Follow the directions in this link
Rockyboy...I have mine (12"x48" & 24"x48") exactly centered on the wall. 24 inches above and below the panel. I have 8 foot ceilings also.
I built a very crude stand for them that could be raised or lowered 6 or so inches at a time. I started them at 24" from the ceiling and lowered them until I found the perfect spot. Actually, I got them too low and had to raise them up. I went up and down several times with them to get them right. It just so happened to be centered on the wall when all was said and done.
I do have panel speakers (Magnepan MG3.6 & Martin Logan CLSIIA).
Like almost all room acoustic treatments let your ears be your guide. Since it is almost completely unpredictable how a particular room will interact with X brand speakers unfortunately the only way to achieve optimum or even good results with panels is to experiment - one panel at a time. I.e., if you try to find the best locations with multiple panels at the same time, with the panels all on in place when you listen, it's like trying to solve three simultaneous equations in four unknowns. Tip: when listening to one panel, keep all the other panels in another room otherwise they'll interfere with your experiment, then introduce one panel into the room at a time and listen again.
"stay at your listening position and have a friend move a mirror along the wall until the speaker sound emitting areas are centered in the mirror. This center point can then be used as the center point of the absorber panels."
The problem with that technique (using a mirror) is that speakers do not have uniform dispersion patterns thus making it difficult to visually obtain optimum results (using a mirror). This is especially true if you are listening to all panels at once initially when trying to make judgements. Not to mention all speakers actually have different dispersion patterns, making generalizations about location of panels kind of irrelevant. If you could obtain the dispersion pattern for your particular speakers you can do the geometry calculations and figure out exactly where the panels should go a priori.
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.
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!
OK, so I've just finished playing - yet again...
A friends place has a 12ft ceiling, so I've been looking at possible solutions.
So I got to thinking about my room with an 8ft ceiling and how a "baffle curtain" might help.
I just happened to have a piece of heavy duty vinyl 2 ft x 6ft and a couple of lighting stands, so I rigged it such that the vinyl hung down from the ceiling like a banner, across the room and tried it in a couple of positions...
1. just behind the speakers...
- There was some improvements in clarity and image
- nothing really to write home about
2. just behind the listening position...
- talk about an OMG moment!
- clarity improved beyond my expectations
- image now easily exceeds the boundaries of the room on most all tracks
- projection of venue acoustics forward now easily envelopes the listener on many tracks
- it's like surround sound with only two speakers
- the improvements can easily be heard outside of the room also, just amazing.
BTW - I also have a couple of 15" x 78"(tall) panels on the side walls behind the speakers to deal with an echo that travelled across that wall behind the speakers
One thing I have found is that acoustic panels improve imaging in the dimension you lay them out. That is, if you don't find very deep imaging, put more behind the speakers. If you find the image is in a line between your speakers, put more dampening above and below the speakers, that is on the floor and ceiling.
Of course, getting rid of primary reflections is the first thing you do, I just thought this might help.