Geometrical details for the numerical example with a mostly rigid box, sized 3 m by 0.2 m, and a small patch with a locally reacting impedance boundary condition. A 2D point source is marked S. In (a), a semicircle of external receivers are placed at a radius of 3.1 m, with an angle step of 1 degree (not all are shown). In (b), two rows of receivers are placed at 1 cm and 2 cm distance (in the x-direction) from the box surface, in steps of 2 cm in the y-direction (not all are shown). An initial test case used an all-rigid box, without the impedance patch. The numerically most challenging receiver directions are marked with darker grey areas, and the more benign challenging directions are marked with lighter grey areas.