My room is 222" long x 147" wide, my ceiling to the bottom of the joist is  80"{ to the top is 90", the cavities between the joists are filled with R=32 mauve tiger. 
 
I have the math down, but I want to know
 
should one mitigate the low frequencies at the bottom of the phase, or at the top, or meet the phase head on at the midsection, in the middle, or floor to joist, or at the shortstop between second and third?
 
I never mix above 75 db at the sweet spot.  But will turn it up to 85 db for a listen, or two.
 
Just thinking out  octopus spider in cyberspace.
 
Jesse Q Screed
 
 

I did a quick conversion to metric (5.64 x 3.74 x 2.03m) ... you have decently sized room, room height will be the biggest issue and cause most problems.
 
Assuming this is a dedicated control room for mixing audio with all options regarding speaker placement (i.e. room not used for anything else, no other fixed installations) I would place speakers at recommended best locations for your geometry (there is plenty of documentation on that), then do the room mode calculation with a simple spreadsheet for all 3 dimensions to see where to expect the biggest problem. Then try to confirm this by measurement (e.g. using Room EQ Wizard 'REW' software) and understand which frequencies are the most serious ones that affect you.
 
Based on that try to build some broadband absorbers. Measure. Analyze. Adopt. Build more absorbers as needed. You can achieve great results in the DYI method ... measure repeatedly. Once the room is half treated and no longer completely empty you can also start listening to reference material as you fine tune further.  This will take you a while to accomplish, but it can be great fun ...
 
BTW, room modes affect you irrespective of mixing level. A decently treated room is also much better at lower levels.
 
 
 
 

These guys will offer fairly good advice based on room dimensions. They helped me...and the advice is free.
 
http://www.gikacoustics.com/
 
 
Regards,
Dan

Hey thanks a lot guys, that information is going to be invaluable!  I'll need to do some more research and work, but it will be worth it in the final phase.
 
Off to work!
 
Jesse Q. Screed

Jesse Screed
should one mitigate the low frequencies at the bottom of the phase, or at the top, or meet the phase head on at the midsection, in the middle, or floor to joist, or at the shortstop between second and third?

If you are unfortunate to have a low frequency strike your trap at the zero-crossing, the absorber will have literally zero effect. At that point in the wave's cycle, there is no energy to absorb. That's why the depth of the absorber has to be a significant fraction of the wavelength of the lowest frequency you need to tame, so that it'll catch at least part of the upper and lower extremes of the waveform.
 
Because you've filled the space between the floor joists, that will give significant absorption down to 550 Hz or thereabouts, with at least some measurable attenuation down to ~100 Hz. Unfortunately, your ceiling-to-floor resonant frequency is 154 Hz. Not much you can do about that with your already-low ceiling. Best you can do is measure the room and find out where your resonances are; just knowing that actually helps a lot.
 
I have a 70 Hz resonance here, floor to ceiling. I have double-thickness pink fluffy in the attic, making it one big bass trap. But even that's not enough to quash that resonance completely, so I just keep it in mind when EQing a mix, knowing my kick drums aren't really quite as thumpy as I hear them in that room.
 
Do you maybe have room in the corners for bass traps?
 
BTW, in terms of room resonances it really doesn't matter what volume you mix at. It's a widely held myth that you can get around room acoustics by mixing quietly. Sadly, it ain't so. What matters is the ratio of the ears-to-monitors distance versus monitors-to-walls-to-ears distance. If the distance to the nearest wall is equal to the distance to your speakers, whatever bounces off the wall will only be 6 dB quieter than the direct signal from the speakers. That's one reason why where you sit and where you place your speakers is a big deal.
 

Hello Bit, your entire response was very interesting, and revealing. 
 
I need to find my resonances. 
 
The one thing I am starting to imagine is that I have the math, and I have reality.  The math can get me so far, but to go beyond the nth degree could be tedious.  In reality, music comes to me like a rush, not just specific frequencies, but all frequencies at the same time, in their place, in their space, and all of this is being constrained by walls, leading to more waves, not at all in relation to their original, but perfect in relation to distance, absorption, and the ability of my hearing to make sense of it all.
 
Visual displays analyzers can help me make a connection between the math, what I see, and what I hear.
 
So I am back to the math, finding that sweet spot, at least as finite as possible,  Trapping (what a funny word for something non carbon based, but they trap quarks don't they?) rogue sounds as much as possible, and financially feasible; as well as learning to listen. 
 
Think, See, Listen
 
Of course to obsess about a room so minuscule, can only get me close, and that would be sweet, but still.........
 
Listening dependent, and because my ears lie, and are insufficient, then I need to see, and think again.  The three together will make it happen.
 
I suppose this is much ado about nothing, and nothing more than my internal mental musings, but I feel confident with all the input from these responses, that I can get as close to sweet as a bear in a beehive; a little sting but worth every precious lick of honey.
 
Thanks Bit, 154 baby, 154.
 
Jesse Q. Screed

It's easy to become obsessive about it, but the reality is that unless you start with a BIG room and an equally big budget, all you can hope for is a frequency response that's flat to 6-10 dB rather than 30-40 dB, and a manageable RT60 of < 500 ms. Truth is, you can train your ears to make do in just about any room, given enough time and repetition. Your brain is an amazingly adaptable organ. Taming acoustics just makes it easier.
 
Here's an old post I made years ago about using any old microphone and SPAN to analyze your room response at the mix position. It's dated, but the principle still applies. For a more detailed picture, get the free REW, which takes it one step farther by showing how frequency response changes over time.

Thanks again Bit, I will be able to work my way through this.  You have been a great help.