Current proposal, with outstanding questions.
Please chime in with your experiences and suggestions for those prefixed with a "Q" !
Four walls:
1) first layer: dampened gypsum boards (air gap filled with dense Rockwool)
2) second layer: metal-plate Helmholtz bass resonators (tuned to the ~32Hz and ~43Hz axial room nodes)
3) third layer: P23 quadratic diffusors with the deepest bass diffusion bandwidth, for the front wall
4) shallower P17 quadratic diffusers for the sidewalls, but believe it's best not to have any closer than 2.0m from the listening position, due to undesirable comb filtering artifacts (which become inaudible at greater distances)
Q1) should the metal-plate Helmholtz bass resonators be mounted directly onto the structural walls, instead on the sheetrock?
Q2) should the sheetrock be mounted over the Helmholtz bass absorbers, and the larger air gap filled with dense Rockwool?
Q3) can the room be over damped with too many Helmholtz bass absorbers, and if so, what might be the optimal % fill factor?
Q4) position/quantity of the sidewall P17 quadratic diffusers - one pair, two pairs, three pairs, or keep all four pairs?
Q5) quadratic 1D diffusers, BAD half-cylinders, or absorption (via dense open-cell foam) at the first reflection zones?
Q6) with so many metal-plate Helmholtz resonators, are corner bass traps needed or can they be disregarded to save space?
Ceiling:
1) first layer: metal plate Helmholtz bass resonators (tuned to the ~49Hz axial room node)
2) second layer: canopy, either P13 quadratic diffusers in a 2D configuration, or /lighter shallower 2D quadratic diffusers
Q1) ~3.5m high false ceilings imply 7.0 roundtrip; as acoustic energy returning from the ceiling are relatively weak and potentially inaudible, are ceiling diffusers a must-have?
Q2) deep, heavy, large P13 1D quadratic diffusers arranged into a 2D configuration?
Q3) or will shallow and light 10x10 or 12x12 "square" 2D diffusors be sufficient?
Floor:
1) base finish: self-leveling epoxy + paint finish
Q1) option 1: vinyl tile flooring (covering the base finish)
Q2) option 2: polyester/nylon carpet "squares" (covering the base finish)
Q3) plywood over concrete, then engineered wood parquet
Carpet/rug:
Q1) option 1: thin rug ("Persian") placed on the rectangle between the loudspeakers and listening position
Q2) option 2: decorative rug made of open-weave fabric/yarn, to discourage over-absorption
Q3) option 3: low-pile carpet (IKEA type, typically available in 2m x 3m size), to encourage absorption of mids/highs
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