Module 4.2 · Tools Track · Compression
Parallel & Sidechain Compression
Two advanced techniques every working engineer uses every session: parallel (heavy compression blended with dry for the best of both) and sidechain (one signal triggering compression on another). Plus bus glue and the EDM pumping effect.
Module 4.1 taught you the basic compressor: threshold, ratio, attack, release, knee, makeup gain. You can now control dynamics on a single track by sending it through one compressor and adjusting those controls until the result sounds right.
This module covers the two techniques that take compression from "controlling dynamics" to "engineering a mix." Parallel compression lets you have BOTH the punch of an uncompressed signal AND the consistency of a heavy-compressed one — at the same time, blended. Sidechain compression lets one signal control the compressor on a totally different signal — the trick that makes kicks and basses coexist, and the basis of the EDM "pumping" effect. Both techniques are in nearly every commercial mix you've ever heard.
They sound advanced. They're not. Both are simple routing tricks once you see them. We'll define four words first, then walk through the practice.
⛓ Where this sits in the signal chain
Parallel and sidechain compression sit inside the compression stage of your channel chain (after subtractive EQ, before additive EQ). Bus compression sits at the bus stage — multiple tracks summed first, compressor on the bus output. Sidechain compression is often used post-compression on the source channel (the kick has its own compression first, then sends a copy to trigger the bass compressor). Parallel compression typically uses an aux/return — the dry signal stays on its track, while a duplicate is sent to a parallel-comp aux for the heavy compression. We'll cover the routing for each below.
First, the words
Four ideas. Two name techniques (parallel, sidechain), one names a use case (bus compression / glue), one names the audible effect (pumping).
Concept 1
Parallel compression
Running a heavily-compressed copy of a track in parallel with the original, blending both at the output.
Think of it like having two of the same singer in the room — one whispering quietly, one shouting passionately — both at once.
A standard compressor is a trade-off. To make a track consistent (smashing the loudest peaks down) you have to give up the transients (the snap of a snare, the click of a kick, the consonants of a vocal). Push the compression hard and you get body but lose snap; back it off and you get snap but lose body. Parallel compression breaks this trade-off. Instead of one compressor, you run two paths in parallel: the original signal (full transients, full dynamics) AND a copy that's been heavily compressed (consistent, all body, no transients). At the output, you blend them. The result has the transients of the original AND the body of the compressed copy — both at full strength, no compromise. Engineers call this the "New York" trick or "drum smashing"; it works on drums, vocals, and the entire mix bus. Most professional records use it on at least one element.
Concept 2
Sidechain input
Telling a compressor to react to a different signal than the one it's compressing.
Think of it like a traffic light controlled by another car's movement.
Normally a compressor's threshold listens to the same signal it's processing — the bass listens to the bass. Sidechain is when it listens to a different signal entirely. The most common use: the bass channel's compressor is sidechained to the kick. Every time the kick hits, the compressor on the bass clamps down for a fraction of a second, ducking the bass to make room — then releases and the bass returns to full level. The kick punches through cleanly, the bass stays out of its way, and they coexist in the low-end without fighting. The bass's own level isn't what triggers the compression; the kick's level is. Sidechain is also used on broadcasting (voice ducks the music behind it), on reverb returns (the reverb ducks under the dry vocal), and as the basis of the EDM pumping effect (next concept).
Concept 3
Bus / glue compression
Light compression applied to a group bus to make multiple tracks sound like one cohesive unit.
Think of it like a band director keeping every player in time and ensemble together.
When you route 8 drum tracks (kick, snare, hats, OH, toms, room) to a single drum bus, you can apply gentle compression on that bus that "glues" them together. The whole drum kit responds as ONE unit — when the band hits a fill, the compressor reacts to all of them as a single signal, and they pull together like a team rather than 8 independent tracks. This is "glue compression" — settings are light: 2:1 ratio, 1–3 dB of gain reduction, slow attack, medium release. The same trick on the master bus (a "mix bus glue" compressor) makes the whole mix feel cohesive. The classic gear is the SSL G-Series bus compressor — every modern DAW has a stock equivalent. Almost every commercial mix uses some form of bus glue.
