Sidechain compression is where one sound automatically turns other sounds down. Nice and simple. A compressor tool reduces the volume when a signal gets too loud. Normally, it listens to the sound it's controlling. With a sidechain, it listens to something else. Feed the kick drum into the sidechain input, put the compressor on the bass, and now every kick hit turns the bass down for a few milliseconds. Kick stops, bass comes back.

So there is now some breathing happening between the bass and the kick.

A great place to hear it in action is any well-mixed dance track anywhere in the last twenty years. Are we saying you will 100% pick it out as you’re listening? No, but it’s fun to try and flex your listening skills.

The trick wasn't invented for music. It came from broadcast radio, where it's called ducking. The DJ speaks, the music automatically drops underneath the voice, the DJ stops, the music comes back up. Purely functional. And, a perfect solution.

Mixing engineers borrowed it for a practical reason: the kick and the bass live in the same low frequencies, and when both play at once, they fight for the same space. We covered what happens when everything is fighting for space a couple of weeks ago in Crushed Headroom.

Sidechain the bass to the kick, and the kick gets a clean pocket to punch through. Done subtly, you can't hear it at all. You just get a low end that isn't mud. And who doesn’t love a crisp low end?

For decades, that was the whole job. Invisible plumbing. You don’t notice it when it is there, but you for sure notice it when it isn’t.

Somewhere around the turn of the millennium, French house producers started showing it off. Instead of a couple of decibels of polite ducking, they cranked the compressor until the entire track pumped like the lungs of someone who just ran a marathon.

Daft Punk built grooves out of it. By the time CALL ON ME arrived in 2004, the pumping was the hook. The compressor stopped being this handy tool and became an instrument.

Then big-room EDM took the instrument and leaned on it until it became a parody of itself. For a stretch of the 2010s, you could not escape it: every festival track gasping like it had just run up the stairs. The trick that once made space in a mix was now flattening everything into the same heaving shape… again.

Sound familiar? It's the Loudness War logic all over again: take a tool built for control, max it out for impact, lose the thing it was protecting, ‘cause we really do need to push every limit we find. And we’re back at crushed headroom.

But here's the part we love. The producers who really understood it kept going deeper instead of louder. deadmau5 sidechains to a ghost kick: a kick drum that is muted, completely silent, existing only to trigger the compressor. The mix breathes around a drum that isn't there.

Kevin Parker pumps entire Tame Impala records through compression as texture. LET IT HAPPEN heaves and swells, and there is no dance floor in sight. Billie Eilish and Finneas duck whole arrangements a few dB under the vocal so quietly you'd never know; you'd just say the voice feels close, and we’ve used Billie’s music as an example of incredible mixing and production before.

The gap in the sound became a compositional choice. Which, if you read Fermata, you already knew was coming. Sidechain is subtractive arrangement, automated. The ear processes every return as bigger/louder thanks to that nanosecond of change.

Threshold is the level where the compressor starts working. Ratio is how hard it squeezes past that point.

Attack is how fast the compressor clamps down when the kick hits, measured in milliseconds. Fast attack, hard duck.

Release is the whole feeling. It's how long the compressor takes to let go, and it is where the breathing bit lives. At 128 BPM, there are roughly 469ms between beats. Set the release around 250 to 350ms, and the ducked sound swells back just in time for the next kick to push it down again. That swell is the exhale. If this bit gets messed up, it will sound and feel too short, or too long.

Gain reduction is how many decibels get pushed down on each hit. 2 to 3dB is invisible glue. 6dB will be noticeable. 10dB and up, you're in CALL ON ME territory, and everyone can hear this one. Give it another listen; it’s on the playlist in first position, and you’ll know what we’re talking about.

There are a couple of edge cases, but you’ll be able to find pockets, spaces, ducking, and a decent soundstage on these ones.

Couple of notes: on the Muse track, the wub is the pump, NIN is kick ducks, and Billie OXYTOCIN is an obvious one. If you want more of a challenge, try YOU SHOULD SEE ME IN A CROWN.