Heavily compressed/limited modern master: squashed to 6-9 dB.
+6 dB = double the voltage/peak (20 x log10(2)). +3 dB = double the power (10 x log10(2)).
High crest factor = needs MORE headroom; low crest factor = dense, already loud.
Amp ratings assume a crest factor (~6 dB); a 1000 W amp may sustain only ~250 W continuous sine.
0 dBFS in digital = absolute ceiling; one sample over clips. Leave headroom for the crest.
How it works
Measure the loudest instant peak of the signal.
Measure the RMS (average) level of the same signal.
Divide peak by RMS, or subtract in dB: peak(dB) minus RMS(dB).
Big gap (15+ dB) = spiky/punchy. Small gap (under 9 dB) = flat/dense.
Set gain so the PEAK stays under 0 dBFS, not the average.
Leave that gap as headroom so transients do not clip.
Real examples
Snare drum: short sharp spike way above its quiet average = high crest factor (~18 dB).
Steady synth pad or sine tone: flat, peak barely above average = low crest factor (~3 dB).
Sine wave: a constant 3 dB crest factor, the textbook reference.
Squashed pop master on radio: crushed to ~6-8 dB so it sounds loud everywhere.
Acoustic finger-picked guitar: snappy attacks give a high crest factor.
How it helps in live sound
Leave 12-18 dB of headroom on drum/transient channels so snare cracks do not clip the desk.
Watch the PEAK meter, not the RMS/VU, when gain-staging punchy sources.
Don't size your amp on RMS alone: pick amps with peak headroom (~2-4x) for spiky material.
A limiter on the master lowers crest factor: flatter, louder, less punchy.
Tune your PA with pink noise (~12 dB crest factor) so the RTA reading is stable.
Transients clip but average looks low? Crest factor is high: pull gain or use a fast peak limiter.
Everyday analogy
Like a calm sea level (RMS) with occasional big waves (peaks): crest factor is how tall the waves tower over the average water line.
Watch out
Myth: gain-stage to the average level. Reality: peaks clip first, so set gain by the PEAK and leave headroom equal to the crest factor.
Fun fact
A pure sine wave's crest factor is exactly the square root of 2 (1.414), which is precisely 3.01 dB, the same number behind the half-power -3 dB point.
Key takeaways
Crest factor = peak ÷ RMS; in dB it's 20·log10(peak/RMS).