Propagation diagram for a 1.7 M_⊙ model during a He-burning subflash. The Brunt–Väisälä frequency is represented by the full red curve, and the l = 1 Lamb frequency S₁ corresponds to the long-dashed blue curve. The horizontal line corresponds to the angular frequency ω = 2πν_max. A wave with pulsation ω propagates where the line is solid, and it is evanescent where the line is dotted. Turning points for the cavities are also depicted by the filled circles. The gray vertical arrow indicates the position of the H-burning shell.