Like I said Mike, my acoustics knowledge is poor but I THINK the answer is that absorbers work because the air particles lose energy by converting motion to heat. By definition, the particle velocity at a solid surface is zero. Some distance away it has speed. I dare say there is an optimum distance? A combination of the material depth, frequency and practicality?
I do know it was proved by the grandfather of speaker design, G A Briggs that cabinet absorbents on walls are far less efficient than those in the centre part of the cab.
My acoustical property knowledge isn't what it should be, either. But I AM an engineer - and Wave Physics was my most-hated physics class at McGill!
We have to remember that sound waves don't move the air particles in a 'velocity' sense (if they did, every sound would make a breeze or wind we would feel - when we feel the air move from a loud subwoofer, we are feeling the physical moving of the air from the physical movement of the speaker, not the actual soundwave, although they are in synch for a short distance), the air particles 'vibrate' - the louder the sound, the more they vibrate, and yes, sound absorbing material converts that energy to heat. The speed of the wave is based on the density of the medium it passes through so the denser it is, the slower the wave propagates (with different frequencies having different speeds in different mediums).
So, if there is no air space at all between the absorber and the wall, the wave moves at a slower velocity out of the absorber, bounces off the wall and back into the absorber at the same velocity. If there's an air gap, the wave 'speeds up' as it leaves the absorber, bounces off the wall, then 'slows down' as it goes back into the absorber. At least that's how my old brain sees it! So, yes, space between absorber and wall means more air particles get vibrated by the soundwave, which does absorb a very small amount of the energy, but I don't think it would be very noticeable.
I wonder if anyone has does measurements of the same panels (at different frequencies) directly against a wall and with air gaps of various amounts?