  Acoustic mode frequencies obtained by applying spherical harmonic
decomposition to HMI, MDI and GONG observations were analysed throughout the
solar cycle. Evidence of a deviation from a linear relation with solar radio
flux was found indicating a saturation effect at high solar activity. The
Gompertz model, which is one of the most frequently used sigmoid functions to
fit growth data, is used. It is shown that its fitting to MDI and GONG data are
statistically significant and a median saturation of four-hundred sfu is
estimated. This saturation level is 50% larger than any obtained in the last
century, hence the small effect observed in the minimum-to-maximum frequency
shift. However, as shown here, it should not be disregarded.
