When mastering, you can introduce low-level compression with various plugins, like the Waves MV2, or utilize the functionality of multi-band compressors. Upward compression or low-level compression can make your master sound full and impressive, without affecting the peaks of your signal.
In my opinion, the easiest way to add low-level compression to your master is by using the Waves MV2 and subtly increasing the slider on the left side of the plugin. You can monitor how much you’re introducing into the signal, and how your dynamics react with the compression.
It’s a pretty inexpensive plugin as well and can be demoed, so it’s worth trying if you’re interested in low-level compression.
The Inflator and Boost are 2 other plugins that can introduce low-level compression during mastering - inflator is more indicative of tube distortion but with upward compression qualities, whereas Boost amplifies quieter details of the signal with the focus slider. With boost, you can limit, compress, and distort the signal.
With the inflator, change the curve to make the signal warmer like tube distortion, or more like transistor distortion with a negative curve.
FabFilter’s MB and other multiband compressors are capable of upward compression - to accomplish this, use the compression mode with a positive range value. Since this is a multi-band plugin, you can create different amounts of upward compression at various frequencies, and connect these bands so they affect one another.
Utilize the release time to affect how long this upward compression occurs.
OTT is a popular free multiband low-level compression, with which you can turn off downward compression at the bottom of the plugin, and emphasize upward compression by dragging the bars in the middle to the right. Then reduce the depth to under 10 percent when mastering.
This plugin is incredibly sensitive so be careful with it.
Parallel compression works by using significant downward compression, then blending that heavily compressed signal back in with the unprocessed signal - Low-level compression works by isolating a quieter part of the signal, compressing it, and amplifying it. Parallel compression is downward and affects peaks, low-level is the opposite.
Try both to see which one sounds better in your session, or maybe combine the 2.
It’s possible to parallel low-level compress by using an auxiliary send, heavily compressing the send, and then blending it back in with the original signal. This may not sound unique from low-level compression, but with this set up you can introduce an EQ or other processor post-compression.
Just be sure to check for phase issues as that might happen with this setup.
With the FabFilter MB, you can upward compress just the side - to do so use the compression mode with a positive range and toggle the expert mode in the bottom right. Then move the slider to the right to have that processing only occur on the side image.
You can now expertly control low-level compression on the side image with the attack, release, and other functions.
Again with the FabFilter MB you can upward compress and control the mid image of your master from within the expert section of the plugin. This works well at making your mids more impressive, or maybe increasing the perceivability and detail of your sub-frequencies.
If you use this on lower frequencies, be sure to use a release time of over 50ms to avoid distortion.
One of my favorite techniques is to distort the signal with the tube or tape distortion and then amplify quieter parts of the distortion with a low-level compressor. This, more times than not, increases the detail and nuance of the distortion, making for a unique, distinct sound.
When mastering, use this effect sparingly.