This is the setup described in
Tam: Computational Aeroacoustics, a wave number approach.
Appendix G.3: Sample Program for Solving the Three-Dimensional Euler Equations
The exact solution is also described there.

One difference is that the domain given by Tam is [-30:30, -30:30, -30:30],
instead of those we use powers of two, to allow the discretization with
the internal generated mesh.
