# Measures & Glossary

Measures

• Average speed in RPM.
• Deviation: Percent deviation of the average vs the correct speed (33.33 or 45).
• Variation: Maximum and minimum speed observed.
• Min%, Max%: Maximum and minimum speed observed as percentage of the average speed.
• High-pass filtered min / max speed variation: Minimum and maximum speeds observed, having removed the slow speed variations (Wow), if detected.
• W&F (no weighting)
• W&F Din: W&F weighted by the IEC / DIN 60386 weighting curve.
• W&F 2-sigma: The 2-sigma means that the lowest 2.5% and highest 2.5% of the speeds observed (the outliers) are discarded for the W&F calculation.
• W&F WRMS: Weighted (IEC / DIN 60386 curve) RMS of the Wow & Flutter values observed.
• Wow: Same as W&F peak-to-peak but weighted by a bandpass filter centered on 2Hz.
• Flutter: Same as W&F peak-to-peak but weighted by a bandpass filter centered on 50Hz.
• Jitter: Average of the total speed variations per second, as a percentage of the average speed.
• Modulation Frequency (primary): If different from zero, represents the periodicity (in Hz) of the low frequency (wow) strongest modulation found (strongest as in more evident) in the signal. A value of zero means the modulation was too insignificant in amplitude, or irregular in period, to be considered.
• Modulation Frequency (secondary): Same as above, but next in correlation power (less evident).

Glossary

• W&F (wow & flutter) – It is a calculated number representing the speed variation relative to the average during the measuring period. It only takes into account the variations from peak to peak, that is, where local extremes (highs or lows) are reached. The W&F designation comes from the fact, it both includes slow (Wow) and fast (Flutter) speed variations. It is expressed as a percentage of the average speed. Obviously, the lower, the better…
• RMS (root mean square) – Contrary to W&F, here all variations are taken into account, not just the local extremes. The squares of each difference to the average speed are added up, and at the end, their mean is calculated through division by the number of readings (samples), and finally, its square root calculated and displayed, also as a percentage of the average speed.
• Bandpass filter centered on X – A bandpass filter lets certain frequencies (the X) pass through, without any attenuation, while all others (both lower and higher) are gradually attenuated, i.e. their magnitude decreased.
• IEC / DIN 60386 curve – A particular kind of bandpass filter where the center frequency and the amount of attenuation applied to lower and higher frequencies agree to an internationally recognized standard (the IEC / DIN 60386). In theory, the resulting attenuation curve mimics the sensitivity of the human ear to changes in frequencies heard.