Jack Hammer said:
Savant, thanks for the link. Good info, it helped, but also added some confusion. They say that you really need a 6" min panel to get any real results and that anyone who claims that thinner ones can do the same is either confused or selling oil - of the snake variety.
I really dont want to lose half a foot of headroom (6'8" ceiling). Can I get good results with a 2" panel? They may not be as good as a thicker panel, but will it make a noticeable improvement or am I just going to have to suck up and get the thick ones to really make it worthwhile? I'd rather do it right the first time, or as close as I can afford.
For treatment of reflections, there is a lot of misinformation on the web (as well as widely differing
opinions 
), I'm afraid. Check out the first slide on
page 3 of this article; it shows about were some common absorbers begin to roll off in terms of frequency response. Personally, I don't even like this slide for a couple of reasons:
(a) There are a wide variety of 1", 2", 4", and other thickness absorbers available on the market. They do not all, universally, exhibit this sort of roll-off behavior. E.g., there are 2" absorbers that perform better and there are 2" absorbers that do not perform as well as the generic curve shown in the slide.
(b) This sort of generalization does not take into account off-axis behavior. In small rooms, sound hits panels at an angle, thus increasing the thickness that is "seen" by an incoming sound wave. The effectiveness shown in the slide was developed based on the
reverberant room behavior of an absorber. This sort of behavior (which is atypical for a small room) is not the same as the behavior observed from a single pass of sound through the panel (which is what happens in a small room). Reflections in a small room like a home theater tend to have a very small number of passes through an absorber. So the behavior in a reverberant field - which assumes sound is passing through the absorber many hundreds/thousands of times - is largely irrelevant to small room applications.
This link shows graphically the point I'm making. Please note that the material shown could be any thickness or type of absorber. The only "catch" is that for flat panels, the amount of
reflection from the panel - relative to the same thickness of sculpted panel (like the one shown) - will increase as angle of incidence increases. This is usually not a bad thing, though, since it often lends itself to just enough reflection to make a room treated with flat absorbers sound "less dead" than a room treated with the same amount of sculpted panels having similar published performance characteristics. The absorption relative to an untreated (e.g., drywall) surface is still much higher. But the slight bit of reflection you get off a flat panel can often be quite useful.
Page 3 of the article referenced also talks about the usefulness of an airspace behind the absorber, which further extends the "roll-off" to lower frequencies.
In short, to answer your question, using a good 2" thick absorber - with or without the airspace - to address the mid and high frequency reflections in your room is a perfectly acceptable application. The reference to a minimum thickness of 6" on the link I gave you is, IMO, in the context of addressing a broader range of frequencies. Which, if you decide on something like GIK panels - again, with or without an airspace - would be more applicable to your situation anyway!
