Since I opened this can o'worms in another thread, I might as well finish the explanation.
Originally Posted by pdxrealtor
What I don't understand is how you calculate the speaker distance based on the results from the calculator.
No more difficult than reading numbers off your room mode chart.
Calculating modes is easy: divide the speed of sound by a room dimension. 1130 ÷ 16 = 70.6 (round up to 71). But 71Hz is not your first width mode because half a wave can bounce back and forth between your side walls and complete a wavelength. So your first width mode is at 35Hz, and multiples thereof (71Hz, 106Hz, 141Hz, etc).
Imagine a ragged landscape with a mountain and a valley. If you slice off the mountain peak and flip it over into the valley, you'll get a flatter landscape. Likewise, if you place your sources of pressure (subwoofers, speakers) at room locations that have the least pressure (nulls), then you can get a flatter response by nixing peaks & dips that were a result of modal problems.
Looking at your width mode chart, notice the numbers at the middle bottom:
Placing a subwoofer 8 feet from your left wall (midpoint of room width) will mitigate the 35Hz width mode. Those numbers are right there in black & white (well, blue & grey). If you've got 2 subwoofers...
...Placing them at 4 feet and 12 feet from the left wall (1/4 of room width from the side walls) will mitigate the 35Hz and 71Hz modes. Above the crossover point, you'll have to use the pressure generators (woofers) in your 3 front speakers, the locations for which are...
...Right there on your chart, at the 3 null points of your 3rd width mode, with the L/R speakers 2.75 feet from the side walls.
BTW, this isn't some theoretical exercise. If you played a 106Hz tone and walked across the width of your room, you would actually hear very noticeable peaks & dips at the locations shown in your chart. Place those 3 speakers at those 3 locations and those peaks & dips would be gone.
You can likewise use your length mode chart to choose seating position. If you're ever tempted to place your seating at the midpoint of room length...
Notice that the frequency response will have some severe peaks & dips.
By comparison, if you place your seating at one of the third divisions of room length...
...Notice how much closer together those frequencies are, giving you a smoother overall response. Even the 3rd lenght mode (in red) is relatively close in level to the other frequencies. Not perfect, but better than other locations in room length.
If you've noticed a pattern, it appears that even divisions (halves, quarters, sixths) work better for speakers while odd divisions (thirds, fifths) work better for seating. Pretty easy rule of thumb to rememeber.