolation you will get from any single-leaf wall. It's not a very heartening principle. If you want to build your studio based on single-leaf mass law, you are going to need an enormous amount of mass. Several meters of reinforced concrete should do the trick....
Fortunately, there's a better way than mass law. There are other principles of physics that apply to walls that are not single-leaf mass.
My friend believes this is overkill and that I may only need a single layer of rubber.
Both you and your friend are wrong: That is "under-kill". Extreme under-kill. It won't even get you one one-thousandth of what you need.
This will be a free standing room within a room.
Fine! That is, indeed, the way to obtain high levels of isolation at reasonable cost, and without being subject to mass law. But you can't guess and "believe" how it should work! You need to calculate it.
Here's the equations you need. First, the equation for MSM resonant frequency:
MSM-resonance-equation.jpg
Then you need to figure out the isolation for each region of the spectrum, using these equations:
Full-spectrum-TL-isoaltion-equations.jpg
With that, you can predict what the isolation of your wall system will be.
Please note that I ask about sound proofing advise relating to the use of rubber.
Sound waves are stopped by mass. Rubber is mass. It stops sound in the same way as any other mass.
So does anyone have any scientific knowledge that could enlighten me as to how much rubber I would need to stop the sound of drums or a band jamming?
According to mass law, and assuming that you want 80 dB of isolation, you would need about 180 cm thickness. Yes, that's right: a bit less than 2m. 2m will give you 81.7 dB....
- Stuart -