If the duck should follow the drums, including fills, missing beats and how hard each kick hits, use a sidechain compressor, because it reacts to the kick that's really there.
If you want exactly the same shape on every beat, or a pump that carries on through a part with no drums, use a volume shaper, which in its usual mode follows the tempo and nothing else.
At a glance
| Property | Sidechain compressor | Volume shaper |
|---|---|---|
| What it listens to | A trigger track, usually the kick | Usually nothing; it follows the project tempo |
| What sets the shape | Threshold, ratio, attack and release | A curve you pick or draw |
| Kick fills and gaps | Follows them | Ignores them, unless it's triggered by MIDI or audio |
| Loud and quiet kicks | Ducks more for louder kicks | Same duck every time |
| Tempo changes | Release stays in milliseconds, so the feel changes | The curve stretches with the tempo |
| Setup | Needs sidechain routing in your DAW | One plug-in on the track |
| Best for | Kick and bass, ducking under vocals, anything off the grid | Designed pumps on pads and chords, and breakdowns with no kick |
What is sidechain compression?
A compressor normally listens to the sound going through it. Feed its sidechain input from a different track and it listens to that one instead. Put a compressor on the bass, key it from the kick, and every time the kick crosses the threshold the bass gets turned down; when the kick fades, the release lets the bass back up.
The duck comes from the kick itself, so it changes when the kick does. A harder kick goes further over the threshold and ducks the bass further, a missing kick means no duck, and a fill means a quick run of them. The sidechain compression lesson covers the rest, including the routing in Ableton Live, Logic Pro and FL Studio.
What is volume shaping?
Volume shaping is ducking drawn by hand. A shaper plug-in turns a track's volume up and down along a curve, and repeats that curve in time with your project's BPM: once a beat for the usual kick-and-bass pump, or once every eighth note, half bar or bar if you want something else. Under the hood it's an LFO with a shape you choose, locked to the tempo and pointed at the volume.
Xfer Records' LFOTool, Cableguys' VolumeShaper (also part of their ShaperBox) and Kickstart, made by Nicky Romero with Cableguys, are well-known examples, and in FL Studio, Image-Line's Gross Beat can do the same job with its volume envelopes. You can also do it with no plug-in at all, by drawing a dip into a track's volume automation and copying it along the song.
In its usual tempo-synced mode a shaper never hears the kick, so it can't tell whether a kick is playing, how loud it is or whether it's slightly late. All it has is the position in the beat, and that's the main difference between the two methods.
LFOTool and Kickstart
Both are volume shapers, so neither is sidechain compression, even though both get used for the sidechain sound. Kickstart gives you a set of ready-made curves and one big control for how much of the curve you hear, which is fast and hard to get wrong. LFOTool lets you draw your own curve point by point, and it can move a filter and the pan as well as the volume. For a pump in ten seconds the simpler one is fine; to design the shape yourself you need one you can draw in.
Shapers that follow the kick
Some shapers can restart their curve on a hit instead of on the grid. LFOTool's Note Retrig restarts the curve on every MIDI note, and its envelope mode plays the curve once per note and then stops, so you can program a MIDI pattern that copies the kick. Kickstart 2 and ShaperBox 3 can also be triggered by MIDI or by audio on a sidechain input, which means the kick track itself.
A triggered shaper gets the timing of a compressor: an extra kick starts a duck of its own, and a late kick takes the curve with it. What it still can't do is react to level. A soft kick and a hard one get the same depth and the same shape, because the curve is drawn in advance. The lab's shaper stays in the usual tempo-synced mode, which is where the two methods sound most different.
How they sound different
On a steady kick
With a kick on every beat and nothing else going on, the two can be almost impossible to tell apart. The compressor reacts to the same hit every time, so it makes the same duck every time, and a shaper set to a similar depth and length lands in the same places. Switch between them in the lab on the plain kick and watch the stacked view: both draw one tidy bundle of lines.
When the kick moves
This is where they split. Give the kick an extra hit, leave a beat out or play one kick softer, and the compressor follows. On the empty beat the bass stays at full level, the extra hit ducks it again, and the soft kick ducks it less, because it goes less far over the threshold.
