3D printing acoustic diffusion panels - diyAudio

I would like to add that for more high end result, of course the backside can also be filled with some more stronger and heavier resin, instead of foam.

I don't know if this is necessary. All that's required is that the material is stiff enough to properly reflect frequencies in the range required. I think this is achievable with a hollow 3D print for most frequencies, maybe painting it would help. If you want to get down into the <<500hz range then you might want to fill it with concrete, or resin, or something.

What if you would print the bottom of each block normally, but the top of each block you would divide to 4 smaller blocks, each different in height?

This type of thing has been done, although I think only in "1D" diffusors, they're called "diffractal" and RPG may still have a patent on it. Basically you put higher-frequency diffusors on top of the lower-frequency ones.

I would not necessarily know how to calculate the design for a 2D "diffractal" design. You might need to put a whole 2D QRD on each square? I think they put a whole N=7 QRD on each panel of the 1D design, so that might be it. 4 levels might not be enough, or would only create random scattering / interference above the maximum frequency... but it might raise the plate frequency, which could be desirable even if you don't get "true" HF diffusion...

Anyway, this is doable with 3D printing and probably not even remotely practical with woodworking, so it would be interesting to look into.