What is multiband compression?

Multiband compression in plain English

An ordinary compressor listens to the whole sound and turns the whole sound down when it gets too loud. That works well on a single vocal or a snare. On a full mix it has a problem: most of the energy in a mix sits in the bass and the kick, so those are what push the level over the threshold. Every time the kick lands, the compressor turns down everything, including the vocal and the chords, which were fine.

A multiband compressor fixes that by splitting the sound first. Filters called crossovers divide it into bands by frequency, for example everything under 150 Hz, everything between 150 Hz and 2.5 kHz, and everything above. Each band goes through its own compressor with its own threshold and ratio, and the bands are added back together at the end. The low compressor only hears the low band, so a big kick turns down the low end and nothing else.

If you've read what compression is, you already know every control on each band. The new parts are the crossovers and the idea that a compressor can be pointed at one range of frequencies.

What is multiband compression used for?

It's for problems that live in one frequency range and come and go with the music. Take a bass line where two or three notes boom out louder than the rest: cutting the bass with an EQ would thin out every note, quiet ones included. A compressor on the low band only acts when those loud notes arrive, and leaves the rest of the line as it was.

The same thinking covers a lot of common jobs:

• Holding down a low end that jumps up on every kick, on a mix bus or a master, without making the whole track pump.
• A vocal that turns harsh around 2 to 5 kHz only when the singer pushes, while quiet lines keep their presence.
• The boom of an acoustic guitar when it's strummed hard.
• Splashy cymbals on a drum bus, pulled back without dulling the snare.
• Squashing every band hard on purpose for a dense, bright effect, which is what OTT does.

De-essing is a special case of the same idea: a compressor that only reacts to the hissy range of a voice, roughly 5 to 10 kHz, so sibilance gets turned down while the rest of the word stays put.

Multiband compression vs normal compression

Say a mix has a kick peaking at −8 dBFS, and you put a single compressor on it with a threshold of −20 dB and a 4:1 ratio. The kick is 12 dB over, so it comes out 3 dB over and the compressor takes off 9 dB. For as long as the compressor holds that gain reduction, the vocal, the chords and the hats are all 9 dB quieter too, then they swell back up before the next kick. That's pumping, and on a mix that wasn't meant to pump it's a sign the compressor is the wrong tool for the job.

Now split the same mix at 150 Hz. In the lab's full mix loop the low band peaks at about −9 dBFS on its own, so with its threshold at −20 dB and 4:1 the kick's low end is 11 dB over and, on paper, the low compressor takes off a little over 8 dB. In the lab the low band's gain reduction only reaches about 4 dB, half the figure on paper, because a 10 ms attack is slow next to the short peak at the start of each kick, so the gain never gets all the way down before that peak has passed. The mid band, with the chords in it, keeps its threshold at 0 dB, so its gain reduction stays at 0 dB and the chords don't move at all.

The cost is that multiband compression changes the tone as well as the level. When the low band is 4 dB down and the mid and high bands are where they were, the mix is thinner at that moment. That's often exactly what you want, but it means you have to listen to the balance of the bands, and to the dynamics within each one.

Reading the lab

The big screen shows the loop after the crossover, one lane per band: violet for low, teal for mid and pink for high. The pale shape in each lane is the band before its compressor and the coloured shape is after it. Each lane is scaled to its own loudest moment, so the high band is still easy to see even though it carries only a few per cent of the energy in most sounds. The amber line under each lane is that band's gain reduction, and hovering anywhere gives you all three figures at that moment.

The meters next to it show gain reduction live for each band, plus the output level. Below, the tone screen draws what the compressors are doing to the frequency balance. The thin coloured curves are the three bands of the crossover, and the pink line is all of them added together, with each one turned down by its compressor. While the loop plays the pink line moves with the music, which is a good way to see that a multiband compressor behaves like an EQ that changes from moment to moment.

