Had friends over for my monthly discussion party tonight, so I was able to make measurements of living room (which requires second person on opposite wall, since there are no straight paths on the floor).
Length: 187.5 inches (15.63 ft)
Width: 157 inches (13.08 ft)
Height: 98 inches (8ft, back), 96 inches (7'10" front), 110.5 (9.21ft, center)
I'm running the program ModeCalc from RealTraps, which produced the plot above. I used 8.6ft average height.
Length has modes which are multiples of 36.15 Hz (36, 72, 108, 145, 181, 217, 253)
Width has modes which are multiples of 43.20 Hz (43, 86, 129, 172, 215, 259)
Heigth has modes which are multiples of 65.70 Hz (66, 131, 197, 263)
Total volume: 1758 cu ft
Ratios: 1: 1.52 : 1.90
Note that the height is an average one, so the 65 Hz sequence, and the ratios, are fuzzy.
If I chose to use max height, for example, the ratios would be:
and the height modes would start at 61.35 Hz.
Given that all rooms have to have modes, it doesn't look bad, the modes are reasonably well spaced and don't tend to pile up much. You notice in all such plots that as you get to higher frequencies, you get more mutual node re-inforcement because on a log frequency scale the frequencies get closer together. So there's a gradual increase up to the maximum calculated 500Hz. Below 125Hz, there is only one area where two node-multiples come together. That is the 2nd harmonic of 36hz (72 Hz) and the primary height mode of 66 Hz, and the height mode should be somewhat soft because of the center-peaked ceiling. In fact, I have noticed a mode in that area. Though RoomCalc doesn't show it as a pileup, you could also argue there is a pile up because of the two lowest principal modes at 36Hz and 43hz (caused by room length and width). That's actually a 1.2 ratio, which is about the smallest acceptible such ratio. Then there is a more pronounced pile-up at 130Hz (2nd harmonic of room width by third harmonic of height). Then the next next pile up is with 215 and 217 Hz. Even "good" ratios have a few pile-ups like that. But in my system, dipolar speakers play above 85 Hz and their output seems to stimulate room modes less.
So, not that I could change the dimensions anyway, they don't look too bad. It looks good enough that you might believe the house developer (low cost San Antonio builder Rayco, which sold out to national developer K&B about 10 years ago) actually considered the acoustic properties in the floorplan, which has no doubt been used countless times. No, it's not one of the "optimal" ratio sets, but it's not bad either.
Anyway, I now agree with what Real Traps says. Regardless of how the room physical or acoustic measurements work out, if you are starting from a pre-existing room, about all you can do is add as much bass trapping as you possibly can. So that is one of the next projects. It will also be useful to run before-and after measurements to see the affect of different materials and different placements.