Concept 4
Pumping
When sidechain compression gets aggressive enough to be audibly rhythmic — you can hear the duck.
Think of it like the rhythm of a heartbeat under the music.
At gentle settings, sidechain compression is invisible glue — the kick and bass coexist cleanly, and listeners don't notice the duck. At extreme settings, the duck becomes the effect: every kick hit slams the bass and pads down audibly, then they swell back up before the next kick. The result is a rhythmic "breathing" or "pumping" — most famously in French house (Daft Punk, Cassius), modern EDM, and some pop production. Pumping isn't a mistake; it's a deliberate stylistic choice. Settings: aggressive ratio (8:1+), fast attack (under 5 ms), release timed to the song's rhythm (often around 200–400 ms for 4-on-the-floor), 6–10+ dB of gain reduction at peaks. Subtle pumping is invisible glue; obvious pumping is a genre signature.
The diagram below shows both routings side by side. Parallel compression splits one signal into two paths and blends them at the end; sidechain compression has one signal triggering the compressor on a totally different signal. Different problems, different tools.
Parallel compression splits one signal into two paths — one dry, one heavily compressed — and blends them at the output. The dry keeps the transients; the smashed copy fills in the body. Result: punch and body, no compromise.
Sidechain compression uses one channel's signal (the kick) to control the compressor on a totally different channel (the bass). The bass's own audio passes through unchanged; the kick decides when the compressor clamps down. Perfect for kick-bass coexistence in the low-end and the basis of the EDM pumping effect.
Going deeper
Why parallel compression works — the engineering
The standard compressor faces a real engineering trade-off. Compressing the loudest peaks (the snare hit, the kick attack, the vocal consonant) shrinks the dynamic range — that's the whole point. But the peaks ARE the transients. Smash the peaks, lose the snap. Pull back to preserve snap, lose the consistency you wanted.
Parallel compression sidesteps the trade-off by processing two copies. The dry copy keeps every peak, every transient, exactly as recorded. The compressed copy gives you the consistent body — what's there between the peaks, lifted up to be audible. Blending them means the transients (from the dry) sit on top of the body (from the compressed), and you have both at full level. That's why parallel compression on drums sounds bigger, more powerful, and more punchy than either path alone.
The classic application: "smash" the parallel hard (8:1 or higher ratio, 10–20 dB of gain reduction at peaks), then dial in just enough of it to feel the body. Most engineers settle around 25–40% blend on drums. Less and you don't hear the difference; more and the dry transients get diluted.
Parallel compression — settings per source
| Source | Ratio | Attack | Release | GR target | Blend |
|---|---|---|---|---|---|
| Drum bus | 10:1 | 1–5 ms | 50–100 ms | 10–15 dB | 25–40% |
| Lead vocal | 4:1 | 3–10 ms | 100–200 ms | 6–10 dB | 15–25% |
| Bass guitar | 4:1 | 10 ms | 100 ms | 6–8 dB | 20–30% |
| Acoustic guitar | 3:1 | 10 ms | 200 ms | 4–6 dB | 15–20% |
| Mix bus | 2:1–4:1 | 10 ms | 150 ms | 4–6 dB | 10–15% |
Why sidechain compression works — the engineering
Two instruments living in the same frequency band fight each other (Module 3.4 covered this). The most common offender is kick and bass — both occupy 60–120 Hz, and at any moment when both are sounding, listeners can't perceive either one cleanly. Standard solutions: cut the bass at 60 Hz to give the kick room, or cut the kick at 100 Hz to give the bass room. Both work; both have side effects.
