EDIT: Aha! It is! And you pointed it out! I calculated the Sa for the glass at 125Hz wrong! ... So after revising everything and getting more accurate results, I redid my RT60s and got an average of 2,1 sec. That's pretty big but for an untreated room like this I guess it's about right.
Glad you figured out where you were going wrong! 2.1 seconds sounds reasonable for that room, but I would have expected even more, around 3.1 seconds for an empty room with hard surfaces all around. Your result is probably due to the floor and wall materials.
And I changed the layout of my control room based on a recommendation from my head lecturer.
I do realize that this is going to be a delicate tight-rope for you to walk, since this is a class where you obviously want to get good marks and not alienate the folks who will mark your assignment! But at the same time, I'm wondering how many studios your lecturer has designed and built, successfully...
Classroom textbook theory is one thing, but real-life design and construction is something else. Years ago I used to sell high-end video post production systems to TV stations and large post production houses (systems that cost hundreds of thousands of dollars) and the comment I most often heard from the technical managers at those places were along the lines: "
Where will I find people to operate advanced systems like this? The graduates coming out of the universities and technical colleges with degrees and diplomas in these areas, actually have no useful skills at all! They know how to pass an exam on the theory, but put them in a real-world studio or control room, and they don't have a clue. I'd rather take a guy off the street at random and train him, since he has no preconceived ideas, and knows that he knows nothing. The graduates think they know everything, but basically they have been taught nothing useful for the real world." Don't tell your lecturer I said that, but it's the truth, and likely is applicable to your school as well. The stuff you are being taught probably comes out of text books written years ago, and is fine in theory... but does not bear much relationship to real-world contemporary studio designs, and does not seem to be bred from real-world experience in building studios. I suspect...
If you are game, and courageous, you might want to mention in your presentation that some of the parameters imposed by the assignment itself are not representative of contemporary design practices, and that the studio you designed could have been much better if it were not limited by unrealistic barriers...
Although judging by your wording, you remember that
Do you mean that there's a better space to put the control room. I imagine the center of the room would be best rather than up near the top as I've done it.
Haas time, and the Haas effect. Look it up. Basically the problem is this: If your ears hear a reflection of the direct sound that arrives within less than about 20ms after the direct sound, and at a similar level, then your ears and brain are incapable of correctly interpreting that. Instead, your ear/brain will judge that you heard something different: You will perceive that the sound came from a different direction from the real one, and had a different frequency and phase spectrum than what it really had. This is a psycho-acoustic problem, due to the way our ears and brain work. If the reflection arrives more than about 30ms after the direct sound, then your ear and brain are able to interpret that correctly, give you the true direction and the true frequency/phase.
Therefore, contemporary studio design places the rear wall as far back as possible, and also arranges the front and side walls, such that the earliest possible reflections cannot ever arrive sooner than about 20ms. Contemporary studio design then also places treatment on some or all of the room surfaces involved in that reflection, such that the reflections that
do get back to your ears, all arrive well after the 20ms Haas time window, and at a level that is considerably quieter than the direct sound, preferably -20dB (but at the very least -10 dB). Those reflections must also arrive as a diffuse field, not specular reflections.
Conclusion: The distance between the rear wall and your ears must be great enough that the diffuse field reflected from the rear wall arrives later than 20ms after the direct sound passed your ears. That implies a "there-and-back" distance of at least 6m, minimum. and preferably more like 7 or 8. Thus, the distance between your head and the front face of the acoustic treatment on the rear wall must be at least 3m, but 4m would be better. And since your head cannot be any further back in the room than about 40% of the total room length, and since the acoustic treatment on the rear wall is at least 50cm thick, that means you need a room that is at least 7m long, front wall to back wall, and certainly not less than 6m. Your diagram currently shows control room depth of 4.7m (if you have the room oriented towards the window), which is too short. Or if you have the room oriented facing up or down the page, then it is barely long enough (6.3m), and in addition the engineer is going to end up with a sore neck, since he'd be turning his head back and forth all the time to look sideways at the LR, then back forward to hear the stereo image properly, then sideways again, then forwards...
