I apologize if I went rogue on the rules. I looked them over more thoroughly and will do my best in this post to stay on track.
You fixed it! Location in profile is The Big One!
I've been researching and accepting guidance from a few regulars on the GS's forum. SBIR is a direction I was guided in from a few fellow GS'ers as I originally suspected my issue was modal but then questioned it upon their recommendations
It's fairly easy to tell the difference in most cases. Modes are standing waves related to room dimensions, and therefore occur at fixed frequencies and fixed locations in the room. The frequency of each mode is fixed, rigidly, because it is directly related to that specific wavelength fitting perfectly into a path between 2 walls (or 4 walls, or all six walls). If a mode is triggered, the frequency will be very stable (unless your walls are wobbly!), but the intensity will be different at different locations in the room, as you move to different parts of the "standing" wave. That's why it is called a "standing wave": it appears to be stationary in the room, not moving (it does, in fact, move, but it appears to be stationary becuase the pressure peaks and nulls always occur at the same spot in the room).
So the sure sign of a mode is that the frequency does not change at different points in the room, but the intensity does, and it also "rings": it carries on going for a short time after then cause has disappeared. That "short time" can be a few milliseconds, or it can also be several seconds in extreme cases, in large rooms.
The other issue, which is very different, is SBIR. As the name implies, it is all about the relationship between the speaker and the room boundary. It is purely a phase cancellation / summing issue. It is caused when the wave laving a speaker hits a wall and bounces back the same way it came. So now you have two identical waves (the original, and the "bounce"). The only difference is that the "bounce" is delayed, since it went out and came back, which requires time. That time delay is exactly the same as a change in the phase between the original source wave, and the "copy". There will be some points where the wave is in phase with its copy, so it will be louder, and some points where it is out of phase, so it will be quieter. In extreme cases, the "quite spots" can be pretty much total nulls.
SBIR also occurs at very specific frequencies, but with a difference: That frequency is related to the distance that the "bounce" wave travels. In other words, it is related to the distance between the speaker and the wall that the wave is bouncing off. If the distance is small, the frequency will be relatively high, because the time delay is short. If the distance is large, then the frequency will be lower, because the time delay between source and copy is longer.
So that's the key; you moved your speaker away from the wall, but the frequency of the problem you are experiencing, did not change. It stayed at 50 Hz. If it was SBIR, the frequency would have gone lower as you moved the speaker away from the wall. Since the frequency did not change, the problem you are seeing is not SBIR. Or at least, it is not SBIR from the front or back wall. It might still be related to the side walls, floor, or ceiling, since those distances did not change. However, the total distance traveled by the wave even to the side walls, did still vary a bit, as you moved the speaker, so you should have seen some variation.
There's a couple of other interesting issues that can help identify SBIR. Firstly, it does not "ring". A mode rings, becuase it is a resonant phenomena: the mode "stores" energy, then releases it again slowly. SBIR is not resonant: there is no stored energy, just phase addition/cancellation, so as soon as the signal changes, it stops, instantly. And secondly, because it is a phase issue, it shows up as comb filtering in the frequency response. If you have SBIR at 50 Hz, then it will also be there at 100Hz, 150Hz, 200 Hz, 250 Hz, 300 Hz, etc. all the way up the entire spectrum. So you will see that as a "comb" shaped pattern of intensity peaks and troughs in the frequency response plots, if you look carefully.
I'm surprised the guys at GS didn't point all of this out for your: There's some good guys over there.
I am attaching a few pictures (adjusted in compliance to the rules).
Great! That certainly helps to understand your room better, and immediately there's several issues that are apparent, but I'll get to those later.
First: . . .
Length - 28' 7"
Width - 11' 9"
Height Left - 6' 4"
Height Right - 8' 1 1/2"
Ouch! That's a long narrow room, with a low ceiling. That's a really tough acoustic environment to deal with. Not only that, but the ceiling slopes in the wrong way! It slopes left to right, but should slope front to back. On the other hand, if you set up your room rotated 90°, that would be even worse, as the speakers would be firing along the short axis, not the long axis, and your head would be too close to the rear wall....
In addition, it is just too long for the width and height. Most of the good ratios have the length less than 2 times the width or height, but in your case it is about FOUR times the lowest height, three times the other height, and still more than twice the width. Not a happy situation at all.
Hmmmm....
But anyway, I plugged those numbers into a room mode calculator, and what is the very first thing I notice? At 48.1 Hz, is your primary width mode! Yup. Your 0,1,0 axial mode is pegged at 48.1 Hz. I'm betting that is your problem. And at 52.0 Hz, you have your 1,1,0 tangential mode, also a primary mode, related to both width and length. That would be my second bet. They are both right there, very close to 50 Hz, where you say your problem is happening.
I would need to see the actual REW data file to be certain, but that's where my money is right now.
