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Old 17th October 2019
Lives for gear

I am not sure I am too familiar with the spring constant (although I assume he is talking to something to do with the tires being quite booyant and **** possibly producing ill effects and doing little to effect any sound) but resonant frequency is something to take very seriously.
You have the basic idea.

It works like this: If you want to "float" a structure (any structure) so that it can't transmit vibrations into whatever it is sitting on (which is what you are trying to do with your "room in a room" that you mentioned), then you need to be sure that it really does "float"! By "float", I mean that it is decoupled from the base that it is sitting on, sort of like a boat is "decoupled" from the river bed under it, by all that water. The boat can rise and fall on the waves, completed unaffected by the river bed. You are trying to do the same with your isolated room: you want it to "float" in the same way as that boat, so that any "rising and falling" from vibrations in the room, cannot get down to the base.

OK, that's the concept, and it's simple to understand in principle.... but its a bit more complex in practice, as you already figured out!

Here's the thing: an object can only float if the "thing" that it is floating on is "soft" enough (water, for the case of the boat, rubber for your room), and it can only float for vibrations above a certain frequency. There is always a lower limit to the frequencies that it can isolate. That limiting frequency is the natural resonant frequency of the object (your room) along with the rubber. It's a resonant system, and it resonates at a specific frequency. In simple physics, it is a "Mass-Spring" system. Just like a weight sitting on a spring. There are very simple mathematical equations for calculating the frequency, and the only two things you need to know are how heavy the mass is, and how "springy" the springy is. That's what the "spring constant" is all about. Its the technical term for "springiness". That's why I asked what the spring constant was, in that earlier suggestion. Because if the spring is too "hard" (not springy enough), then the boat won't float!

Think of this: if you have a heavy weight sitting on a spring, and the weight is so heavy that it squashes the spring completely flat, then obviously, that isn't going to float! Its not going to isolate. Also, if the weight is so light that the spring isn't compressed at all (maybe with just a feather on it), then it also won't float... and it won't isolate. You have to use just the right type of spring for the weight, in order to make it "float". In fact, you need to use a spring that will compress by about 10% to 30%. If it compresses more than 30%, then it doesn't float. If it compresses less than 10%, it doesn't float. And that range also depends on the type of spring: for example, neoprene rubber has a different range than EPDM rubber, Sorbothane rubber yet another range, and a steel spring is different again.

As you can see, it's not just as simple as putting any old spring under any old weight! there's math and physics to think about, equations to do. It is very important to get the right spring for the weight of your room. If not, then either your room will over-compress the spring, and it will "bottom out", meaning that it doesn't isolate... or it will "under-compress" the spring, so it "tops out", and it won't float... and won't isolate. You have to get it right, if you want your room to actually be isolated.

It would be rather sad if you spent all that time and money and effort to build your floating room, then when it is finished you find that it does not actually isolate, because the spring you used was too hard, or too soft...

If you look at YouTube, you'll find a whole bunch of videos there of people who are trying to float their rooms with a couple of 2x4s resting on rubber pads, and a plywood deck on top... thinking that they are "floating their floor"... then at the end wondering why their room sounds like garbage, and the neighbors are still complaining! The sad truth is, that in order to float a floor properly, you need a huge amount of mass, and the exact right spring. 2x4 joists on rubber pucks is way, way short of what is needed. I'm posting a graph below that shows the actual, real, true, tested, situation, not YouTube fantasy. You can see for yourself why bits of rubber and 2x4's don't work.

That graph is for the case where you use air as the spring. Air is actually a great spring, believe it or not: far better than a coil of steel, or a piece of rubber. It is much "softer", and that's a GOOD thing! You want the spring to be as soft as possible. The harder the spring, the less it isolates.

So, the graphs shows four different curves, for four different weights ("psf" means "Pounds per Square Foot"). On the left side, it shows the resonant frequency that you would get for various combinations. As you already realized, what you need is to get your resonant frequency lower than the lowest frequency that you'll have in the room. In fact, for complex mathematical reasons, you need to get the resonant frequency one octave lower, which means half the frequency. You used the example of 33 Hz, which is very valid, so let's go with that: if you need to isolate frequencies at 33 Hz, then you need to make sure that the resonant frequency of your floor is less than half of that, or less than 16.5Hz. So look at the left side of the graph, locate the spot where 16.5 Hz is (call it half way between 10 Hz and 20 Hz), and draw an imaginary line across the chart there. You can immediately see that your line will NEVER meet the curves for 5 psf or 10 psf, and only just touches the curve for 30 psf, way over at the far right. So the only real option you have, is the last curve: 60 psf. Your line will hit that somewhere around the middle of the graph, at roughly the 3.0 mark. So, that means that if you ONLY used air as the spring under your floor (large air bags filled with compressed air, for example), you would need to have 3" thickness of air... and you would also need to have a floor that weights 60 pounds per square foot! So if your room is 100 square feet, the floor would have to weigh six thousand pounds! That's what you'd need for 60 psf. Now, consider that a sheet of really thick 3/4" plywood weighs about 3 pounds per square foot... I think you see the problem! You would need TWENTY LAYERS of 3/4"plywood to get enough mass on your floor! Now you can understand why those folks on YouTube don't have much success... The only reasonable way to get enough weight there, is by making a 4" thick reinforced concrete slab. Concrete is about 4 times more dense than plywood, so it can be one quarter the thickness for the same total weight.