The Listen to buttons solo a band, so you can hear exactly what each compressor is working on. A/B switches all three compressors off and turns the result to the same loudness, and a soloed band stays soloed, so you can compare one band compressed and uncompressed. The panel on the right explains your current settings with numbers measured from the loop, including how much of the sound's energy is in each band.

The loops come from Sounds Like You packs, and the full mix is the default because that's where multiband compression earns its place. You can also upload your own file or record eight seconds with your microphone, and it loops through the lab instead. Nothing leaves your device.

How the crossover splits the sound

A crossover is a pair of filters at the same frequency, a low-pass that keeps everything below it and a high-pass that keeps everything above. Three bands need two crossovers. Each filter slopes away gradually, so the bands overlap a little around each crossover frequency.

The lab uses a Linkwitz-Riley crossover, the type many multiband compressors use. Each side falls at 24 dB per octave, and both sides are 6 dB down at the crossover frequency itself, which is exactly the amount that makes the two halves add back up to the original level. With a 150 Hz crossover the low band is already about 25 dB down one octave above, at 300 Hz, so very little of the mid range reaches the low compressor.

Adding the bands back together restores the level of every frequency, but the phase near each crossover comes out shifted slightly in time. On most material you won't hear it. Some multiband compressors, FabFilter Pro-MB and the Dynamics module in iZotope Ozone among them, offer a linear-phase mode that avoids the shift in exchange for extra latency and a little pre-ringing before sharp hits; the linear-phase EQ lesson explains that trade.

The controls

Each band has the same controls as a normal compressor, so a three-band compressor has three thresholds, three ratios and so on. Some plug-ins share attack and release across the bands to keep things simple, and the lab does too; most give every band its own.

Crossover frequency

This sets where one band ends and the next begins, and it matters more than any other setting. Put it between the things you want to treat differently. To control a kick and bass without touching the rest, a low crossover around 100 to 150 Hz is a good start. To catch a harsh vocal, you want a band that starts around 2 kHz and ends around 5 kHz, and to control cymbals, one that starts around 6 to 8 kHz.

Avoid putting a crossover on top of the main notes of an instrument. If the crossover sits at 110 Hz and the bass line lives at 110 Hz, each note is split between two compressors that behave differently, and the bass can sound uneven in a way that's hard to pin down. Solo the band and listen: if half an instrument is in it, move the crossover.

Threshold

Each band's threshold is set against the level of that band alone, which is lower than the level of the whole sound. That catches people out. In the lab's full mix loop the low band peaks at about −9 dBFS and the high band at about −7 dBFS, but the high band's average level is far lower than the low band's, because hi-hats are short spikes. A threshold of −20 dB means something different on each band, so set each one by watching that band's own gain reduction meter.

Ratio

Ratio works exactly as it does on a single compressor: at 4:1, a band 8 dB over its threshold comes out 2 dB over. Multiband compression usually wants gentler ratios than you'd use on one instrument, around 2:1 to 4:1, because each band is a slice of a whole sound and a heavy ratio on one slice is easy to hear as a change in tone. The threshold and ratio lesson goes into the curve itself.

Attack and release

Low frequencies have long cycles. A 55 Hz bass note takes about 18 ms to complete one, so a low band with a release shorter than that can start following the waveform up and down, which sounds like distortion. Give the low band slower settings, something like a 20 to 30 ms attack and a release of 150 ms or more. The high band can be much faster, with an attack of 1 to 5 ms and a release of 50 to 100 ms, because the sounds in it are short and the cycles are tiny.

As on any compressor, a slow attack lets the front of each hit through. On a drum bus, a 30 ms attack on the high band keeps the snap of the snare while still pulling down the cymbal wash behind it. Attack and release has the full picture.

Band gain

Every band has an output gain after its compressor. It's makeup gain for that band, but it also works as an EQ: +3 dB on the low band is a broad boost to everything below the crossover. This is where multiband compression gets useful for shaping a sound. You can compress the low end so it stops jumping around, then turn the band up so it sits fuller than before, and the low end ends up both louder and steadier.

