Top: root-mean-square brightness fluctuation σ as a function of ν_max. The symbols have the same meaning as in Fig. 1. The red curve is a power law of the form with the slope p =  −0.51 obtained by fitting the power law to the theoretical values of σ (filled red squares). The filled green square corresponds to the value σ = 39 ppm found for the solar 3D model. Bottom: σ as a function of the quantity z₃ given by Eq. (19) where ℳ_a and are here supposed to scale according to Eq. (17) and Eq. (22), respectively. The green line corresponds to a linear scaling with z₃ and the red one to a power law of the form where p = 1.11.