flutter

Coincidence fluctuations, flutter, are rapid changes in signal parameters such as amplitude, phase or frequency.

Flutter can be triggered by rapid field strength changescaused by atmospheric disturbances, the movement of mobile antennas, or reflections and absorptions in the ionosphere. In tape recorders, variations in synchronism of the drive motors cause frequency changes and thus variations in pitch.

The human auditory system is able to perceive synchronization fluctuations that cause frequency changes of a few parts per thousand.