Power spectrum of the radial velocity of 18 Sco represented in logarithmic units. The contributions to the noise from granulation (G), supergranulation (SG) and from a third Harvey-like component are represented as dashed lines. The photon noise is a dot-dashed line. The p mode component of the spectrum appears as a dotted line. The full red line shows their sum. The black dots are the points of a binned power spectrum that has been effectively used to perform the fit.