Translate This Page
Darn! I'm glad you are only asking easy questions here!!!Would love to hear Stuart's breakdown of the fabric issue ...
Whew! At least it's nothing complex....
That's a relief! 
OK, there's several things at play here. I've covered a couple of them already, such as: GFR = Gas Flow Resistivity = acoustic impedance. Measured in the cryptically named units of "MKS rayls", or also pascal-seconds per meter (Pa * s * m^−1). Which, even more confusingly, is the same as newton-seconds per cubic meter (N * s * m^−3). Or alternatively, kilograms per second per square meter (kg * s^−1 * m^−2).
Now that I have completely confused the hell out of us all, what that really means (as I alluded to above), is that you apply some air pressure to one side of your sample of material, and see how much air flows out the other side. That's it. The relationship between the pressure and the flow, is GFR. It's that simple... (or not!)
What that tells you is how much the material resists air flow (duh! kind of implied by the name: GFR!). When you think about it, sound is just flowing air too (in the sense that the molecules of air don't actually flow very far, but they do "flow" back and forth due to the vibration that we call "sound"). So if you measure the resistance to air flow, you are actually measuring the resistance to sound flow. It's the same thing. Except that more correctly we are talking about how the material "impedes" the flow of sound, rather then "resisting" it... technically, it is not the same thing. In common speech, "resisting" and "impeding" are pretty much synonyms, but not in electronics, physics... and acoustics. But it has basically the same meaning: the material somehow stops some of the sound getting through, or slows it down, or whatever, so that less of it gets through. That's what GFR is all about: acoustic impedance: it answers the question: "to what degree does this material inhibit the flow of gas, or air, or sound?". (To be entirely accurate, I'm taking about specific acoustic impedance here, not just plain acoustic impedance... but don't worry about that yet.... maybe some other day).
So, a high GFR number means "sound has a hard time getting through", while a low GFR number means "sound can get through more easily". Curiously, even air has a GFR number! It is about 400 MKS rayls... which sounds rather silly, as that basically means that air resists the flow of air
but it's not so silly when you think about it a bit more. But I digress!
If you know about about electricity or electronics, then this might help (if not, then skip to the next paragraph...). Acoustic impedance is very analogous to electrical impedance. With electricity, you apply a voltage to a resistor, and you get current flow. How MUCH flow you get, depends on the impedance value of the resistor. Another word for "voltage" is "electrical pressure" or "potential difference". This is identical with acoustics: you apply an air pressure (pressure difference) across the material, and you get air flow. How MUCH air flow you get depends on the impedance value of the material. With electrical circuits, you measure impedance in "ohms". With acoustic impedance, you measure it in "rayls". But they are very much analogous to each other.
OK, so now that you are even more confused(!) lets simplify: you blow through it, and see how much air comes out. Period. That's about as simple as you can state it. If you blow hard and very little air comes out the other side, then it has "high GFR", or "high acoustic impedance". If you can just blow a little bit and already get a lot of air out the other side, then it has "low GFR", or "low acoustic impedance". If you can't get air to flow at all, no matter how hard you blow, then basically the GFR is infinite, and thus the acoustic impedance is infinite.
So far so good!
Now for the next step: If sound cannot get through, then either it must get reflected back the way it came, or it must be absorbed by the material itself, in one way or another. The sound energy has to go SOMEWHERE! One of the most fundamental principles of physics is that it is impossible to create or destroy energy: only God can do that, but we mere mortals, and all of the processes in the universe, cannot do that. Energy cannot be created or destroyed. Fundamental law of physics. All that is possible according to the laws of physics, is to convert energy from one form to another (or in extreme cases, to convert it to or from mass, in nuclear reactions). So, since there are no nuclear explosions going on inside your studio fabric (hopefully not, anyway!!!), the sound energy that arrives at the face of the fabric MUST go somewhere: it cannot just disappear. The only real options here are: 1) it gets converted into low-grade heat energy (several mechanisms can do that, but basically think: friction). 2) It gets converted into "kinetic energy", meaning that it makes something move, 3) it gets converted into some form of potential energy (complicated!! not going there for now!), or 4) it stays as sound energy, but going in a different direction from where it would have gone (usually meaning that it reflected off the material).
So, to summarizes; the sound that hits the non-breathable fabric and can't get through as moving air, can do one of these: 1) be converted into heat, 2) be converted into movement, 3) be stored, 4) be reflected. That's it. There are no more options.
Right, so the simplest one of those four to explain here is number 4: reflection: the sound hit the material, could not get through as air molecule movement because of infinite impedance, so it bounced right back. That's the most likely, and simplest to understand: simple analogy: the rubber ball hit the wall, could not break through the wall, so it bounced right back.
