Crest factor is the gap between your peak level and your RMS level, and for vinyl it’s one of the most useful single numbers for working out how audio is going to behave on the lathe. If audio is slammed with a brickwall limiter it will mean a low crest factor, this means it is literally squashed, the ‘body’ of the audios volume is right up against its peak level. A problem with this can be groove distortion, we have an analogy to try and understand it. We know this isn’t technically what’s happening in the groove, but as an analogy it works.
Imagine you’re not playing a record, you’re playing the waveform on your screen. Your imaginary stylus tracks along the top of it. If the waveform is a solid block, flat at the top, the stylus slides rather than grips, and that plays as distortion. If the waveform has dynamics and the stylus has something to track, it grips the movement and plays back cleaner. Not the science, but a decent way to picture it.
With crest factor and vinyl mastering, 6-10db is usually a good spot. Lower numbers can mean it’s dense, limited material, a higher number can mean extreme peaks and quiet sections, which is fine, but also needs managing in its own way, which is part of mastering…