a) SDO/AIA 304 Å image of sunspot NOAA 11131 on 08 December 2010 at 02:00 UT. The dashed line shows the umbra’s outer boundary, the solid line shows the outer boundary of the penumbra. The vertical line shows the slit of the time-distance plot. b) The time-distance plot constructed for the time interval 02:30–03:30 UT in the south-north direction through the sunspot centre. The intensity is given in the logarithmic scale. The horizontal dashed lines show the umbral boundaries. The white triangles labelled with digits show the instants of time when the 3 min oscillation power reaches the local maxima.