The shaper does none of that. It ducks on every beat whether a kick is there or not, so on an empty beat you hear the bass dip for no reason, and an extra kick between the beats gets no duck of its own. If your drums have fills, ghost notes or gaps in the pattern, reach for a compressor.
With a whole drum loop
Key a compressor from a full drum loop and the snares and hats can trigger it as well as the kick, as long as they cross the threshold. The duck then moves with the groove, which can sound alive or messy depending on the loop. If you only want the kick, key the compressor from the kick track alone. A shaper ignores the loop either way.
The shape of the duck
A compressor's duck has the shape its design gives it. You set how fast it drops with the attack and how fast it comes back with the release, but the curve between those is fixed: the gain recovers quickly at first and slows down as it gets close to full level.
A shaper can make any shape you draw. It can hold the bass right down for most of the beat and then snap it back just before the next kick, a hard, obvious pump that a compressor can't copy. It can also duck on the off-beats, or follow a shape that runs across a whole bar.
Where the duck starts
A compressor can only start ducking once the kick has crossed the threshold, and its attack adds a little more delay. With a 1 ms attack that delay is too short to hear, but with a 20 ms attack the front of the kick plays over a bass that hasn't moved yet. That can be what you want on a plucked bass, since the front of each note keeps its bite before the duck pulls it down.
A shaper drops exactly on the grid, which is ideal when the kick is exactly on the grid too. If the kick sample has a few milliseconds of silence at its start, or the kick is swung or played by hand, the shaper ducks a little before the kick arrives and comes back a little early.
Tempo and timing in numbers
One beat lasts 60,000 ÷ BPM milliseconds. At 120 BPM that's 500 ms, at 128 BPM about 469 ms and at 174 BPM about 345 ms. Any duck has to fit inside that window if the bass is going to be back at full level before the next kick.
Suppose a short kick leaves the bass 16 dB down as it ends, and the compressor's release is 150 ms. The lab's compressor recovers exponentially from there, as many do: the bass is about 6 dB down 150 ms later, 2 dB down after 300 ms and under 1 dB down after 450 ms. At 120 BPM the next kick is 500 ms away, so the bass is nearly back. At 174 BPM a kick lasting about 45 ms leaves roughly 300 ms to recover, so the bass is still 2 dB down when the next one lands and the track pumps harder, even though nobody touched the compressor.
A shaper set to recover over half a beat takes 250 ms at 120 BPM and about 172 ms at 174 BPM, because its curve is drawn against the beat. The feel stays the same at any tempo, which helps if your song changes tempo or you move a preset between genres. In the lab the shaper's length is in milliseconds so you can compare it directly with the compressor's release; most shaper plug-ins set it as part of a beat instead.
When to use each
| Job | Sidechain compressor | Volume shaper |
|---|---|---|
| Bass under a kick that changes | Best | Ducks on missing kicks |
| Bass under a plain four on the floor | Good | Good |
| A big, obvious pump on pads or chords | Works | Best, since the shape is yours |
| Keeping the pump going with no drums | Needs a muted kick in the sidechain | Just keeps going |
| Music ducking under a voice | Best | No, speech isn't on the beat |
| Ducking a reverb or delay | Best | Only for rhythmic effects |
For kick and bass, start with a compressor. The bass is there to support the kick, so it should get out of the way only when a kick is really playing, and a compressor keyed from the kick does exactly that. The kick and bass lesson covers what to do alongside it.
Reach for a shaper when the pump is part of the sound you're designing, like chords that swell up between the kicks, or when the drums drop out and you still want the track to breathe. Because the curve never changes, it's also the easier one to keep consistent across a long song.
How to set up volume shaping
The names vary between plug-ins, but these steps work in any of them, or with volume automation.
- Put the shaper on the track you want to duckUsually the bass, or a bus holding the bass, pads and chords. Leave it off the kick.
- Set it to repeat once a beatMost shapers call this the rate or the grid, and 1/4 means one curve per beat. Check it's synced to the project tempo and not running free in hertz.
