Comparison between the sunspot number (SSN) smoothed over 13 months (A) and seismic travel times, τ_s, for skip number s = 19 (B) measured in the red VIRGO/SPM channel data. Vertical dashed lines indicate cycle maximum and minimum. The red line represents the filtered data, τ_s, smooth(t), which we use to measure the amplitude of the travel-time variations due to the solar cycle A_s (black arrow). (C) Residuals between τ_s and τ_s, smooth. (D) Power spectrum of τ_s, normalized to the power of the 11-yr harmonic. A Butterworth filter (red line) is used to extract τ_s, smooth(t).