Hi there "Novicedrummer", and Welcome!!!
You deserve that extra warm welcome, because you have clearly done your homework very well before posting!
That's a refreshing change from many new members.
room in a room with nearly ~0.3m air gap.
Each leaf mass ~40-45kgm2 (from several layers of OSB & plasterboard),
No floating floor
just a concrete slab base.
I'd like some glass in my garden drum room, laminated due to the damping advantages as well as the safety and UV protection from the inter-layer.
Does anyone know if the critical frequency "dip" is significant at these laminated glass thicknesses?
Are you referring to the coincidence dip? It's significant, yes, but not overwhelming. The dip can be as much as 15 dB if you do a lot of things wrong, but more like a small ripple if you do lots of things right. For example, 1) use glass of different thickness: thicker in one leaf than the other leaf. that implies that the coincidence dips for each pane will be slightly different, so the two dips won't "line up" with each other. The difference won't be huge, but can still knock off a few dB. 2) Use glass that has an "acoustic" PVB interlayer, which provides much more damping internally than the normal PVB. That can knock off quite a few extra dB. 3) Get the laminated glass itself made with layers of different thickness. Eg, if you need 19mm glass, then get it made with one layer of 6mm and one layer of 13mm (plus the PVB). That can knock off a few more dB. 4) Make the air gap between the panes larger than the air gap between the wall leaves. 5) Place abundant insulation all around the periphery of the air gap between the panes, to provide as much damping as possible in the air gap. 6) If you REALLY want to get fancy (and expensive!) use multi-layer laminate glass, where there are more than two layers of glass and more than one acoustic PVB interlayer. Eg, for you 19mm glass, get it made from 3mm+6mm+10mm, with 2 acoustic PVB interlayers. Probably not necessary, but it is an option.
If it isn't I could just use regular 21mm laminated for both leaves (good mass), but an expensive mistake if it is!
Don't forget that the thicker the glass is, the LOWER the coincidence dip goes, in frequency! Thicker panel = stiffer = lower critical frequency. Yes, it is also more massive, but mass and stiffness do not change at the same rate with thickness. So you probably don't need to go to 21mm, an you definitely should not make them BOTH 21 mm. You could probably get away with 1 pane at 19mm and the other at 16mm, for example, if the air gap is big enough.
One other thing: the coincidence dip is going to be fairly high up the spectrum: top end of the mid range / low end of the high range. Isolation in that part of the spectrum is going to be much better than the low end, and your drums put out much more energy in the lows than they do around the coincidence dip, so it's unlikely that you'll have a problem with coincidence. If there is a significant problem at coincidence, then there will also be a MUCH more significant problem in the low end!
Finally, is this wall density over-kill (as it is driving the glass thickness) or is good low frequency attenuation outside of my budget!
Do the math!
It's not that hard... But first, define your numbers: how much isolation do you need (in decibels)? You should be able to get "good low frequency attenuation", yes, but that depends very much on your definition of "good". That one person considers "fantastic", you might consider "lousy"... That's why you need to define your actual goal for isolation, in terms of how many decibels you need.
(I know you might ask me what numerical dB isolation do I need - and the answer is I don't know at low frequencies.
Yup! That's what I am asking, for sure!
If you don't know, then you need to figure it out...
Barely audible in a quiet garden at 5m, has got to be ~60dB attenuation at mid/high frequencies and should be achievable.
That's guessing, not defining...
You need to get out your sound level meter and check some levels for a typical situation. And also get a copy of your local noise regulations, to find out what your legal limits are. All of that will help you define your number. With that number in hand, you can then look at various building materials and techniques, to figure out what method will provide the number you defined, within your budget.
I have chosen a mass-air-mass that gives a genuinely low frequency resonance ~25Hz -
You may not be aware, but at the MSM resonant frequency, your wall does not isolate at all! Rather, it passes sound through uninhibited, and in fact can even amplify sound at that specific frequency. It only starts to isolate at 1.4 x the MSM resonant frequency, and does not isolate well until about 2 x that frequency, and really well at about 3 x and above. So if your wall is tuned to 25 Hz, it only starts isolating decently at 50 Hz, and really well at 75 Hz.
However, I think your calculations are off: with 45 kg/m2 on each leaf and a 30cm air gap with good insulation, I get much lower resonance.
and a wall construction targeting >STC 66.
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 how it is isolating at 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.
Sorry for the rant, but STC is just no use at all here.
My only reference point is my tutors drum room, which has 50% lighter walls & slightly smaller air gap - isolation is very good except there is still noticeable bass.
The poor isolation in his room might be due to many other things, not just the mass and air gap...
- Stuart -
!!
(to spend elsewhere!)