Case of a varying thin film placed over a poro-elastic slab with a rigid backing. Comparison between Monte Carlo envelopes (realisations in blue/green, realisation-wise standard deviation envelope with bullets and extremal envelopes with triangles ▲) and envelope from the proposed method (dashed red, n being the multiplier to the standard deviation of the varying parameter). Absorption coefficient (top most axis) and centred envelopes (two lower axes) for two angles of incidence (0° in blue and 60° in green).