Sidechain compression is a different solution: time-domain separation. Instead of cutting frequencies, you cut time. The compressor on the bass listens to the kick. When the kick hits, the bass briefly ducks (gain reduced 3–6 dB) for a tenth of a second. The kick passes through cleanly with all its frequency content intact. Then the bass returns to full level until the next kick. You haven't EQ'd anything; both instruments have all their original frequencies. They simply don't both happen at full volume at the same moment.
The technique works because the human ear doesn't notice a 3–6 dB duck that lasts 100 ms — but it absolutely notices a missing 60 Hz on the bass. Sidechain achieves what subtractive EQ achieves, without the tonal compromise.
Sidechain compression — common use cases
| Use case | Trigger | Compressed | Settings |
|---|---|---|---|
| Kick → bass duck | Kick | Bass | 4:1, fast attack, ~150 ms release, 3–6 dB GR |
| Kick → pad pump | Kick | Synth pad | 8:1+, fast attack, release matched to song tempo, 6–10 dB GR (audible pump) |
| Vocal → reverb duck | Lead vocal | Reverb return | 4:1, fast attack, 200–400 ms release, 4–8 dB GR |
| Vocal → music duck (broadcast) | Voice | Music bus | 2:1, slow attack, 1–2 sec release, 6–10 dB GR |
| Snare → ambient pad ducks | Snare | Pad/atmosphere | 4:1, fast attack, fast release, 3–5 dB GR |
| Lead vocal → backing vocals | Lead vox | Backing vox bus | 3:1, slow attack, 200 ms release, 2–4 dB GR |
"Sidechain isn't a special effect — it's a routing trick that solves a frequency problem in the time domain. Once you start hearing the duck on commercial records, you can't unhear it." — FTM, on the ubiquity of sidechain in modern mixing
Bus / glue compression — making tracks feel cohesive
Bus compression is the most-used type of compression on a finished mix. The principle: route a group of related tracks to a bus, then put a single compressor on the bus output. The compressor reacts to all the routed tracks together as a single signal, so when the kit hits a fill or the band peaks together, the compressor reduces gain on the WHOLE group equally. The result is cohesion — the tracks feel like they're moving together rather than as independent pieces.
Common bus compression chains
| Bus | Source tracks | Settings |
|---|---|---|
| Drum bus | kick, snare, hats, OH, toms, room | 2:1 or 4:1, slow attack (10–30 ms preserves transients), medium release, 2–4 dB GR |
| Vocal bus | lead + all backing vocals | 2:1, slow attack (10 ms), medium release, 2–3 dB GR |
| Mix bus / 2-bus | everything that's NOT the master | 2:1, slow attack (30 ms), medium release, 1–3 dB GR — gentle, glue only |
| Group bus (e.g., guitars) | all electric guitars | 3:1, medium attack, fast release, 2–4 dB GR |
The classic gear for bus compression is the SSL G-Series Master Bus Compressor (a stereo VCA compressor built into SSL consoles). Plugin equivalents: Waves SSL G-Master, UAD SSL 4000 G, Cytomic The Glue, and the bus-comp section of Logic's Compressor (set to "VCA" mode). The settings are conservative — bus compression's job is glue, not control.
The "pumping" effect — when sidechain becomes the sound
French house in the late 1990s (Daft Punk's "Da Funk," Cassius's "Cassius 1999") and modern EDM made the sidechain duck audible — and called it a feature. The bass and pads sidechain heavily off the kick, so every quarter-note kick produces an audible "ducking and breathing" in everything else. Listeners hear the kick punching through and the rest of the mix swelling back up between hits.
For audible pumping:
- Aggressive ratio — 8:1 or higher.
- Fast attack — 1–3 ms (you want the duck to land instantly when the kick hits).
- Release tied to song tempo — at 120 BPM with a 4-on-the-floor kick, each kick is 500 ms apart, and you want the bass back at near-full level just before the next kick. Aim for 300–400 ms release for clean breathing.
- Aggressive gain reduction — 8–12+ dB at peaks.
- Often applied to a parallel pumping bus — send the pad/bass to a "pump bus," apply the sidechain there, blend with the dry to taste.
