Lots of variables and details. Gross room dimensions, ceiling shapes, adjacent spaces -- size and shape and dimensions of doorways, wall construction material (concrete or wallboard), etc. For complex spaces, calculations get complicated.
The size and shape of the space relates to bass resonances and to midrange and treble reflections. For an existing room, I would run a bass sweep with a mic at the listening position(s) to identify resonances, and experiment with bass traps or equalizer. First reflections for midrange and treble are easily spotted by moving a mirror along the walls to identify the reflection points, then apply an absorbant panel or material. While these are not a complete solution, room calculations that may be difficult to perform theoretically can be dealt with using relatively standard procedures.
The size and shape of the space relates to bass resonances and to midrange and treble reflections. For an existing room, I would run a bass sweep with a mic at the listening position(s) to identify resonances, and experiment with bass traps or equalizer. First reflections for midrange and treble are easily spotted by moving a mirror along the walls to identify the reflection points, then apply an absorbant panel or material. While these are not a complete solution, room calculations that may be difficult to perform theoretically can be dealt with using relatively standard procedures.