So, if you do want to have a control room that is wider than it is long (which is not a bad idea, and many rooms are built like that), oriented to face the LR, then you'll have to make it much bigger, such that the front-wall-to-back-wall distance is no less than 6m, absolute minimum, and preferably more like 7m.
That's contemporary design, based on real-world up-to-date acoustic and psycho-acoustic principles, not text-book theory principles from 30 years ago.
I changed the material for the floor to "wood on joists" because a floating floor is needed in the brief
Same as above! The same "text-book vs. real world" syndrome applies to light-weight wooden floating floors. You could bring this up too: Floating floors are usually not necessary, are rarely used in contemporary studios, and in the few cases where they are used, they are done correctly, with massive concrete slabs floated on properly designed and calculated springs, not with light wooden decks on 2x4s and rubber pucks! That's doomed to failure. It won't isolate well, will probably end up actually amplifying some frequencies as they go through it to the other side, and in addition will act as a "drum head" on top of which the room is sitting, vibrating with some frequencies, not with others, absorbing some frequencies, amplifying others and in general wreaking havoc with the low-end response of the room.
Here's why:
http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173
And here's the long, boring, detailed technical explanation:
... starting with a graph that explains the problem in simple terms:
resonant-frequency-of-floating-floor-by-mass-and-gap-Graph---GOOD!!!.-S02.jpg
That shows how much mass you need on your floor, and how much air gap you need under it, to get the right resonant frequency. What I mean by "right resonant frequency" is simply the one that will allow your floor to actually isolate! Your floor is a resonant system. It will resonate naturally at a certain frequency that is governed by the mass (weight) of the floor, and the depth of the air cavity under it. At that frequency, and for one octave above it, the floor will NOT isolate. In fact, not only does it not isolate, it can potentially
amplify sounds at that frequency. And because this problem extends to one octave higher, obviously you want your floor's resonant frequency to be at least one octave lower than the lowest frequency you need to isolate. So if you need to isolate kick drums, which are often tuned around 80 Hz, then your floor should be tuned no higher than 40 Hz, which is one octave lower. If you want to isolate bass guitar, which easily goes down to 36 Hz (5 string bass), then you'd need to tune your floor no higher than 18 Hz. Let's assume this is the case, and now we can look at the graph.
The graph shows the frequency up the left hand side. You need something at 18 Hz, so draw an imaginary line across the graph a bit less than 20 Hz. You can now see that no matter how deep your air cavity is, the top two dashed lines are no use: you can never get a low enough frequency if your floor only weighs 5 PSF (pound per square foot) or 10 PSF. Not possible. However, at 30 PSF it is possible (the dotted line, third from the top): it looks like you would need to have an air cavity that is at least 4.5 inches deep, so you can't do it with 2x4's, as they are only 3.5" deep. You'll need to use 2x6's (which are 5.5" deep). Your other option is to go with an even heavier floor: the bottom curve on the graph, labeled 60 psf (solid line, not dashed). With that option, you can get a frequency of 18 Hz. with a cavity about 2" deep, so you could use 2x4s there.
So those are your options: you can build up your floating floor on 2x4s with a 60 PSF floor, or 2x6s with a 30 PSF floor.
So that brings up the question: What would you need to do, to get a 60 PSF floor? Well, lets' assume that you wanted to use OSB. The density of OSB is roughly 610 kg/m3, which works out to about 3.2 PSF for every inch of thickness. So to get 60 PSF using OSB board, you'd need to make it about 19 inches thick!
In other words, you'd need to have 31 layers of 5/8" OSB on your floor, to get enough mass.
But if you wanted to go with the 30 PSF option, you'd "only" need 16 layers of OSB to get there....
As you can see, it is physically impossible to float a light-weight deck consisting of just a couple of sheets of OSB on 2x4 studs. If you did that, the resonant frequency would be around 42 Hz, so the floor would amplify kicks, toms, bass guitar, electric guitar, and keyboards! It would only isolate from about 84 Hz upwards.
