There are a dozen node and null center location points in any room. The math is simple for the most basic border reinforcement...
1128 / distance in feet from wall (surface) = wavelength of frequency of distance.
1128 / D = F
Full reinforcement starts at the quarter wave frequency, or...
F / 4 = start of reinforcement
So, if your ears at 3 feet from the floor, the bass below
1128 / 3 = 376 Hz
376 / 4 = 94Hz
So, the floor reinforcement starts at 94 Hz where the acoustic energy is 6 dB SPL louder than beyond 3 feet.
If you are sitting exactly 3 feet from the floor, then there will be a null at half the frequency of that wavelength, or 188Hz. There will also be nulls at every octave above that, or 372Hz, and so on.
It gets complicated when you look at all the resonant distances between all the larger reflective surfaces. In a perfectly rectangular "cube" room, you have 3 perfect resonant frequencies. However, sound also travels in tangential directions, but with less reflective surface area. So it is very complicated.
So, if you are seated directly between the two side walls which are 15 feet apart, you will be sitting in a null between the two walls, or 75.2 Hz, and 150.4 Hz, and 300.8 Hz. Then you can add the reinforcement from 0 Hz to 94Hz from the floor and a null at 188Hz and 376 Hz from the floor. It is a mess when one attempts to do all that in your head.
I have two means of discovering the best results in a given room (beyond my instinct based on experience and internalizing all this math and theory)…
1) Use a RTA to measure the bass up to about 500Hz in the current listening location and then make similar measurements with the same settings from logically and aesthetically appropriate locations you'd like to experiment with. Pick one you like, then play a good test track that accentuates what you love and hate about the current location and then move to the new potentially ideal location and listen to the tracks again - preferably within a few minutes of each listening test. If one is obviously better, move there for awhile and see if the issues are less prominent (there are always issues, so the goal is minimize them as best you can).
the other method:
2) Just wing it and try different listening positions for awhile and decide which you like the best - or hate the least.
Personally, I go by the rule that since my ears are likely to be approximately 3 feet from the floor and thus 5 feet from the ceiling (for an 8 ft ceiling), then the rear wall should be 2 feet or 4 feet or 7 feet from your ears. Ideally longer distances are better, so 4 feet or 7 feet would be preferred. Basically, you do not want the same distance to the rear as to the floor or ceiling, nor do you want a perfect mathematical multiple of those two distances.
