Schematic illustration of the light-ray geometry probed by the composite spectra. A^*_K reflects extinction constrained mainly by the near-IR, while $A_K^{\mathrm{M}\mathrm{I}\mathrm{R}}$ is determined primarily from the mid-IR. The quantity $A_K^{\mathrm{f}\mathrm{g}}$ represents the foreground extinction estimated from nearby source-free regions, which accounts for dust located along the line of sight in the Galactic disc and within the CMZ. The envelope-only extinction, $A_{K,0}^{\ast }$, is defined as $A_K^{\ast }-A_K^{\mathrm{f}\mathrm{g}}$, isolating the extinction produced by material within the protostellar envelope along the sightline to the background star.