Crest Factor / Headroom
Definition
The crest factor is the ratio between the peak level of a signal and its RMS (average) level, expressed in dB. A sine wave has a crest factor of about 3 dB. Music with strong transients has a much higher crest factor, while heavily compressed or limited music has a lower one. Headroom is the margin between the nominal operating level of a system and its maximum level, and it must be at least as large as the crest factor of the programme for peaks to pass without clipping. An amplifier or converter that clips the peaks alters the transients even when the average level appears safe.
In the field
Music commonly has a crest factor of about 10 to 20 dB, so a sound system needs comparable headroom above its nominal RMS level. A system whose average meters look correct can still clip snare and kick transients when its headroom is smaller than the crest factor of the programme.
Related lessons
Sources
- Crest factor (Wikipedia) : rapport crête/RMS, √2 soit environ 3 dB pour une sinusoïde · en.wikipedia.org
- Headroom (Wikipedia) : marge entre niveau nominal et niveau maximal · en.wikipedia.org
- ProSoundWeb, « Understanding The Nuances Of Crest Factor » : facteur de crête de la musique typiquement de 10 à 20 dB · prosoundweb.com
- MC2 Audio, « Crest factor and amplifier power » : la grande majorité des musiques entre des facteurs de crête de 4,8 et 7,8 (environ 14 à 18 dB) · mc2-audio.co.uk