Background medium considered in this study. In the solar interior (up to r_a), it is characterised by its density, sound speed, attenuation, and sources that can depend on the 3D vector r. In the atmosphere (r_a ≤ r ≤ r_t), the background medium is radially symmetric and does not contain sources. The boundary condition is usually applied at r_t, but can be directly applied at r_a using learned infinite elements.