Dependence of observed silicate-feature optical-depth τ_Si and mid-IR spectral index α on model parameters. The grids show how τ_Si and α change with a (dotted lines; from a = 0.0 to  −2.0) and N₀ (dashed lines; from N₀ = 2.5 to 10) for different half-opening angles of the torus (red: 60°, green: 45°, blue: 30°). Left: type 1 AGN with i = 0°; Right: type 2 AGN with i = 90°. The grid lines are results for one random arrangement of clouds for the given set of parameters.