Originally Posted by dragonfyr
Just a few comments...
The primary problem you will experience will be with LF room modes. Use Room EQ Wizard to generate waterfall plots to identify the modal pattern.
I would seriously suggest you begin with 'pink fluffy' fiberglass 'superchunk' style vertical corner traps* as well as with corner traps around the 4 ceiling/wall junctions. I would also face the bass traps with no thinner than a 6 mil plastic face in order to make the trap frequency selective - to absorb the LF and to retain the mid and high frequency energy in the room.
And then, after all of the room treatment is complete, I would follow that with and identification of the remaining LF anomalies and use RoomEQWizard to establish EQ filters to further address the remaining LF issues below 80 Hz.
As far as the walls being concrete, the only issue they exacerbate is the LF modal response - as the y are massive enough to reflect all of the high energy content LF energy. As far as specular reflections above the modal region, their behavior will be effectively the same as for any other reflective surface.
For this behavior, the use of the ETC response will identify the precise paths of any high gain early (or late) reflections. And from this information, you can easily employ absorptive panels to treat the ACTUAL high gain reflections.
The most cost effective treatment for specular energy would be fiberglass or rockwool panels. Making a framed broadband absorptive panel is extremely simple and cost effective. For broad band absorbent panels, the best material density characteristics are either ~3lb/ft^3 Fiberglass or ~4 lb/ft^3 mineral wool.
And the minimum configuration that should be considered for a broadband panel is 4" thick with a 4" boundary gap. (Any thinner and the effective low frequency extension suffers. In other words, they are effective against high frequencies but their absorptive ability does not extend low enough to address all of the reflected problem energy.)
This configuration effectively behaves similarly to an 8 thick panels placed flush to the wall.
Forget the foam. It simply does not present the same value equation in terms of effectiveness per unit cost compared to Fiberglass or Rockwool.
And I would not simply proceed to allocate the preponderance of your funds for broadband panels, as the modal response will constitute the preponderance of the immediate room problems.
Instead, you will want to ONLY surgically use the fewest broadband panels necessary to treat ONLY the REAL high gain focused reflections, and not simply apply them as one's intuition, imagination or a mirror may dictate that will simply result in a dead room.
Thus, the balance of the attention to the room will be easily 3/4 towards the modal behavior, and the remaining 1/4 for specular control.
* Superchunk traps that have 2' wall interfaces and an ~34" face filled with fluffed 'pink fluffy' fiberglass - generally employed insulation used for attics that has been fluffed for maximum loft.