- Choose the depthFor bass under a kick, 6 to 12 dB is often enough to clear room. For a pump you want people to hear, go much deeper, as far as silence at the bottom of the curve.
- Shape the way back upA curve that's back to full level half way through the beat keeps the groove tight. One that holds low and returns late makes the pump obvious.
- Line it up with the kickZoom in on the kick and check that its start lands on the grid line. If it starts late, move the kick or shift the curve so the duck begins with it.
- Check the parts where the drums changeListen through fills and breakdowns. If the shaper ducks where there's no kick, automate its depth down there, or use a compressor for that section.
- Compare at matched loudnessDucking makes a track quieter on average, so turn the shaped version up to match before deciding whether it's better. The lab does this for you.
Using both together
They don't have to compete. One common split is a sidechain compressor on the bass, keyed from the kick so the low end stays clear wherever the kick goes, and a volume shaper on the pads and chords for the pump you hear. If the kick drops out for a bar, the bass stops ducking while the chords keep breathing.
Keep the two off the same track unless you mean it. A compressor and a shaper both ducking the bass on every kick add up, so a 10 dB duck from each becomes 20 dB, which usually sounds like the bass is cutting out.
Common mistakes
Using a shaper over a kick that isn't four on the floor
A shaper set to every beat ducks on every beat. Over a breakbeat, a trap pattern or anything with gaps, that means dips on empty beats and no room on the kicks between them. Use a compressor there. To keep the shaper, draw a curve that matches the pattern over a whole bar, or switch it to MIDI or audio triggering if it has one.
A curve that's slightly off the kick
A few milliseconds is enough to blur the front of the kick. Kick samples with silence at the start and swung or hand-played drums both put the kick off the grid, while a tempo-synced shaper stays on it.
Forgetting the release after a tempo change
A compressor's release is a time, so speeding a track up shortens the gap between kicks without shortening the release. If the pump suddenly sounds heavier after a tempo change, shorten the release.
Ducking further than the mix needs
For bass under a kick, a few decibels in the right place usually clears the room. A 20 dB duck on every beat makes the bass sound like it's switching on and off, which works as an effect but gets tiring when you only meant to make space.
Train your ears
The quiz under the lab plays the kick with fills and ducks the bass one of the two ways, with the screens covered and the levels matched. With a kick on every beat it would be close to a coin toss, so the fills are what give it away: listen to bar 3, where a beat has no kick, and to the quick hits at the end of bar 4.
Questions people ask
Is Kickstart sidechain compression?
No. Kickstart is a volume shaper: it turns the sound down along a fixed curve in time with your project's tempo. In that mode it never hears your kick, which is why it needs no routing and why it keeps ducking when the kick stops. Kickstart 2 can also restart its curve from MIDI or from audio on its sidechain input, but even then the depth doesn't change with how hard the kick hits.
Is LFOTool better than sidechain compression?
It's better at a fixed, designed pump, because you draw the shape you want and it never changes from beat to beat. A sidechain compressor is better when the kick pattern changes, since it reacts to the hits that are really there. Plenty of mixes use both, on different tracks.
Can I sidechain without a kick track?
With a volume shaper, yes, since it only needs the tempo. With a compressor you need something feeding its sidechain, but you don't have to hear it. The usual trick is a muted kick track, often called a ghost kick, that only exists to drive the compressor.
Which is better for kick and bass?
Start with a sidechain compressor on the bass, keyed from the kick, so the bass only gets out of the way when a kick is actually playing. Switch to a shaper if the kick is a plain four on the floor and you want the duck to be exactly the same every time.
Can I do volume shaping with automation?
Yes. Draw a dip in the track's volume automation over one beat, then copy it across the song. It works in any DAW and costs nothing, but changing the shape later means redrawing or re-copying every beat, which is the work a shaper plug-in saves you.
Why does my bass still clash with the kick when I use a shaper?
Usually the curve and the kick aren't lined up. If the kick sample starts a few milliseconds late, or the kick is swung or played off the grid, the shaper ducks in the wrong place. Zoom in and check where the kick starts, then shift the curve or the kick so they match.