Underruns occur in real-time audio processing software and software synthesizers (such as FruityLoops) when the program can not mix the song fast enough. They are likely to happen on slow computers and non-DirectSound enabled sound cards. Underruns are heard like clicks and pops during real-time playback. They can be avoided by setting longer buffer for the DirectSound driver. Underruns may occur ONLY in real-time playback. They will not happen while exporting to wave or mp3 file.