Acoustic panels - how high?

It is possible since there was no difference in sound that the room is fine without them.
Another + for Elwood though. I used to have a few roomtunes etc..until I rebuilt the room with Crown moulding and added art to the walls.

Regardless of a speaker system's dispersion pattern, sound waves travel in a straight line and their angle of reflection will equal their angle of incidence. Thus, an easy way to find the first reflection point, of the waves that will actually reach your ears, is via the method mentioned by Elwood. (http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reflec.html) Yes- a panel that will disperse or absorb those reflections must be chosen.

Further info, here: (http://realtraps.com/rfz.htm) & (http://www.gikacoustics.com/video-early-first-reflection-points/)

Thanks, good info. I have treated first and second reflection points on side walls and improved things quite a bit. Think I will now experiment with a panel on my ceiling. No mention anywhere about treating the front wall either directly between or behind the speakers. Any thoughts?

It all depends on the geometry and the physics. In reality the second reflection points can be worse than the first ones. It all depends on the radiation pattern of the speakers, the dispersion pattern. That's why attention to the placement must be done by ear not some generalized instructions. Otherwise you're only finding the local maximum. Same goes for Tube Traps, you will not find the optimum locations by following the instructions of just plopping them down in the room corners since it depends on where the standing waves are actually located which might easily be two feet from the corner! Hel-looo!