Hi, first of all I wanna say I’m enjoying calculix as an FEA newbie, and I’m wishing I can get some help to solve this issue.
I’m open for other workarounds 
Unlike Abaqus, CalculiX doesn’t have such impedance boundary conditions or infinite elements. Its acoustics analysis capabilities are minimal. It might be better to use more specialized codes like Elmer FEM or code_aster. If you want to stay with CalculiX, you would need some workarounds like a large (artificially extended) domain, high material damping or even a bunch of dashpot elements attached to the boundary.
Thanks for the suggestions.
I will look at the workarounds within calculix first, they seem to be feasible.
In case that doesn’t quite work out, is it possible to import calculix .inp to Elmer FEM?
Elmer has a completely different input deck syntax. You could only transfer the mesh using some universal format. FreeCAD supports both solvers in its FEM worlbench but doesn’t support acoustics.
All right.
I looked at *DASHPOT, it is not implemented yet with explicit dynamics.
High material damping, haven’t done that yet.
What do you mean by large domain? Let’s say if I want a surface to be nonreflective, I just extrude it into a very large value?
Thanks again for your help!
There are no infinite elements so you can approximate an infinite domain with a very large region where boundary effects fade out. Like an infinite plate with a hole in solid mechanics that can be replaced by just a very large plate or very small hole. But of course, this approach will be computationally costly so it might be better to investigate other solutions involving damping.
The explicit dynamics procedure in CalculiX is still very immature and buggy. It might be another reason to switch to a different solver. Btw. here’s an interesting website about acoustics with open-source software: Computational Acoustics with Open Source Software | Computational Acoustics with Open Source Software
I will definitely take a look at that. Thanks for your suggestions!