7. Multi band compressors
Compressors, limiters, EQs, filters—all of these tools are used in audio mastering as well as in mixing. But there are some specialised tools for the final step in production, which really step up the game.
When working with an ordinary compressor, you may encounter the problem that bass-heavy signals tend to trigger the compressor more strongly than mid- or treble-heavy signals. For example, the bass drum may trigger the compressor more heavily than the snare, even though the latter is perceived as „louder“. One option to tackle this problem is using sidechain filters. Dynamic processors like compressors analyse the audio signal in order to determine whether the threshold is exceeded. If you eliminate the bass frequencies from the analysed signal using a low cut filter, the compressor will no longer react to the low frequencies of the bass drum. The low cut filter cannot be heard, as the sidechain signal is separated from the audio signal path. Many dynamic processors have a sidechain input, which lets you manipulate the sidechain signal using the tools of your choice before feeding it to the compressor.
A specialised version of a sidechain compressor is the De-Esser. While it was originally conceived as a tool for reducing sibilances in vocal recordings (especially „S“ and „T“), it has its uses during mastering, e.g. to tone down sharp snare drum signals.
Multiband compressor in Presonus Studio One
A multiband compressor goes even further. It consists of several separate compressors, each of which works on a portion of the signal only. To split up the input signal into several bands for processing, the device or plug-in contains adjustable filters. After that, the frequency bands are compressed individually, and joined back together before the output. A 3-band compressor can be used to compress bass, mids, and treble frequencies separately. This allows you to maximise the energy in the stereo signal—modern rock and pop productions would not sound the same without multiband compressors. Keep in mind that multiband compression always affects the volumes of the various frequency bands in relationship to each other, depending on their dynamics. It requires experience and a good ear to be able to compensate for this and use this powerful tool effectively.
