The crest factor describes a signal's dynamic range as a single number: the difference between its peak level and its RMS (average) level. It is one of the most useful ideas in mastering.
Peak and RMS
Dynamically, audio can be divided into two aspects:
- Peak - the highest point of the signal in a single moment: a drum hit, a transient, a sudden burst of energy. Peak is what you hear - it happens in an instant.
- RMS - the average level over a short window, roughly a third of a second. It includes peaks but expresses the signal's actual power. Higher RMS feels louder. RMS is what you feel.
Controlling the relationship between them shapes everything from the punch of drums to the sustain and power of a sub-bass.
Calculating the crest factor
Crest factor = peak level - RMS level (in dB, a positive number).
Example:
- Peak: -6 dB
- RMS: -14 dB
- Crest factor: 8 dB
A high crest factor means big peaks relative to the average - dynamic and punchy. A low crest factor means peaks close to the average - dense, compressed, loud.
In production
Keeping an eye on crest factor helps you produce with a consistent sound from track to track - for example, aiming for a similar crest factor across an EP.
In mixing
Crest factor helps you judge whether a premaster is healthy and whether the low end is under control. A crest factor of roughly 6-12 dB, with RMS levels somewhere around -18 to -9 dB, usually makes a good premaster.
In mastering
In mastering, crest factor is arguably one of the most important numbers. Looking at peak and RMS separately splits the music into two aspects; the crest factor combines them. Clients, genres and formats imply different targets. As rough illustrations:
- Dynamic jazz or acoustic music - 8 dB or more.
- Commercial pop - around 5-7 dB.
- Loud trap, dubstep or EDM - 3-5 dB or even less.
Measure the crest factor of your reference tracks to set targets for your genre.
A dynamic standard
Paying attention to crest factor in every session builds your own dynamic standards, in line with modern releases, across the genres and tempos you work in. It helps keep the low end healthy and the high end under control - and the result sounds more balanced.
Do this now
- Measure the peak and RMS of three reference tracks from different genres and calculate each crest factor.
- Measure your premaster's crest factor.
- Master your track to match the crest factor of your main reference, and listen to the result.
Adapted, with thanks, from the lesson notes of J.M. Brooklyn Audio Mastering.