So how do you get such a high mass? If you can't do it with OSB, then what do you need? Simple: Concrete. The density of concrete is around 2400 kg/m3, which is roughly 12 PSF for each inch of thickness. So a concrete slab just 3 inches thick (36 PSF) would let you do it with a 3.5" cavity, and if you went up to 5" thick concrete slab, you could do it on a 1.5" air cavity.
That's the plain, hard, cold facts from the real world, not from a pure theory text book. You cannot float a light-weight deck and expect to get good isolation for low frequencies. The laws of physics prevent it. You might want to point out this to your lecturer, and ask why you are having this silly situation imposed on your assignment, when it would create a terrible room and is not even necessary, give the basic scenario!
Now, all of the above assumes that the "deck" is fully isolated from the underlying subfloor, and that the only "spring" in there, is the air in the cavity. In real life, that is not possible: you need some type of resilient mounting to hold the deck up while decoupling it from the sub-floor below: it might be rubber pads, or metal springs, or something else, but there has to be something that disconnects the deck from the subfloor, mechanically. Which makes things
worse! That rubber or metal spring works in
parallel with the air spring, and that REDUCES the total "springiness". So you actually need a deeper cavity to get the same frequency...
Now for the kicker that really dooms this whole light-weight deck concept: Whatever it is that you use as the spring to decouple the deck (rubber, metal springs, snake-oil), you have to ensure that it really will float! If you put too much weight on a spring, then you flatten it out completely, and it is not "springy" any more: it bottoms out, and does not float. On the other hand, if you don't put enough weight on it, it is also not "springy"! It "tops out" and does not float. So you have to ensure that you put the right amount of weight on each spring, such that it has the optimal amount of compression, and really does float. For each type of spring, there are tables and equations that allow you to do that, but for most springs, you need to compress it about 10 to 20% to make it "float". Less that 10% "tops out" and more than 20% "bottoms out".
Great So let's go back to the light-weight deck (pretending that the above graph does not exist, and imagining that it might be possible to magically get the right frequency with just two layers of OSB). We already know that two layers of OSB weighs about 6 pounds per square foot, so let's say we do some calculations for magical rubber pads, made of purest snake oil, and arrive at the conclusion that we need four pads of two square inches each for every square foot of floor, and with a load of 6 PSF, that will float just fine, with exactly 15% compression. Great! Amazing! The floor floats! ... Until you stand on it.... Assuming you weigh about 180 pounds, and that your weight will be spread across four square feet of floor, just by stepping on that floor you increase the loading from 6 PSF to 51 PSF
Gulp! I think you see where this is going.... You just flattened your rubber pads into oblivion! They are now squashed flat, and don't float.
So you think creatively, and decide that you don't need the floor to float when you are
not in the room, so you re-design it to float only when you are standing there, and the load is 51 PSF. Fantastic! Wonderful! It floats! .... until you bring in your guitar, amp, a couple of pizzas and a crate of beer... now the load is 63 PSF, and the floor doesn't float....
So you wrack your brains, and re-design the rubber pads yet again, so they float at 63 PSF.... But then you invite your buddy over to join you for a jamming session, an he brings his girlfriend, another amp, more pizza, and a suitcase, since he's going to stay the night.... and now you have a load of 85 PSF....
OK, I can keep on adding scenarios here, such as the desk, chair, couch, console, DAW, other gear, more people, etc. etc., but you can see the problem: The load on a light-weight deck varies so enormously that it just is not practical. But with a concrete deck, which has a much, much higher density, this is not a problem. Putting all that extra load on the floor, or taking it off, only changes the total mass by a few percent, and the floor still floats: the springs are still operating inside their optimal range.
So that's the issue. Floating a light-weight floor is not a viable solution. You need huge mass to float a floor successfully. It is certainly possible to float a floor, and companies like Mason Industries make devices to do that, but it only works with very high mass for the floor deck, such as 3 or 4 inches of solid concrete.
