Multiband compression is a complex and subtle tool. Compression itself is one of the more complex processes commonly used in mixing. Multiband compression multiplies that complexity. Compared to regular compression (also called ‘full band’ compression), multiband compression is much more complex because it works by applying several compressors in parallel, each operating on its own frequency band. Because the audio is split by frequency, multiband compression is best suited for processing complex audio with varying dynamic behaviour across the frequency range. Generally, this would be a full mix – either on the mix bus or in mastering.
Multiband compression is best used for one of two purposes – surgical problem solving or subtle levelling.
Multiband compression is ideally suited to some kinds of problem solving because it allows compression to be applied to a specific frequency range without altering the rest of the audio. For example, a mix with uneven bass guitar playing could be tightened by using multiband compression on the mix bus (or in mastering) to reduce the dynamic range of the low frequencies. This can work better than full band compression because the rest of the mix would not be affected. Another example could be a mix where the vocal is uneven and alternates between being too quiet and too loud. Depending on the mix, multiband compression could be used to even out the vocal in relation to the rest of the mix. In both these examples, multiband compression would be used in mastering – only if these problems couldn’t be fixed in the mix by processing the individual channels. While multiband compression can be a very exact tool, it is always better to fix these kinds of problems in the mix (or earlier) if at all possible.
Another use of multiband compression is subtle levelling. Rather than using a single band to solve a specific problem, all bands are activated and configured to gently ride the audio level. In this case, the bands are always being compressed, but only gently. This approach works best for mixes that are rather weak overall and not well-balanced. It can improve the overall tonal balance and dynamic behaviour in a much less damaging way than full range compression. As always, however, this approach is only appropriate if it’s not possible to go back and revise the original mix.
?One of the important things to be aware of when using multiband compression is that even small adjustments in one area can cause a perceived change in other areas. As with most other mastering procedures, try to keep the processing as subtle as possible.