Multiband compression is often used in mastering to give a final polish to the overall mix. It turns out that using a full-band compressor to compress a complete mix is not ideal because the low-frequency sounds in the mix will determine the compression
applied to everything else. The kick drum and
bass line would probably dictate how the mix will be compressed if you used a full-band compressor.
A multi-band compressor comprises a
set of filters that splits the audio
signal into several frequency bands. After
passing through the filters, each frequency band is fed into its own compressor,
after which the signals are recombined. The main advantage of multi-band
compression is that a loud event in one frequency band won't trigger gain
reduction in the other bands. For example, a loud kick drum will not pull the whole mix down with it, only the low-frequency sounds (kick and
bass instrument) will be compressed, leaving the mid-range and high frequencies
untroubled. Similarly, a loud event in the mid-range won't affect the extreme
high or low frequencies for the same reason.
The 4-band multi-band compressor in Cubase allows you to control the range of frequencies in each band to be compressed, the type of compression to be applied to each band (i.e. the threshold, ratio, attack and release) as well as the overall level of gain to be applied to each frequency band. A multi-band compressor is therefore a very powerful tool that can provide an improved finish to the final mastered track. Using a multi-band compressor's controls appropriately is absolutely critical. A multi-band compressor can ruin a mastered track if it is not used correctly.
