A compressor is an audio tool that helps to alter the dynamic range of a signal by reducing the volume of louder parts or by amplifying quieter parts. It is without a doubt one of the most important tools in the modern audio world.
This blog will discuss a very special type of compressor called the ‘Multi-band Compressor’.
What is Multi-band Compression?
A Multi-Band compressor works by splitting the frequency spectrum into a certain number of frequency bands (Usually 4) using band pass filters or crossover filters. Each band is then provided with its own compressor having functions like Threshold, Attack, release, ratio and makeup gain. This way one can have different compression settings for different frequency ranges, hence making it a very useful device. A lot of these compressors also provide a solo button for each band. This is very helpful because you can easily hear and manipulate the compression on each part of the frequency spectrum.
Why is it helpful?
For audio signals with narrow bandwidth like vocals, guitars, bass etc. full bandwidth compression is very helpful. The reason being it affects the whole frequency spectrum and does not cause any anomaly on signals with a narrow frequency range.
On the other hand, if the signal has a wide frequency range we need a compressor that can have different settings for different frequency ranges hence enabling effective compression without any anomalies.
For example:
There is a two track recording a kick drum and bright acoustic guitar. If we put a full bandwidth compressor on it with a short attack, the guitar will sound fine but the kick drum will sound boxy and choked. The reason being, the low-end of the kick drum will not have enough time to be heard before the compressor kicks in.
On the other hand, if the same compressor is set to a longer attack, the kick will sound fine but the guitar will sound harsh because the peaks will not be tamed with the high attack time.
Now if we introduce a Multi-Band Compressor. We can have a longer attack in the lower mid band for the kick to sound right and shorter attack for the upper mid band for the acoustic guitar to sound right. Hence solving the issue.
Common controls on a Multi-Band Compressor
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The frequency band selector:
The part helps to select the frequency ranges for different bands. Thus giving a customized control over the bandwidth. It also has the makeup gain function for each band.
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Compression settings for each band:
Each has a setting like Attack, Release, Ratio, Threshold, solo, and bypass.
Some application of a Multi-Band Compressor
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Mastering
It is a very common tool used for mastering process. Some people also refer to it as the ‘secret tool’ of a mastering engineer. It adds an interesting flavor to the whole mix. Especially to boost the low end and make the high end crisp and smooth. One important thing to remember is to use lower ratios because a little can sometimes be too much.
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Drums
Drums have a wide frequency spectrum and are quite often processed using multi-band compressors.
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Vocals
Even though vocals have a narrow bandwidth but still multi-band compressor can be used to alter a very specific range of frequencies thus have a very cool effect on the vocals.
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De-essing and De-popping
Multi-Band compressor has a wide application as a ‘dynamic equalizer’. It can be used to cut down the sibilance within a specific range of frequencies only if the signal exceeds a certain threshold. This is not possible with a ‘static equalizer’ because it will do so for the whole track.
Additional Resources & Source Texts
https://ask.audio/articles/mixing-tips-understanding-multiband-compression
http://www.soundonsound.com/techniques/multi-band-compression
http://www.recordingmag.com/resources/resourceDetail/126.html
