Significance of the maximum excess over 4 Hz in the 790−810 Hz band as a function of time in the dynamical PDS resulting from the stack of the 4U 1608-52 observations (see text). The solid line corresponds to significance values measured in the data while the dashed line is the level expected from the burst light curves and QPO profiles measured before the burst. We note that the plot starts at −43 s because of an observation gap for burst 21.