Wavelet power of the velocity at a randomly chosen umbral location. Top panel: wavelet power of the photospheric velocity derived from the Si I 10 827 Å line. Middle panel: wavelet power of the chromospheric velocity derived from the He I 10 830 Å line. bottom panel: power of the WCS between the photospheric and chromospheric velocities. The gridded region indicates the parts of the spectra outside the cone of influence. Solid black lines mark the 95% confidence level. The vertical dashed blue lines in the bottom panel indicate the times illustrated in Figs. 11 and 12.