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Measurement of wow and flutter is carried out on audio tape machines, cassette recorders and players, and other analog recording and reproduction devices with rotary components (e.g. movie projectors, turntables (vinyl recording), etc.) This measurement quantifies the amount of 'frequency wobble' (caused by speed fluctuations) present in subjectively valid terms. Turntables tend to suffer mainly slow wow. In digital systems, which are locked to crystal oscillators, variations in clock timing are referred to as wander or jitter, depending on speed.
While the terms wow and flutter used to be[when?] used separately (for wobbles at a rate below and above 4 Hz respectively), they tend to be combined now[when?] that universal standards exist for measurement which take both into account simultaneously. Listeners find flutter most objectionable when the actual frequency of wobble is 4 Hz, and less audible above and below this rate. This fact forms the basis for the weighting curve shown here. The weighting curve is misleading, inasmuch as it presumes inaudibility of flutters above 200 Hz, when actually faster flutters are quite damaging to the sound. A flutter of 200 Hz at a level of -50db will create 0.3% intermodulation distortion, which would be considered unacceptable in a preamp or amplifier.