Different Frequencies, Same Compressor Settings
If a recording of a bright guitar and a bass drum is sent through a compressor with a short attack time, the transients of the guitar will be tamed, but the bass drum will lose its punch and feel choked. With the short attack time, the low frequency transient information in the bass drum does not have enough time to make it through before the compressor kicks in. If the attack time is set longer, the bass drum may have the desired punch before the compressor kicks in but the guitar will sound harsh. Its high frequency transients take a far shorter time to get through — the compressor will not compress the guitar much at all.
Regardless of whether a shorter or longer attack time is used, the compressor will favor different sections of the frequency spectrum in the recording and change the relationship between elements in the mix. In order to effectively compress the wide range of frequencies contained in a recording, different attack and release times would be required for different parts of the frequency spectrum. Multiband compression makes this possible.
What is multiband compression?
Multiband compression divides the frequency spectrum into different sections, or bands, so that each has its own unique compression settings. This allows a longer attack time for the low band of that bass drum to punch through, while keeping a shorter attack time in a higher band to keep the guitar in check. By using a multiband compressor, it is possible to closely tailor the compression to the different elements in a mix and compress the recording more transparently than with a standard single-band compressor.
Most multiband compressors feature either 3 or 4 different bands. Since the crossover frequencies define the range of frequencies contained in each band, getting them right can have a big impact on the effectiveness of the compression. Many multiband compressors have a solo function to focus on one band at a time — this can be very helpful when setting the crossover frequencies by revealing exactly what each band contains. Additionally, soloing individual bands can be helpful when setting attack and release times to make sure the transients in each band are shaped as desired.