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The ceiling plays a lesser role in changing the nature of the system in most small rooms. It's often acceptable to leave it alone.
In larger rooms or rooms with tall ceilings, it may be the source of unwanted added reverberation.
The ceiling and the floor have a similar relationship between each other as do the front and back walls.
Any room that does not have carpeting, has thin carpeting or only patches of carpeting may benefit from some absorption on the ceiling.
Diffusion in the rearward area of the ceiling and overhead the listening area can be beneficial.
If your testing reveals a high Reverberation Time (RT) above 500 ms, adding absorption where possible on any surface may be beneficial.
If you follow the guidelines here for all surfaces, in a typically sized room, added absorption beyond these recommendations is normally not required and may be detrimental.
