Bus compression and glue

What it is and how it works

Bus compression is applied to a group of summed tracks (drum bus, vocal bus, music bus, mix bus). A single compressor reacting to the sum makes all the elements move together: when the kick hits, the others dip a little too. This bond makes the group more cohesive, as if it had been played in the same room: that is the “glue”. You need little reduction (1–4 dB), low ratios and musical timings.

Bus structure in FL Studio

• Create mixer tracks for the groups: Drums (kick, snare, clap, hi-hat, percussion), Bass (bass + 808), Music (synths, guitars, piano, pads), Vox (lead vocal, doubles, backing vocals), FX (risers, impacts) and the send effect tracks (reverb, delay).
• Route each track to its bus: select the source and, on the bus track, right-click the route arrow → “Route to this track only” (removes the direct send to the Master). The buses then go to the Master.
• The tracks remain visible and you can work by groups (bus fader = group volume).

Parameters and typical values

• Ratio: 2:1 on the mix bus, 2–4:1 on subgroups, rarely 10:1 (effect).
• Attack: slow-medium, 10–30 ms: lets the transients through (punch) and compresses the body. Attacks of 0.1–1 ms flatten the mix.
• Release: Auto (widely used on the SSL) or in time: 100–300 ms. The release determines the “breathing”: watch the GR meter, it should return to zero between the strong beats.
• Threshold / GR: 1–2 dB on the mix bus, 2–4 dB on drum bus and music bus, 2–3 dB on the vocal bus.
• Sidechain HPF: 60–150 Hz. Prevents kick and 808 from controlling the whole bus (pumping).
• Mix: 50–100%; with the mix at 50–70% the glue is more transparent.
• Knee: soft on the mix bus.

The classic: SSL G Bus Compressor

Ratio 2, 4 or 10:1; attack 0.1, 0.3, 1, 3, 10, 30 ms; release 0.1, 0.3, 0.6, 1.2 s and Auto. Starting setting for the mix bus: 2:1, attack 30 ms, release Auto (or 0.1 s), GR 1–4 dB, then make-up for equal level. For the drum bus: 4:1, attack 10–30 ms, release 0.1 s or Auto, GR 3–4 dB.

Mix bus: before or after?

• Mixing “into” the compressor: you insert it early (after the rough balance) and mix while already hearing its action. Many engineers do this: the mix adapts to the glue.
• Inserting it at the end: you risk changing the balance as soon as you insert it.
• Either way: light GR, comparison at equal volume, and no limiter “to hear it” when you evaluate the compressor.

Starting settings by bus

By genre

• Pop/K-pop: SSL mix bus 2:1 1–2 dB, carefully crafted vocal bus.
• Rock: drum bus 3–4 dB that “breathes”, mix bus 2 dB.
• Hip hop/trap: drum bus often with saturation and clipper instead of compression; mix bus 1–2 dB or nothing, loudness achieved in the master.
• EDM/house: the groove comes from the sidechain; light mix bus compressor with release in time (1/8 or 1/16).
• Jazz/orchestral: mix bus almost nil (0–1 dB), light vari-mu.

Combinations on the mix bus

1. Light corrective EQ (rarely).
2. Glue compressor (1–2 dB).
3. Color EQ (Pultec, Baxandall: +0.5/+1 dB air).
4. Light saturation (tape or console).
5. Clipper/limiter only for reference listening (remove it before exporting the premix for mastering, or leave it if you do the master yourself).

Common mistakes

• Attack too fast on the mix bus: the mix flattens and loses punch.
• Too much GR (over 4 dB on the mix bus): pumping and closing down.
• No sidechain filter with a very strong 808: the whole bus dips on every 808 note.
• Glue on buses with tracks that are already heavily compressed: little effect, more noise and flatness.
• Uncompensated make-up: the glue “sounds better” only because it is louder.

How to hear it

• Temporarily bring the GR to 6–8 dB and listen to the pumping; move the release until it breathes in time; then reduce the threshold until you get 1–2 dB.
• A/B at equal volume: with the glue the elements seem to be “in the same space”, the tails come closer to the hits, the mix sounds more finished.