February 12, 2024
Compression is one of the most important and misunderstood tools in an audio arsenal. In previous posts, we’ve talked about using compression on stage and compression tips specifically for bassists, but here we’ll introduce a slightly more esoteric tool – multiband compression.
Multiband compressors have the same settings as single band compressors – attack, release, threshold, ratio, makeup gain, knee – but instead of being applied to the entire frequency range, they divide the signal into several bands, each with their own compressor. So, in essence, a multiband compressor is several compressors in one.
This means you can compress, say, low frequencies with different settings than mids and high frequencies, or you could compress certain frequencies and leave others alone.
Multiband compressors usually have 3 or 4 bands, but more wouldn’t be out of the question. Often these bands are adjustable, so you can decide which frequencies are affected. They’re commonly used in mastering, but they can also be used to judiciously tame or shape single tracks such as vocals or bass.
Multiband compression is especially useful if you need to compress something complex like a full mix, but you don’t want to mess up the transient character of one element verses another. This comes up especially with something like bass versus guitar.
This is because bass waveforms are longer than higher sources like lead guitar. They need a longer time for transients to develop, and you might not want the compressor to chop that transient off. So, a slower attack time might be good for the bass, but not so great for a bright guitar which might be better served by a shorter attack.
Using multiband compression, you can set different attack and release times for these two elements.
Of course, if you’re mixing and these tracks are separate, you might as well compress them separately. If you’re mastering a stereo track, though, you might not have the luxury of separating these tracks.
Another common use of multiband compression is in mixing bass parts. For example, with a multiband compressor, you can compress the low end of a bass without affecting the transient character of the mids and highs, which can help keep the bottom end even. This is helpful with live bass parts.
Vocals can also benefit from multiband compression sometimes, for example to push highs just a bit more than the rest to lift them over clashing synths, or to tame harsh frequencies in the high mids. As it turns out, a de-esser is simply a band-limited compressor which you can recreate by using a multiband compressor and only using the high-mid band to home in on sibilance.
Multiband compression can also be useful in mastering to achieve a louder sounding end result, especially if the mix is not quite balanced – say if it’s a little boomy and hard to tame on the low end.
Finally, you can try multiband compression anytime you’re having trouble dialing in the right settings on a single band compressor, or if you want to try something wild to seriously change the nature of a track.
Multiband compressors employ filters and crossovers to split the signal into multiple frequency bands. Any time this happens, noise, ringing, or other undesirable artifacts can be introduced. Multiband compression can also severely alter the tonal balance of an instrument, making it sound unnatural. This is more problematic with acoustic music or music with “real” instruments than it is with electronica and other already “unnatural” sounds.
Mastering engineers often use multiband compressors, but some of the best in the business only use them sparingly for this reason.
All in all, it’s not a good idea to reach for a multiband unless it’s really needed – or if you want to do something crazy.
Multiband compression can be a life saver in some situations, and if you’re having a hard time achieving what you want with a regular compressor, you might want to try a multiband. But they’re not a work-a-day type of tool. Think of multiband compression as a specialty tool – one that can be quite useful at times.
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