Another treatment option is this hybrid absorber / polycylindrical diffuser. A QRD pattern is cut into softwood fir 2x stock (similar to RPG’s Flutter-Free) and then arranged on a large radius similar to a large polycylindrical diffuser. Air spaces are left between the mini-QRDs to allow low frequencies to pass while scattering mids and higher frequencies. The cavity of the treatment is filled with rigid fiberglass to reduce low-frequency build up.
This is a very nice treatment for rear walls of critical listening spaces and for general treatment of live and performance spaces.