Perhaps you could bring up this with your lecturer, or in your presentation, showing why it is no feasible to float a light-weight deck as a studio floor, firstly due to the impossibility of getting a low enough resonant frequency with low mass, and secondly due to the wildly varying loading that would put the springs outside the usable range, simply by having people and gear moving in and out of the room.
Suggest to your lecturer that you should be allowed to assume a floating concrete deck, at least 3" thick (15cm) and a damped air gap that is also at least 3" deep, as that is a realistic situation, found in real studios in the real world.
Reality meets text book, once again.
I'll definitely take your advice about the iso booths. Should I put an air gap between the two booths though, or make them share a wall as you said?
The reason I suggested that is to save on building materials, simplify construction, and also have better acoustics. The inner-leaf wall of the LR will follow the contour of those two booths, in the form of a large angled "wedge" that bulges out into the room, and each iso booth will be constructed as an independent, stand-alone single-leaf structure within that "wedge". The "wedge" basically acts as a very low frequency diffuser, which will greatly help to break up the modes along that axis of the room, and will also eliminate SBIR and LBIR associated with that wall and certain locations in the room. I did exactly this for a facility I designed a few years ago... Two booths, back-to-back on the far wall of the LR, with careful use of glass on the sides facing the LR and facing each other, and suitable treatment on the other walls, as well as above and below the glass.
A chunk of the project is put towards getting an average STC of 55 so I need to be careful.
Another text-book error! Here's why you should
never use STC as a parameter for determining studio isolation:
STC 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 very 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 around 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 about 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.
Once again, you could mention this in your presentation, and state that your studio is designed to provide 55 dB of transmission loss, as measured with a calibrated type 1 or type 2 sound level meter, set to "C" weighting and "slow" response, using full-spectrum pink noise, played at 100 dBC inside the studio and measured at 45 dBC or less outside the studio. That will be considerably better than STC-55! And that's the way that a typical contemporary studio would be specified in the brief, and designed.
Text-book. Real-world.
I think the lounge mostly got expanded because I reduced the size of the live room.
That begs the question: Why did you make the LR smaller? You should be trying to make it bigger, not smaller.
Here's the issue: Volume. Most instruments love a room that has as much volume as possible, within reason. Put an instrument in a small room with a low ceiling, and it will sound bad. Raise the ceiling, and it will sound better. Move one wall further away, and it will sound better still. Move all the walls further back, and the ceiling still higher, and it will sound great! The more volume you can put in your live room, the better. If the decay times start getting too long, or too unbalanced, then add treatment: probably diffusion, and a bit of absorption.
There's a general rule of thumb that studio designers use (but that you won't find in a text book!
) : The volume of the LR should be at least 5 times the volume of the CR: And since the floor area of the CR should be around 30m2 or so, and the height around 2.5m or so, minimum (call it 3 to be safe), you can set a rough parameter that your CR should be around 90 m3, and your LR around 450 m3. Your CR should definitely never be less than about 45m3, so your LR should not be less than around 225 m3. At just over 300m3, yours is OK, but only just, and bigger would be better.
There's a real-world reason for this as well (not a text-book reason... ): Since you do want to be able to hear what the LR sounds like when you are sitting inside the CR, listening to the sound of the mics that you have set up on the instruments, then the decay time of the LR must be considerably longer than the decay time of the CR. If not, then the "reverb tails" of the LR would be lost: you would not hear them in the CR, as the CR itself would have longer tails. And since the decay time of your LR needs to be in around 200 to 400 ms (depending on size, volume, etc.), the decay times of the LR need to be substantially longer than that.
I could go into more detail, but I don't want to bore you, nor to get you kicked out of the course for defying your lecturer and your text-book at every turn!
The moral of the story is that if you're going to work while sleep-deprived, make extra sure you know what you're doing
Welcome to my world!
- Stuart -
EDIT: You're right, that is the Sabine variable, oops.
I love being wrong sometimes! But now the RT60 is only slightly better than before - 0,85 sec. Am currently looking for calculation errors and inaccurate absorption coefficients