A phenomenon that occurs in analog audio reproduction systems (tape machines mostly). Flutter is caused by speed inconsistencies in transport systems and is basically a form of frequency modulation that imparts a vibrato effect to the audio. Slow variations are known as wow, where fast variations (more than 5 Hertz) are called flutter. In cheap tape machines flutter can sometimes be very apparent on long sustained sounds such as piano (which normally has no other pitch modulation). It almost sounds like a grainy distortion in some cases. Wow and Flutter are generally listed in equipment specs where they are noted as a percent of deviation from the specified tape speed. There are several methods of measuring flutter that produce different results. The DIN spec is different from the IEC spec, which is different than JIS. Some produce an RMS result while others produce a peak result. Most equipment in the United States is specified with the RMS (more tolerant) result.
A flutter is also a visual cue used in film music scoring. It is a group or cluster of three to seven visual “punches” (in film a punch is a small circular flash in the film made by literally “punching” a hole in the film) with each punch separated by an unpunched frame thus producing a flicker effect over the picture. Typically, a flutter is placed on the downbeats of specified bars of music to provide a “visual metronome” for the conductor so that he or she can, in conjunction with streamers, correctly pace the performance of the music to be sure that the length of the music and the events in the music end up matching the picture as compositionally intended as the performance proceeds.