a comparison to the observations but they do not influence the qualitative results presented here.

Pulsation modes before and after core helium ignition {#models}
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![image](fig5a.eps){width="88mm"} ![image](fig5b.eps){width="88mm"}

Here, we compare pulsation properties of two representative models with similar positions in the H-R diagram but in two different evolutionary stages, one during the shell hydrogen burning phase ($16 M_\odot$, $\log T_\mathrm{eff} = 4.343$, $\log L/L_\odot = 4.705$, hereafter Model1) and the second during core helium burning ($15 M_\odot$, $\log T_\mathrm{eff} = 4.244$, $\log L/L_\odot = 4.815$, hereafter Model2). They are calculated with OPAL opacity tables, AGSS09 metal mixture and $Z = 0.015$.

Propagation of pulsational modes in the radial direction is confined to the regions determined by the two characteristic frequencies (e.g., Unno et al. 1989), i.e., the Lamb (acoustic) frequency ($L_{\ell}$) and Brunt-Väisälä (buoyancy) frequency ($N$). The values of $L_{\ell}$ depend mostly on the mode degree, $\ell$, whereas the Brunt-Väisälä frequency is very sensitive to the structure of a star (e.g., a presence of convective zones or chemical gradients).

In Fig.\[fig3\], we present propagation diagrams, i.e., $L_\ell^2$, $N^2~~vs.~~\log T$, for Models 1 and 2 (the top and bottom panels, respectively). The main difference between these models is the presence of 