What is bus compression?

Bus compression in plain English

A bus is a channel that other channels feed into. In Ableton Live it's a group track, in Logic it's usually a summing stack or an aux, and in FL Studio it's a mixer track that several others are routed to. The master channel, where everything ends up, is a bus as well, and engineers call it the mix bus, the stereo bus or the master bus.

Put a compressor on one of those and it hears the sum of everything arriving there. It works out one gain change from that sum and applies it to the whole group. When the snare hits hard enough to cross the threshold, the compressor turns down the snare, and the kick, hats and room mics with it, all by the same few decibels.

That one shared gain change is the difference between bus compression and compressing each track. It's also why the settings are so much gentler. On a single vocal you might take 6 dB off the loudest words. On a whole mix, 2 dB is already plenty, because every part of the song moves when the compressor does. The compression lesson covers what each control does in general; this page is about using them on groups.

What does glue mean?

Glue is the word engineers use for what bus compression does to a mix: the parts stop sounding like separate recordings and start sounding like a band playing together.

When the drums hit, the bus compressor turns down everything on the bus a little, including the chords and the bass. Between hits it lets them back up. So the chords pick up a small, regular movement that follows the drums. Every part on the bus now rises and falls together, in time with the loudest thing in the mix, and your ear hears that shared movement as the parts belonging together.

The figure shows it on the loops from the lab. Drag the threshold and compare the two lines. On their own, the chords would get compressed whenever the chords themselves got loud. On the bus, the dips line up with the drums. At −16 dB the chords move by around 1 to 3 dB over the bar, which you'd hardly notice as a change in volume, but you do notice the chords and drums sitting closer together.

A slow attack adds to the effect. With 30 ms of attack the front of each drum hit gets through before the gain comes down, so the drums keep their punch while the sustained parts underneath are held back a touch. Push too hard and the same mechanism turns into pumping, where you can hear the whole mix ducking on every beat.

Reading the lab

The lab opens on a short full mix of drums, bass and chords, because that's what a bus compressor hears. You can switch to the drum loop on its own to try drum bus settings, or upload your own bounce of a mix or a group. The knee is fixed at 4 dB, a gentle setting typical of bus compressors, so the page can stay on the controls that change the most.

The pale shape on the Whole loop picture is the original and the pink one is the compressed mix. The amber lane along the bottom is gain reduction, and on this page the useful thing to watch is its movement more than its depth: whether it snaps back between hits or hovers at one level. When you stop the loop, the Most turned down readout shows the most the compressor took off at any moment. The Original and Compressed switch takes the compressor out and matches the loudness. Use it every time on a bus, because the compressor changes so little that a decibel of level difference would decide the comparison for you.

Bus compressor settings

Threshold and gain reduction

Set the threshold by watching the gain reduction meter. On a mix bus, aim for 1 to 3 dB on the loudest moments, which in a pop song is usually the chorus. On a drum bus you can go further, to 3 to 6 dB, because drums can take more movement before it sounds wrong.

On the full mix in the lab, −16 dB at 2:1 gives about 3 dB. Lower it to −24 dB and the maximum climbs past 6 dB, which is already more than most mixes want. The threshold number on its own means little, because it depends on how loud your mix is. Read the meter. The gain reduction lesson goes into reading it.

Ratio

Bus compressors use low ratios. 2:1 is the usual choice on a whole mix and 4:1 is common on drums. At 2:1, a peak 6 dB over the threshold comes out 3 dB over, so loud moments are brought in a little while the quiet ones are untouched. Many bus compressors only offer a few fixed choices, often 2:1, 4:1 and 10:1, and the highest is rarely used on a mix.

A low ratio with a lower threshold sounds smoother than a high ratio with a higher one, even at the same gain reduction, because the compressor works a little on most of the mix instead of a lot on the peaks.

