Snapshots of spatial distribution of 3 min oscillation power measured in 304 Å in the umbra of sunspot NOAA 11131, a) at the time instant of the maximum 3 min oscillation power, 02:49 UT; and b) at the minimum power, 02:35 UT, showing the shape of wave fronts. The dashed curve indicates the umbral boundary. The signal is shown in the logarithmic scale. The black dotted curves highlight the apparent wave fronts. The bar shows the normalised logarithmic power, black indicates the high power, and white the lower power.