a) AIA 171 Å image of active region NOAA 11330 observed on 27 Oct. 2011 at 04:30:01 UT is shown with the flux on the logarithmic scale. A cut that was taken to make the time-distance plot is indicated with a black bar. b) The running difference R₁ of the time-distance plot started at 04:30:01 UT. R₂ is the first half of R₁, to the left of the white dashed line. It covers about 10 cycles of the propagating features. Panel c) shows the background-subtracted time-distance plot D₁. The first half of D₁ on the left of the white dashed line is D₂.