First, let’s get through a little nomenclature,
dBFS
Deci-Bel (one tenth of a Bel) Full Scale, on the digital recording scale, -0 dBFS is the hottest signal you can have.
“All ones.” Top of the scale, can’t get hotter, etc. Always “minus” as you can never go higher so the reading will always be a specified amount below 0.
Line Level / 0dBVU
Just what it says. Line level. 0 dBVU on an analog VU (volume unit) meter. Pro (+4 dBu) or consumer (-10 dBv) level, it’s line level.
We can also refer to this as RMS (Root Mean, Squared), or a level over a specific amount of time. You can go above or below 0 dBVU.
It’s simply a nominal level to which basically everything audio is related to.
Headroom
The space between a nominal signal (in this case, line level) and the point where the circuit fails.
In digital, basically anything under full scale (-0 dBFS) would be considered headroom.
In analog, it’s the space between 0 dBVU and the point where the circuit clips (failing completely). In analog, there can be a big difference between “headroom” and “usable headroom.” We’ll get into that in a bit.
Steak
From the old Norse “steik” meaning “roast”. A slice of meat, typically beef, usually cut thick and across the muscle grain and served broiled or fried (thank you, Wikipedia).
So…
You have a microphone and a preamp going into a converter or sound card. Those converters are calibrated at line level.
In most cases, over the last several years, most I’ve seen are calibrated to -18 dBFS = line level (or 0 dBVU).
In other words, if you run a steady signal (a sustained note on a keyboard for instance) through a preamp and turn up the preamp gain until the VU meter reads 0 dBVU, at the converter, it will read -18 dBFS (or -18 dBFS(RMS) – full scale, but measured over time).
This is where your gear is designed to run. This is where it’s spec’d at. You will have a decent amount of headroom, the lowest distortion, the best signal to noise ratio, etc., etc., etc. around this level or lower.
Some gear – usually very high-quality stuff, has a good amount of usable headroom above this level.
A lot of “budget friendly” gear does not. So all of this advice is more important if you’re using “okay” gear at the input. Even your digital converters are analog components up to the converter itself. They don’t want to be “beat up” all the time either.
Let’s look into headroom. Above that 0 dBVU/-18 dBFS range, digital headroom is simple – perfect, perfect, perfect, perfect, CLIP.
The signal is “what it’s supposed to be” until the point of failure. Analog gear (your preamp, compressors, outboard signal processors, etc.) isn’t like that…