Solo and bypass

Almost every multiband compressor lets you solo or bypass each band. Solo is how you find the crossover frequencies: listen to the band on its own and move the crossover until only the sound you want is in it. Bypassing a band switches its compressor off while keeping the crossover, which is the fairest way to hear what one band's compression is doing.

How to use a multiband compressor, step by step

1. Name the problem firstSay in words what's wrong and where: the low end jumps on every kick, the vocal turns harsh on loud notes, the cymbals splash. If it's wrong all the time, use an EQ instead.
2. Set the crossovers by soloingSolo the band and move its crossover until only the problem is in it. In a three-band compressor you may only need one crossover in the right place, with the other left out of the way.
3. Leave every other band aloneKeep their thresholds at the top so their compressors do nothing. It's normal for only one band to be working.
4. Bring that band's threshold downAim for 2 to 4 dB of gain reduction on that band's meter when the problem happens, and close to 0 dB the rest of the time.
5. Set attack and release for the bandSlower on low bands, faster on high ones. Watch the band's gain reduction recover between hits.
6. Match the level with band gainTurn the band up until it sits as loud as before on average, then decide whether you want it louder still.
7. Compare at the same loudnessBypass the whole compressor and back. If the version without it sounds as good, take it out.

Multiband compression settings for common jobs

These are starting points for the one band doing the work, with the other bands left uncompressed. Use the band's meter and your ears to finish the job.

Multiband compressors in your DAW

You probably have one already. Ableton Live's Multiband Dynamics has three bands and can compress upward as well as downward. Logic Pro's Multipressor has up to four bands, FL Studio's Maximus has three plus a master section, and Reaper comes with ReaXcomp, which lets you set as many bands as you like. Among third-party plug-ins, Waves C4 is a long-standing four-band design and FabFilter Pro-MB, with up to six bands that can each sit anywhere in the spectrum, is a common choice for mastering.

They differ in how the bands are drawn and how many you get, but the controls on this page are on all of them.

Multiband compression vs dynamic EQ

A dynamic EQ attacks the same kind of problem from the other side. It skips the crossovers and uses normal EQ bands, such as a bell or a shelf, which cut or boost more as the level in their range goes up. That makes it the better tool for something narrow, like a resonance at 3.2 kHz that only rings on loud notes. Multiband compression is better for broad ranges, such as the whole low end of a mix. Multiband compression vs dynamic EQ compares the two side by side.

Common multiband compression mistakes

Using it to fix a static problem

If a sound has too much 300 Hz all the way through, that's an EQ job. A multiband compressor only helps when the problem rises and falls with the music. Using one on a static problem just adds a compressor that's always on, which flattens the band and changes its tone at the same time.

Compressing every band because it's there

Three bands of controls invite you to use all three. Most of the time one band needs work and the others should do nothing at all. If you can't say what a band's compressor is fixing, put its threshold back up.

Judging it by loudness

Band gain makes it easy to turn the result up, and louder nearly always sounds better in a quick comparison. Match the level before deciding, with the A/B in the lab or the bypass in your plug-in with the output gain set carefully.

Fast releases on the low band

Settings that sound fine on a high band can distort a low one, because the compressor follows the waveform of the bass instead of its level. If the low end turns buzzy or grainy when you compress it, slow the release down.

Losing the punch

Squash every band hard and the sound gets dense and bright, with nothing standing out from anything else. That's the OTT sound, and it's great on a synth pad and wearing on a whole mix. Keep gain reduction to a few decibels unless you're after that effect.

Train your ears

The quiz under the lab compresses one band hard and leaves the other two alone, then asks which band it was. The band's gain is set so its average level stays where it was and the only clue is which part of the sound has stopped jumping out. Start with the full mix, where the low and high bands are easiest to tell apart, then try it on the vocal.

Questions people ask

Keep going