Treatment Priorities That Change With Section Role
Not all acoustic treatment is created equal, and not all sections need the same treatment. The hierarchy depends on what your arrangement is doing.
When drums, percussion, plucked strings, or stabs carry the section, treat the first reflection points immediately. Those are the spots on the side walls where you see the speakers reflected when seated at the listening position. A 2x4-foot panel at each point cleans up transient smearing more effectively than any other treatment you can add.
808s, sub basses, and kick drums make corner bass traps non-negotiable. Without them, the low end shifts character depending on where you stand in the room, and mixing decisions won't translate. Corner traps at least 4 inches thick, preferably 6, flatten the low-end response enough to trust what you hear.
Panned guitars, wide pads, and layered background vocals call for diffusion behind the listening position. Absorption kills reflections, but it also kills the sense of space. A diffusor, or even a bookshelf with unevenly spaced books, scatters sound energy without absorbing it, leaving a natural reverb tail that helps you judge how wide your stereo elements really are.
A lead vocal or spoken word needs the front wall behind the speakers treated. This reflection point is the one that consistently messes with vocal intelligibility. A thick broadband absorber, 4 inches or more, across the front wall gives vocal clarity that translates to any playback system.