Coefficient ζ (see Eq. (13)) as a function of L/M for sub- and red giants. The filled circles correspond to the values, ζ_nad, obtained with the MAD non-adiabatic pulsation code (see details in the text). The empty squares correspond to the adiabatic coefficient Kjeldsen & Bedding (1995) (see Eq. (16)). The red line corresponds to a power law of the form ζ₀ (L/M)^k with k = 0.25. Both intensity-velocity relations ζ have been calibrated so that for the Sun ζ = 1 (see text).