Dependence of $c_1{\breve{v}}_1(x=0)$ on density contrast ρ_i/ρ_e for several values of Λ/d as labeled and highlighted by line styles. The results for the step case (μ = ∞; black curves) are compared with those for (a) μ = 2 (red) and (b) μ = 1 (blue). The axial wave number is fixed at kd = π/20. We note that $c_1{\breve{v}}_1(x=0)$ measures the magnitude of the asymptotic variation at the slab axis (see text for details).