An apparatus for decomposing an audio signal into a background component signal and a foreground component signal, comprises: a block generator (110) for generating a time sequence of blocks of audio signal values; an audio signal analyzer (120) for determining a characteristic of a current block of the audio signal and for determining a variability of the characteristic within a group of blocks comprising at least two blocks of the sequence of blocks; and a separator (130) for separating the current block into a background portion (140) and a foreground portion (150) wherein the separator (130) is configured to determine (182) a separation threshold based on the variability and to separate the current block into the background component signal (140) and the foreground component signal (150), when the characteristic of the current block is in a predetermined relation to the separation threshold.