Crest Factor
Peak-to-RMS ratio — the number that clips amplifiers and fools meters.
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The engineering
Crest factor is headroom accounting. A true-RMS meter specified to CF 3 misreads pulsed waveforms beyond it; a shaker amplifier rated in RMS watts still clips on the peaks. Random vibration tests commonly demand CF ≥ 3 capability — that's 9× the RMS power on the peaks.
High CF with a modest RMS is the signature of impacts, inrush, and PWM edges.
Where this math comes from
The ratio fell out of AC metrology in the 1890s — Fleming's and Steinmetz's generation had to reconcile peak-reading electrostatic instruments with heat-reading RMS ones, and 'form factor' and 'crest factor' named the disagreements.
It graduated to a design constraint with random-vibration testing and PCM audio: Gaussian noise theoretically has unbounded peaks, so every shaker spec and every digital recorder quietly chooses where to clip.
- 1892J. A. FlemingAC instrument era forces peak-vs-RMS bookkeeping.
- 1955Random vibration testingCrest factor becomes a shaker-system spec.
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