Attack

The attack is the control that decides whether bus compression adds punch or removes it. At 10 to 30 ms, the first part of each drum hit passes through at full level and the compressor clamps down on what follows. Under about 3 ms it catches the transient itself, so the kick and snare lose their snap and the mix sounds smaller and further away.

The lab shows the difference in numbers. On the full mix at −16 dB and 2:1, changing the attack from 30 ms to 1 ms more than doubles the most gain reduction, from about 3 dB to just over 6 dB, and nearly all the extra comes off the front of the drum hits. The crest factor shows the same thing from the other side. The loop starts at about 16.4 dB between peak and average; a 30 ms attack raises that to 17.3 dB, because the drum fronts get through while what follows is turned down, and a 1 ms attack leaves it at 16.5 dB. Start at 30 ms on a mix bus. Go faster only if a few peaks are escaping and making the mix jumpy, and listen to the drums while you do.

Release

The release sets how the compression moves, and that's most of what glue sounds like. A short release, around 100 ms, lets the gain recover between hits, so the amber lane in the lab bounces with the beat. That keeps the mix lively, and pushed harder it becomes pumping. A long release, 600 ms or more, never quite lets go between hits, so the compressor holds the whole mix down by a steady couple of decibels and the movement gets slow and smooth.

The tempo is a good guide. At 120 BPM one beat lasts 500 ms, so a release somewhere around 300 to 500 ms recovers roughly in time with the beat and breathes with the track. Shorter than that and it moves on every hit; much longer and it rides across whole bars.

Many bus compressors have an Auto release. It changes the release time with the material, letting go quickly after a short peak like a snare hit and more slowly after a long loud section. On a mix bus it's often the best place to start, because a full mix has both kinds of peak. The lab uses fixed release times so you can hear each one, and the attack and release lesson goes further into both controls.

Makeup gain

With a long release the full mix in the lab loses about 2 dB most of the time, so it comes out about 2 dB quieter, and 2 dB of makeup puts it back where it was. Match it carefully. A mix bus compressor makes small changes, and an extra decibel of level will sound like an improvement whether the compression helped or not.

Mix

A mix knob on a bus compressor turns it into parallel compression on the whole group. It's common on drum buses: compress the drums heavily, with a fast attack and a ratio of 4:1 or more, then blend in 30 to 50% under the dry drums. The transients stay on top and the room sound and ghost notes come up underneath. On a mix bus it's less common, though a mix of 50 to 70% can soften a compressor you like the sound of but find too strong. The parallel compression lesson has the details.

The SSL bus compressor

When people say bus compression, a lot of them mean one specific design: the stereo compressor built into the master section of Solid State Logic's large-format mixing consoles, from the SL 4000 series of the late 1970s onwards. It's a VCA compressor, so it reacts quickly and predictably, and its controls are a short list of fixed choices.

A common starting point on it is a 2:1 or 4:1 ratio, a 10 or 30 ms attack and the Auto release, with the threshold set for a few decibels of gain reduction. Those are the values the lab starts from, apart from the release, where 300 ms stands in for Auto.

You don't need the hardware to get the sound. Ableton Live's Glue Compressor was built with Cytomic and is modelled on a 1980s British console bus compressor, Waves has the SSL G-Master Buss Compressor, and Universal Audio has the SSL G Series Bus Compressor for UAD. Other compressors that get used across mixes include the API 2500 and vari-mu designs such as the Manley Variable Mu, which is slower and smoother and turns up in mastering. Any compressor that can do a low ratio and a slow attack will work, including the one that came with your DAW.

Drum bus vs mix bus

The same compressor behaves differently depending on what's on the bus. A drum bus has nothing but transients and short tails, so it can take faster settings and more gain reduction, and the result is a kit that sounds like one instrument instead of several microphones. A mix bus also carries the vocal and every sustained part, so anything heavy-handed shows up as the vocal ducking under the kick. The drum bus lesson covers the whole drum chain, with saturation and clipping as well as compression.