OK, that's for an air spring: what about a rubber spring? Or a steel spring? That's a bit more complicated, but it turns out that you still need a huge amount of mass to make it work... and there's one other important point here: Even if you use rubber pads under your floor, you still have an air gap down there! And the same graph applies to that air gap! In fact, you need a larger air gap now, because the rubber and the air are in parallel, and the rubber is a worse spring than the air, so you need to have more of both, and softer rubber, to make it work.

Yes, it's complex. Floating a floor, or a room, is a Big Deal. It can't be done successfully by just putting down some random bits of rubber thingies with a wooden deck on top! There's no way that can work. For people who do know how to float a floor and how to do the math, suggestions of rubber pucks, tennis balls, bicycle inner tubes, and other types of rubber, are just plain silly. Especially when the floor on top of that is just a layer or two of plywood with some laminate flooring, or vinyl tiles on it! It's a joke in bad taste. Anybody who actually did that in real life will end up with a failure: a floor that does not float, and is probably illegal anyway! There's a thing called "building codes", which are laws and regulations that govern how you are allowed to build something. Putting unapproved materials under your floor is going to cause you major problems with that!

Ok, so getting back on track: floating your floor, or your entire room, might not even be necessary. The vast majority of home studios do NOT have floated floors... Firstly because it is usually not needed, secondly because it is really, really hard to do right (as you can see from the very simple explanation above), thirdly because you need a huge amount of weight.... that your current floor probably could not handle anyway, and would collapse... and fourthly, because it is insanely expensive to do it.

Let's start by figuring out if you even need to think about floating your floor, and if it is physically possible to do that: What type of floor do you have at present? Is it a concrete slab? A wooden floor? Something else? And what is underneath that floor? Does it rest directly on the ground, or is there another room down there below you?

If you have a wooden floor, then it is pretty much impossible to float your studio, as a wooden floor would not be able to support the huge weight of a fully floated studio. Even if you have a concrete floor, it still might not be possible, if you are on an upper floor with other rooms below you: even concrete has limits. You would have to hire a structural engineer to come take a look at your place, and tell you how much additional weight you could safely put on that floor. The only time you would probably be OK, is if your floor is a concrete slab that sits directly on the ground. And it that is the case, then you don't need to float the room in any case! A concrete slab on grade is an excellent studio floor, with good isolation all by itself, so there would be no need to float your room.

So that's the situation: You probably don't need to float, or cannot float anyway, so I would not bother wasting too much time on that. Rather, look at what you CAN do, which is to build your room such that the walls, ceiling, windows, doors, electrical system, and HVAC system are all isolated. You can still get good isolation for your studio like that, even without floating the floor.

But basically what he is saying that light materials bad, heavy materials good.
Yep! Exactly.

Everything in the world has a resonant frequency and when anything is exposed to it's resonant frequency it will experience certain phenomena mostly meaning it will start to produce excess vibrations itself thus defeating the properties of the noise cancellation for a given frequency band and harmonics and also exaggerating all sounds of a similar frequency in close proximity.
Exactly! Spot on! You obviously already understand a lot of this stuff. So it's just a matter of figuring out how to put that into practice, to isolate your room.

And so if you didn't know audible sound generally ranges from ~20hz ish i believe to some number around or above 20Khz (maybe higher say scientist) but also most studio monitors even in the 8 inch class struggle at producing bass frequencies at that level accurately so your bass is likely to be mute before it is to do this in a noticeable
Right. The audio range is generally considered to be 20 Hz to 20 kHz. There's actually not much going on at the extremes: the only two instruments that get down below 30 Hz, are the cathedral organ, and the concert grand piano: It's more realistic to think of the lower limit as maybe 28 Hz or so (a six-string bass gets down to about 31 Hz).

That said, even though small speakers don't have good specs for low frequency, they can still put out a lot of energy below their so-called "cut-off" frequency. A speaker where the spec says it goes down to 40 Hz, for example, could still put out a fair amount of power even at 20 Hz, which is only one octave lower.

unless you are applying an ungodly amount of EQ and as a result really driving your amps to push in the low end (probably slowly killing them)
Right! And if you do that, then your mixes sound terrible when you play them any other place outside the room!

I just need an incredibly small sound proof space for mixing and studio monitors that is as neutral as possible acoustically
Those two concepts don't go together, unfortunately. You can't have a really small room that is acoustically good for mixing. That's sort of like wanting a bottle of water that is dry, or a match that burns cold... In order to get the neutral sound that a control room must have (you are totally right about that: it must be neutral), then room has to be large enough for the acoustics to work. The specs for pro control rooms say that you need at least 215 square feet of floor space, to do that. Now, don't get too worried: you can actually get decent sound in a room that is smaller than 215 ft2: it's just easier to do it if the room is bigger. I have done rooms down to about 110 ft2 that turned out quiet successfully. But I would not go much smaller than that. The smaller it is, the harder it is to treat, and the less good the final result will be. Once again, it's all about the laws of physics... there are limits on what can be accomplished. So make your room as big as you can.

Anyone please correct me if anything here is f'ed up beyond reconciliation I am doing my best...
You are doing fine! You are asking the right questions in the right place, and you already have a good grasp of some of the concepts. It's just a matter of working through the right procedure now, step by step, and things will start to make a lot more sense.

- Stuart -
Attached Thumbnails
I'm tryna build something crazy - would like input pleez-resonant-frequency-floating-floor-mass-gap-graph-s02.jpg  

Last edited by Soundman2020; 18th October 2019 at 05:55 PM..