What is it you are trying to do?
If the point is to reduce room modes and uneven bass due to standing waves, you want your absorber to be 1/4 wavelength out from the surface where the air velocity max (and pressure min) occurs in the standing wave. You don't want the absorber at the surface where the pressure max and velocity min occur. If the absorber is placed closer or farther from the surface, absorbtion will be less effective. Obviously 1/4 wavelength is a different physical distance for each frequency, so an absorber at a given distance from the surface is more or less effective at different frequencies. If you want a "bass trap" to absorb best at say at 100 Hz, 1/4 wavelength would be 1130/100 x 1/4 = 2.8 feet out from the wall. To absorb 200 Hz, ideal spacing would be 1.4 feet out from the wall. Thicker absorber, like 6" or 12" fiberglass or rockwool batting will absorb more effectively than thinner, because it is close to 1/4 wavelength out for more different frequencies and also because it attenuates the sound more. Multiple layers of heavy drape stage curtains serve this broadband absorption purpose very well.
For a much more narrow purpose, for example to kill early reflections from sidewall or ceiling, as in a recording studio control room, you want to absorb mainly higher frequencies. Wavelengths are much shorter, so a 2" thick fiberglass panel placed 4" out from the wall or ceiling will be very effective for a broad range of higher frequencies, although it won't be a very effective bass trap. It all depends on what your goals are. Lately, more studios are using diffusors to reflect sound in many directions and let the room sound more live rather than absorbers that make the room sound more dead.
I also highly recommend F. Alton Everest's Master Handbook of Acoustics as a nonmathematical and very readable reference book on these topics.
If you are serious about acoustics and are investing significant money and time in a sound space, it would probably be worth your while to hire an acoustician to advise you or to design the acoustics. Low budget, check out Everest, or other books by Rod Gervais or Philip Newell. If you want to know all there is to know about diffusors and like math, check out Cox and D'Antonio's book on absorbers and diffusors.