filete
Spacing them out from the wall, within reason, increases the LF absorbtion -- which is usually a good thing.
Rule of thumb, airspace the same thickness as the absorbing material gives a good absorbtion for the dollar, because air is cheap. With the exception of Jacques Cousteau, the french haven't started selling Bottled Air internationally yet. (I bought some bottled water this week, and I'm not quite over the disbelief that I purchased it yet)
There are four techniques I've seen:
1) frames. The absorbing material is at the room side of the frame, with an air space behind it.
2) blocks. Auralex
TruSpacers are an example of this. TeaBags (from
modularguy) are mounted with velcro, and 2x4 blocks with velcro on them could hold them out from the wall.
3) suspension. Cables from the ceiling. You don't want these hung too far from the wall or they become less effective. (I don't have any measurements for that statement, so I'm not sure where the transition is, or in which frequencies it applies, although I'll bet it's in the LF.)
4) french cleats. These occupy a bit of space, so end up holding it out from the wall.
16" down from the ceiling is a standard size for ceiling tiles. So there's a few examples of the same material on the wall vs 16" away from the wall here
http://www.bobgolds.com/AbsorptionCoefficients.htm
as well as several examples of absorber building techniques (links)