Number two is also easy to understand: the sound wave hit the fabric, but could not get through so it made the fabric move... just a little, of course: it's not going to blow it across the room! (unless we get back to the nuclear explosion thing...). In fact since the air molecules are just vibrating in place as the sound wave moves along, then the force of those air molecules as they rush in, can cause the fabric itself to vibrate, at the same frequency as the sound wave was vibrating. (Sound is just air molecules vibrating in place: the sound wave rushes along at the speed of sound, but the individual air molecule stay where they are, vibrating around their original location). So that's what would happen in this case: the air molecules hit the surface of the fabric, move it a tiny fraction of an inch, stretching it just tiny amount, but then the molecules run out of steam, so to speak, and the stretched fabric "un-stretches" again, pushing the molecules back where they came from... but then the next wave front arrives, and pushes the molecules the other way again, they hit the fabric, stretch, un-stretch, lather, rinse, repeat... In simple terms, the vibrating air makes the fabric itself vibrate.... and here's the kicker... because the fabric is vibrating, then it causes the air on the OTHER side to ALSO vibrate! Tah-dah! The sound wave gets through, even though the air molecules did not!!!
OK, so now we are all confused again! I just said that infinite impedance means that sound CAN'T get through, and now I'm saying that sound CAN get through!
Explanation: this is a totally different mechanism. With this situation, the sound can only get through if there is enough energy to make the fabric vibrate, and that depends on OTHER properties of the fabric: not the GFR. GFR only tells you about how the air molecules themselves will move through the material, bumping each other along. But when they CANNOT get through it (because the material does not allow them to move through), then obviously that GFR thing is not happening any more! So now we have gone beyond simple porous absorption, and we are looking at something else: resonance. We are making the entire sheet of fabric vibrate, but it will ONLY do that at one specific frequency, or a set of frequencies, and none of that is related to GFR. Now it has to do mostly with two other parameters: the mass of the material (surface density), and the elasticity: how tightly it is stretched. That's even more complex to get into, but the basis is simple: pluck a guitar string and listen to the note. Tighten the string a bit, and the note goes up. Ditto for a drum head: tap it with a drumstick and it makes a tone: tighten it a bit, and the tone goes up. Loosen it and the tone goes down. Loosen it a LOT, until it is limp, and there is no tone at all! Your drum head is now a "limp membrane"....
So, your non-breathable fabric on your bass trap is very much like that drum head: if it is stretched even just a little bit, it will have a resonant tone. If it is hanging loosely, it won't: it will be a "limp mass"... and will look rather ugly, since it will be all wrinkled and floppy! For any reasonable arrangement, you will undoubtedly stretch the fabric, at least a little. So it will be a membrane, not a limp mass. And thus, it will want to resonate at a specific frequency (or set of frequencies). An when it DOES resonate, it will allow that frequency to get through to the other side, because it vibrates in sympathy with that tone, and thus it transmits that sound to the other side. However, for other frequencies, it wont vibrate, and won't transmit sound. In other words, you have a tuned trap! It is tuned to some frequency(ies)!!!!
For a bass trap, that might, or might not, be a good thing. Probably not.
Now, the interesting thing about membrane traps, is that they can be tuned "tight" (high Q) or "loose" (low Q), depending on a number of things.... which are a bit complicated to go into at 3 AM!!!!. And in general, they transmit not just their own resonant frequency, but also all frequencies lower than that, while NOT transmitting al higher frequencies.. which they simply reflect back the way they came, or turn into low grade heat, or nuclear explosions...
And now we get back to what I said about plastic foils earlier: in essence, non-breathable fabric IS a foil! It behaves just the same. It allows some range of low frequencies to get through and reflects back the others. The only real difference here is that the fabric is stretched tight, so it also has a resonant frequency due to that. Which is why I also said before that the equation I gave for plastic foil assume that it is NOT stretch tight, but rather is limp Because if you stretch it tight, even a little bit, then you have a membrane trap! Exactly the same as the fabric.
OK, so after waffling on for many paragraphs, I'll try to get to the point: Non-breathable fabric reflects sound. But it does so differently for different frequencies. It might also absorb some sound (by converting it into low-grade heat), and it might also transmit some sound to the other side, which it does through resonance if it is stretched a little, thus acting as a tuned membrane trap.
Not sure if any of that helped at all!
I think I just confused myself, actually....
- Stuart -
And I also agree that everyone has been super helpful and offering great advice and valuable insights. I've already had so many questions answered!
hahah I'm humbled. Glad my OCD comes in useful every now and again. Definitely wish I had taken this to heart sooner. You would've thought after that first studio design you guided me through 8 years ago, I would have made something better out of my house. But I was too caught up in thinking I needed to build from the ground up to get it right when in reality I could have use many of the same principles to make this room WAY better than it is currently. I sort of just let go of the whole idea of having a properly tuned room... 
(Famous last words. 
Just make the whole theme of the place an illusion. "Trompe L'Oeil Studios - You Won't Believe Your Ears"