When using multi-band compression on your master bus, you should know how to create 4 forms of compression with the plugin - downward and upward compression and downward and upward expansion. Furthermore, utilize the attack and release to control the timbre, and use mid and side processing when possible.
When creating bands in a multi-band compressor, they can be separate from one another or overlap in a way that can impact the sound of your master. Delink bands for the most isolation, increase the slope between them some isolation, and lower the slope for the bands to crossover.
Isolation gives you more control but can sound a little less natural, whereas crossover emulates natural musical dynamics amongst frequencies, but with less control.
To create typical downward compression create a band and select the compression mode or create a compression ratio, and use a negative range. When the threshold is crossed by the signal, attenuation of the signal will occur, which helps both control dynamics and possibly balance the frequency response.
For example, if 200Hz is making the master muddy, I’d use downward compression to attenuate it when it’s too loud.
To create less common upward compression select the compression mode and use a positive range. Whenever the signal falls below the threshold it will be amplified from the noise floor up, in turn adding body and power to that frequency range - while leaving the tracks peaks unaffected.
So say I wanted my mids to have more body - around 800Hz or so I’d use upward compression to increase its amplitude whenever it got quiet.
Downward Expansion is often called gating and doesn’t often play a big role in mastering, but while we’re on the topic of multi-band compression, it’s good to cover - also if used subtly it can increase dynamics. To create it select the expansion mode and use a negative range.
So say I wanted the snare’s frequencies to be quieter until the snare hits - I’d very subtly use downward expansion to decrease the range’s volume and set the threshold to trigger only when the snare hits.
Upward expansion is a more common use for multi-band processing during a mastering session and can be created by using the expansion mode with a positive range value. Whenever the signal goes above the threshold the signal will be amplified, which is great for kicks and snares.
For example, if I wanted my kick to be more prevalent I could upward expand 80Hz. Maybe I’d combine them with downward compression on the bass guitar’s frequencies and use crossover to tie these two effects together.
Attack and release play a large role in establishing the timbre of your compression or expansion and can be combined to achieve particular effects. For example, a short attack and long release cause a smooth, possibly blurred sound - while the opposite settings can cause increased clarity.
So if I wanted to smooth out the high frequencies, I could use downward compression with a quicker attack and slower release.
How much compression or expansion occurs once the signal goes above or below your threshold depends on your ratio and your knee, which work in close relation to one another. A harder knee results in less compression but with a more aggressive sound when it does occur.
A softer knee will cause more compression but the compression will be more gradual and less noticeable.
Some multi-band compressors allow for mid-side routing, meaning you can expand or compress just the mid or side, or a combination of the 2 - in turn narrowing or widening your stereo image. Typically mid-only bands should be used on lower frequencies and side-only ones on higher.
Say I want to increase the width of the stereo image, I could create a band on just the side and expand some of my higher frequencies.
The phase settings of your multiband compressor become increasingly important if you want to use it on your mixes or mastering session. Typically it’s best to avoid minimum latency filters since these will result in the most static phase changes to your signal, which can have a negative sound.
Linear phase results in the least phase changes, but can cause pre-ringing that smears transients.
Dynamic is a good blend of the 2 and reduces static phase changes with very modest pre-ringing.