Pick Drums in the lab to try the drum bus row. At −18 dB, 4:1, 30 ms and 200 ms, the drum loop gets up to about 4.5 dB of gain reduction, and the hits still crack because the attack is slow.

How to set up bus compression, step by step

1. Route the tracks to one busSend everything you want compressed together to a group or aux, or use the master channel for the whole mix. Put the compressor first on that bus, before any limiter.
2. Start from bus settings2:1, a 30 ms attack and an Auto release, or 300 ms if there's no Auto. On a drum bus, start at 4:1.
3. Lower the thresholdPlay the loudest section and bring the threshold down until the meter shows 1 to 3 dB of gain reduction on a mix bus, or 3 to 6 dB on drums.
4. Check the drumsListen to the kick and snare while you try a faster attack. If they get softer, go back to 30 ms.
5. Set the release by the movementWatch the meter. If it bounces hard on every beat and you can hear the mix ducking, lengthen the release. If it sits still and the mix feels lifeless, shorten it.
6. Match the level and compareUse makeup gain until bypassing the compressor doesn't change the loudness, then switch it in and out. Keep it only if you can say what got better.

You can put the mix bus compressor on near the start of a mix and mix through it, or add it at the end. Mixing through it means your fader moves already include the compression, so the balance doesn't shift later. Adding it at the end makes it easier to hear exactly what it changed. Either way, set it once and leave it; if you find yourself chasing the threshold every few minutes, the balance underneath is what needs work.

Bus compression vs compressing each track

They do different jobs, and most mixes use both. A compressor on a single track evens out that one part: the vocal's loud and quiet words, the bass player's uneven notes. It reacts only to that track, so nothing else in the mix moves when it works.

A bus compressor reacts to the whole group, so one loud part moves everything. That's the point of it, and also its main risk. If the kick is much louder than everything else, the kick decides all the compression, and the vocal, the bass and the pads all dip whenever it plays. Fix the levels and the worst peaks on the individual tracks first, then use the bus compressor for the small shared movement.

This is also why bus compression is different from sidechain compression. A sidechain compressor on a bass track listens to the kick and ducks only the bass. A bus compressor ducks everything on the bus, by much less.

Common bus compression mistakes

Too much gain reduction

If the meter on a mix bus regularly passes 4 dB, the compressor is changing the balance you set with the faders. The last step in the lab shows where that leads: at −24 dB and 4:1 the full mix loses about 6 to 10 dB all the time, and the chords and bass sink every time the drums hit.

A fast attack on the mix bus

A fast attack takes the front off every drum hit at once, so the whole track loses punch, and it's easy to miss because nothing sounds broken. If the drums seem smaller after the bus compressor goes on, slow the attack before you touch anything else.

Letting the low end run the compressor

Kick and bass carry most of the energy in a mix, so they trigger most of the gain reduction. If the mix pumps with the kick, look for a sidechain high-pass filter on the compressor. Set at around 60 to 100 Hz, it stops the lowest frequencies triggering the compression, though they are still compressed along with everything else.

Judging it louder

Take 2 dB off the peaks and add 3 dB of makeup, and the compressed version comes out at least a decibel louder, so it wins a quick switch whether the compression helped or not. Match the level before you decide, either with the lab's Original and Compressed switch or by setting makeup gain until bypass makes no change to the meter.

Using it to fix the balance

A bus compressor can't fix a vocal that's too quiet or a kick that's too loud. It applies one gain change to everything, so the relationships between parts stay roughly where you left them. Sort the balance with faders and track compression first.

Train your ears

The quiz under the lab hides the controls, picks a setting and asks you to name it: fast or slow attack, or a ratio of 2:1, 4:1 or 10:1. It uses heavier settings than you'd put on a mix bus so the differences are clear, and it works on whichever sound you have selected. Start on Drums, where the attack rounds are easiest, then switch to the full mix, which is closer to what you'll hear on your own mix bus.

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