Always compare the differences made to a track with the levels matched, I.E., eliminating the variable that is playback level. Otherwise, the increase in level will fool your ears into thinking something sounds better when it's just louder.
We're addressing the peak level with a limiter, rather than the RMS (average level).
Integrated LUFS — This is the measurement that all the streaming services use to determine whether your song needs to be increased or decreased in level. It measures the whole file and attaches an integrated LUFS value to the track that you cannot alter.
If you want to end up at an average of -14 LUFS, find the loudest section of your song and ensure it's around 4dB over that. For example, let's say the chorus is the loudest section, ensure it's sitting around -10dB LUFS to achieve an average of -14dB across the whole track. These values are only a guideline but can help you set up your limiter.
The sinister aspect of using a limiter is losing some low-frequency information and gaining a tiny amount of high-frequency distortion. The distortion is correlated to the input signal, but not in a musical or pleasing manner.
Because the highest amplitude portion is usually in the low end, it makes sense that the bass will be driving the limiter. Therefore, the more you run your track into a limiter, the more you're going to restrict the bass in relation to everything else.
Limiters are used with a fast release time in mastering, as a slow-release will hold on to the signal for a long time and tends to have a dulling or blurring effect on the material. But, very fast release times can turn into high-frequency distortion, so there's a balancing act to find the sweet spot. Take a moment when you're using a limiter to check your kick drum, ask: is the hat coming up? Is the sibilance coming out? An increase in high-frequency distortion is often a side effect from limiting, and it's not necessarily a bad thing, but it's a change you should listen out for when using limiters.
You can decouple the L/R channels when limiting to give yourself more control over the stereo image.
IRC 1 = Similar to an analogue limiter. Very little look-ahead. This mode reacts almost instantaneous to an input signal, which provides more transient reduction, a warmer and punchier sound.
IRC 2 = More like a look-ahead digital limiter. A look-ahead means the plugin has a buffer of memory to look ahead at the incoming signal, to better anticipate and react, allowing for a smoother style and added warmth in the low end.
IRC 3 = Crazy sophisticated limiter. For every instance of limiting this mode will access a look-up table, choosing the limiting algorithm that will produce the lowest amount of distortion. The result is a cleaner sound that can be brighter and is guaranteed to have the least amount of distortion associated with it. This mode will also impart the least amount of colour on to a signal.
IRC IV = This is IRC modes 1, 2, and 3, combined. But in every case, it has a spectral shaper (which you can think of as a sophisticated dynamic EQ) that sits in front of the limiter. This tool pulls back where you have peaks in the spectrum, ensuring that the limiter doesn't have to work as hard.
As you lengthen the release time, it will hold on to a signal for longer producing a smoother overall sound which reduces the excitement somewhat. Do you want to maintain the sharpness of a track, or something a little mellower? An extended-release time will also produce less distortion, giving you a darker, more pleasing sound.