Diffusers

If sound waves hit a smooth, sound-reflecting surface, they are reflected back unchanged.
This is called a hard reflection. High-energy, hard reflections are very similar to the direct signal, except that they reach the listener with a certain time offset and come from a different direction. Both signals overlap and lead to a very unfavorable comb filter effect, which distorts the sound, but also to a loss of the stereo image, because the brain locates the sound source from a different direction.
These problems can be solved by absorbers. However, since a room can easily be overdamped with absorbers, diffusers come into play. Thanks to their mathematically optimized surface structure, they can perfectly disperse the sound, i.e. diffusely reflect it.