Subtle pumping (3–6 dB GR, fast release) is invisible glue. Audible pumping (8+ dB GR, tempo-matched release) is a stylistic choice. Use intentionally.
Common parallel and sidechain mistakes
- Parallel comp set too loud. The blend overwhelms the dry, and the result loses transients. Pull the parallel down until you hear the body fill in under the dry, not on top of it.
- Forgetting to bypass the dry. If your parallel aux is processing the same signal as the original AND the original is unprocessed, both get summed twice. Make sure the parallel-comp bus output is treated as a SEND-only path, not a parallel feed of the same signal getting muddled.
- Sidechain release too long. If the bass is still ducked when the next kick hits, the sidechain just stays pulled down constantly — the bass never returns to full. Match the release to the song's rhythm so the duck recovers between kicks.
- Sidechain release too short. The duck snaps back immediately, creating a "blip" that can sound choppy. Pull the release up until the recovery is smooth and musical.
- Sidechain attack too slow. If the attack is 30 ms, the kick is past before the bass even ducks — the duck is happening behind the kick. Fast attack (1–5 ms) is the rule.
- Pumping when you don't want pumping. If the sidechain is audibly breathing on a non-EDM mix, it's too aggressive. Pull the GR back to 3–6 dB for invisible glue.
- Bus compression too aggressive. Glue compression is supposed to be invisible. 6+ dB of GR on a bus comp sounds squashed and lifeless. Stay at 2–4 dB for actual glue.
- Bus compression on top of full-track compression. If every individual track is heavily compressed AND the bus is heavily compressed AND the mix bus is heavily compressed, you're stacking 6+ stages of compression and the dynamics are gone. Pick the level where compression matters most and ease off elsewhere.
In your DAW
Setting up sidechain routing varies more by DAW than any other technique in this course. Here's where to find it:
Sidechain & parallel routing per DAW
Logic Pro
Sidechain: open the compressor on the bass track, click the "Side Chain" dropdown in the top-right of the plugin window, choose the kick channel as the source. Works on stock Compressor and Logic's other compressors. Parallel: route source to a bus, create an aux track receiving from that bus, insert heavy compressor on the aux, fade aux up alongside the source.
GarageBand
Sidechain: the stock Compressor doesn't expose a sidechain input; you'll need a third-party plugin (Logic Pro's compressor if available, or TDR Kotelnikov free). Parallel: not natively supported as easily — duplicate the track, compress one, blend faders.
Ableton Live
Sidechain: click the small triangle in the bottom-left of the Compressor device to expand the sidechain section. Set Audio From: kick channel. Pre-FX or Post-FX as needed. Parallel: use a Return Track. Send the source to the return; load heavy compressor on the return; bring up the return knob on the source.
Pro Tools
Sidechain: on the bass channel, insert a compressor with sidechain key input. Set the kick channel's output to a bus that's also routed to the bass compressor's key input. Use the compressor's "Key" or "External" sidechain selector. Parallel: route source to a bus, create an aux input from that bus, insert compressor on the aux, blend.
Reaper
Sidechain: Reaper's track routing is flexible. Send kick to bass channel via the Send window with channels 3-4 selected (channels 1-2 are the bass's main audio; 3-4 are the sidechain inputs). Set the compressor's sidechain to listen to channels 3-4. Parallel: route source to a bus track, parallel comp on the bus, blend.
FL Studio
Sidechain: right-click the compressor → "Sidechain to this track" → choose the kick mixer insert. Or use Fruity Limiter / Fruity Compressor, both of which support sidechain input. Parallel: route the source to a parallel mixer insert via send, apply compression, blend the parallel insert.
Studio One
Sidechain: on the compressor, click the "Side Chain" button (a small chain icon) and select the trigger track from the dropdown. Parallel: use a Bus Channel; send the source to the bus; compress on the bus; bring up the bus's "Send" volume on the source.