Compression is a common technique used by all audio engineers and producers, and is one of the only processing techniques used throughout the entire recording process.
Most of the common uses for compression use regular compressors, which act on the entire frequency range of human hearing (20Hz – 20kHz). And though for most of our needs mixing needs these compressors do a superb job and getting the job done. There are instances where one might need to compress a specific frequency range of a sound or instrument, and that’s where multi band compression fits into the equation (Refer to: 3 Commonly Used Types of Dynamic Compression & 5 Waves Compressors to Help You Achieve Better Dynamics)..
So what is a multiband compressor?
Exactly what it sounds like!
It’s a compressor that is split up into different frequency bands with crossovers. (Usually 3-4-5 band compressors.) This can be extremely helpful when you’re looking to compress a certain frequency range of an instrument, without affecting the other frequencies within it.
Since regular compressors affect the entire frequency range, they can create a pushing and breathing effect from the high and low end frequencies, which can be unwanted at times. Having the ability to dissect a sound wave at its different frequency ranges allows for a much more controlled mix.
For instance, a five band multiband compressor might can process the bass range from 0-150 Hz, the low mids from 150-500 Hz, mid range from 500-3 kHz, high mids from 3-8 kHz, and highs from 8 kHz and up.
Think of your multiband as – multiple compressors combined into one in order to control different frequency ranges. A four band multiband compressor is basically four compressors combined into one. (One compressor for lows, one for low mids, high mids, highs). Every band is completely controllable with threshold, attack, and release. Some multibands will even have a knee parameter; which will adjust the harshness of the compression.
Now the thing to remember about multiband compression is just that. There are multiple bands you are compressing; which means multiple ways to mess things up… Though a very versatile and powerful tool, multiband compression can quickly turn ugly and demoralizing if done out of context or unconventionally.
Since you are compressing different frequency ranges of a single signal, you want to make sure the instrument you are compressing has a fuller frequency range. (I’m not going to use multiband compression on a kick drum that is mainly made up of low mid and low-end frequencies.)
The one individual instrument I do find multiband compression useful on from time to time is vocal tracks. Now I’m not saying that I mix every vocal I record with multiband compression; (in fact; there’s very few times I do.) but when I do it’s to change to actual tone of the singer’s voice.
Since I have control over the different frequency bands; I can make the vocal track have more treble or low end depending on how much the singers performance varied during tracking.
Note: you’d always want to hopefully achieve a solid vocal recording to begin with; but sometimes you just have to work with what you got.
Another typical (more common) use for multiband compression is using it to help control frequencies within a group; or stereo mix bus of instruments. Because you are dealing with a larger group of instruments with a larger range of frequencies; multiband compression is a much more practical use of processing here.
One very practical example of this would be if you were trying to tame a really heavy kick drum that is muddying up your drum sub mix. Some may want to argue or question why not use an EQ? Becasue then you are not taming the low frequencies of the kick, you are actually deleting them. You are not only getting rid of the low end from the kick; but from the snare, and toms, and all the other instruments as well. Being able to compress the band and control the attack and ratio is a mix-saving advantage in a situation like this one.
Mastering would probably be where you would see multiband compression most commonly used. Very similar to when compressing a stereo mix bus; multiband compression when mastering is finely tuning the frequency bands for the finished product.
When it comes to frequency processing everyone’s mind goes straight to the EQ plugins. A lot of what you may by trying to achieve through basic equalization might be able to be done through multiband compression as well.
The great thing about dynamic processing is that is really allows you to control your overall headroom of the mix; which will return deliver a high quality sound across multiple monitor systems. Multiband compression is definitely not for engineers who are just starting out with compressor. I would suggest experiment around with EQs first, and then dip into the multiband compressors to see what similar and different effects you can achieve out of your instruments!