The current treatment in the room:
. . . is overdone! There's too much raw absorption in there: I have not yet seen the data, but I'm betting that room is dead, muddy, lifeless, unpleasant. I'm betting the decay times are around 100 ms, or less. I'm betting it is difficult to mix in there, and fatiguing....
Most of your treatment is actually correct, but there's too much of it that is killing the room. You need to do things to add some life back again.
I later discovered that Knauf does not provide G.F.R. details on their products
That's OK. There's a reasonably reliable relationship between the density of each type of insulation, and the GFR. If you know the density, you can roughly figure out the GFR.
I mistakenly assumed any fluffy insulation would provide desired results.
A common mistake!
This has left me wondering if I should simply rebuild my corner traps.
Perhaps, perhaps not. What is the density of the insualtion you used for your superchunks? What type is it? (fiberglass, mineral wool, something else). Is it fluffy, semi-rigid, rigid?
the golden cuboid setup listed on the Cardas site
Well, there's your problem! There is no such thing as a "golden cuboid", outside of the realm of one person who invented it, and a bunch of his followers. If you don't believe me, Google it, and try to find some actual mathematical research paper published in a respectable journal, that presents the theory behind this concept. Try to find something that does not trace back to that one single original source... (Good luck!
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In reality, there are many good ratios for a room, and several possible speaker/mix position layouts that work well. There is no such thing as a "perfect" room ratio, or a "perfect" setup in a small room. There just isn't, because the laws of physics prevent it! There cannot be. THe room has to be large (very large) before you can get close to that ideal, but for pretty much all normal studios (both home and pro), there simply is no possibility of having a "perfect" situation, where there is true statistical reverberant behavior across the entire audible spectrum. In practically any studio, there will always be an uneven spread of modes in low frequencies, and therefore uneven frequency response across the low end of the spectrum, regardless of the dimensions, ratios, speakers, locations of the speakers, location of the mix position, or anything else. It simply is not possible.
That said, a lot of real scientists and acousticians have done a lot of real research on the math and science behind this subject, and come up with a set of rooms ratios and layouts that are pretty good. Not perfect, but plenty good. People with names like Louden, Sepmeyer, Bolt, and others have published papers and equations that they derived, with experiment and math to back them up. Those are the room ratios you should be looking at, not some mythical "cuboid" that nobody every heard of.
Unfortunately, the dimensions of your room put you way, way outside the scope of all of the "best" ratios, and all of the "good" ratios. There might be a couple of "mediocre to poor" ones in that region, but I doubt it. I'd have to check, but I don't think it is worth it. Even if there is one, it won't be "good".
All of these have displayed Null's in the lower frequencies.
Yup. Because it is a small room that is very long and thin, with a very low ceiling! No matter where you go in that room, there will always be modal issues.
63hz was the culprit in the golden cuboid position
... Yup. That would be your 2,1,0 tangential mode, at 62.3 Hz. Also related to width and length...
Moving the speakers and mix position rather close to the front wall brought the low end loss back.
I could give you my suggestion for that room, but I'm not sure that it is worth your while even trying to do it. It won't be a huge difference.
The big issue here is that you are fighting the room itself, not the treatment, layout, or speaker ports.
What I woulds suggest is two things, and they both involve a lot of work.
1) Make the room shorter. Build a wall across the back at a point that would give you a good ratio, or at least a much better ratio than what you have now.
2) Make the ceiling symmetrical, such that it is the same on the left and right. Yes, that would imply making it lower on the right to match the left, reducing room volume, etc, but the difference at present is just too great. Symmetry is critical for a control room: your left ear must hear the same acoustic "signature" as your right ear hears from the room, and the way you have it right now, there's a huge difference. Even though you would lose height, you could minimize the loss by having a gabled ceiling, with each side sloping down the same form a central peak. This is not the issue that you are dealing with right now, but it will be. After you solve the low-end mulchy mess from the room length, that would be your next problem. You might as well deal with both issues at once.
Yep. Probably not what you were hoping for! Nobody likes it when they hear "Your room is terrible. There's no hope. Rip it apart and build it again." That's NOT something that any studio owner wants! But in your case it is the only realistic solution I see.
I'm not sure if you are prepared / willing / able to do that, but its the realistic solution. If you can't do that, then is there another room in your house where it might be possible to set up your studio? Even if it is small, it would probably still be better than this.
Sorry to be the bearer of bad tidings,....
I'm more than happy to take REW measurements and post them.
Let's see the one where you had the mix position at 37.5% of the room length. Upload that one to a file sharing service, and post the link here. (MDAT files are too big to upload onto the forum.)
I'll take a look at that file for you, but I can pretty much predict what it will show. I could tell you how to improve some of it, but there's no treatment or tuning that can fix a low ceiling and a drastically long thin room. In medical terms: I can show you how to put a band-aid on your scraped knee, and put your sprained arm in a cast, and take a pain killer for the headache, but there's nothing I can do for the real problem: That gaping hole in your chest where you just got shot with a bazooka! (Sorry to be a bit dramatic....)
- Stuart -