High-bay and atrium ceilings are 20 ft and above. Very large volumes need large amounts of absorption, and the ceiling is the only place to put most of it.
At 500 Hz the bare ceiling absorbs about 5% of the sound that reaches it. In a room with a hard floor, the ceiling and floor reflect sound back and forth, so treating the ceiling is often the single most effective step.
Options: clouds, baffles or ceiling tiles.