Propagation diagram for a 20 M_⊙ MS star model. The x-axis represents the radius in solar radii, and the y-axis shows the cyclic frequency in microhertz. The left and right panels show the full radius, while the middle panel shows the external 10% of the radius. The blue line represents the Brunt-Väisälä frequency N, the brown dashed lines correspond to the Lamb frequencies S_l for different spherical harmonic degrees l, and the black dashed line shows the profile of the acoustic cut-off frequency. The blue-shaded region shows the g-mode cavity, and the brown-shaded region – p-mode cavity. The right panel shows profiles of tunnelling ω_min for l = 1,2,5 in dashed red lines and one horizontal value of ω_min taken near the convective layer (see text for details). Profiles of ω_min collapse to an apparent vertical line in the He convection zone, and this particular zone is better resolved in zoomed version of the figure in Appendix D.