Top: thermal frequency (1/τ) computed from Eq. (9) versus the cut-off frequency computed following Eq. (1), normalized to the solar values, for models with masses ranging from M = 1   M_⊙ to M = 1.4   M_⊙ (with an increment of 0.05   M_⊙) and from the ZAMS to the ascending vertical branch. Bottom: the same as for the top panel, except the thermal frequency is divided by the Mach number to the third versus the normalized cut-off frequency.