Compression

Lab: Bigger or just louder? · Listen and compare

Starts with: the shared drum loop playing through Compressor at Threshold −24 dB, Ratio 4:1, Attack 20 ms, Release 80 ms, with makeup gain. A/B switch between "Off" and "On".

Reader controls: A/B switch, then a Match loudness toggle.

Shows: level meter for each version.

Ask: "Which one sounds better? Now turn on Match loudness. Still sure?"

Fallback: three clips (off, on, on matched) with a short written description.

Owner: modules/compressor; loudness matching through loudmatch or a pre-rendered matched pair.

Play the loop and flip between the two versions. Most people pick the compressed one straight away: it sounds fuller, punchier, more finished.

Now turn on Match loudness and listen again.

The difference gets much smaller. Some of what you liked was the compressed version being louder, and our ears almost always prefer louder. That's the first thing to know about compression, and it's why every comparison in this guide is level-matched.

But a difference is still there. The kick feels more solid and the room sound between hits is more present. A compressor did that by turning the sound down. Let's find out how turning something down can make it sound bigger.

Try it: watch the compressor work

Lab: Gain reduction you can see · Change a signal

Starts with: the drum loop, Threshold −24 dB, Ratio 4:1, Attack 5 ms, Release 80 ms.

Reader controls: Threshold, Ratio, Attack (three controls only). "Open the full Compressor" for everything else.

Shows: the input waveform with the threshold drawn as a line, and a gain-reduction trace underneath that dips whenever the compressor acts.

Ask before playing: "If you lower the threshold, will the compressor act on more of the loop or less?"

Fallback: a labelled image of waveform, threshold line and gain-reduction trace at two threshold settings.

Owner: modules/compressor (feed-forward, peak or RMS detector).

Lower the threshold slowly. At first only the loudest kick and snare hits cross the line, and the trace dips only on them. Keep going and the hi-hats start crossing too, until the compressor is working almost all the time.

What just happened

A compressor is an automatic volume knob with two rules:

Threshold is the level where it starts to act. Below it, nothing changes.
Ratio is how hard it pushes back above the threshold. At 4:1, every 4 dB the input goes over the threshold comes out as 1 dB over.

So the loud hits get turned down and the quiet parts are left alone. The gap between the loudest and quietest moments shrinks. Engineers call that gap the dynamic range, which is where the name comes from: the compressor compresses the dynamic range.

Then comes the step that makes it sound bigger. Because the peaks are lower, you can turn the whole thing up (that's makeup gain) without the peaks getting any louder than before. The quiet parts, like the room sound, ghost notes and the tail of the snare, come up with it. That's the extra "fullness" you heard.

Do the maths (optional): a hit peaks at −6 dB with the threshold at −18 dB, so it's 12 dB over. At 4:1 it comes out 12 ÷ 4 = 3 dB over, at −15 dB. The compressor turned it down by 9 dB. A quiet ghost note at −30 dB is below the threshold, so it isn't touched. Add 6 dB of makeup gain and the hit is at −9 dB while the ghost note rises to −24 dB: the gap went from 24 dB to 15 dB.

Why attack and release matter so much

Threshold and ratio decide how much the compressor turns things down. Attack and release decide how quickly, and that's where most of a compressor's character lives.

Attack is how fast it turns down once the sound crosses the threshold.
Release is how fast it lets go once the sound drops back.

Lab: Shape the hit · Listen and compare

Starts with: a single snare hit repeated, heavy gain reduction (Ratio 6:1), Attack at 0.5 ms.

Reader controls: Attack only, from 0.5 ms to 50 ms.

Shows: the hit's waveform before and after, overlaid.

Ask: "A slower attack lets more of the hit through before the compressor reacts. Will that make the snare sound softer or punchier?"

Fallback: three clips at 0.5, 10 and 40 ms with overlaid waveforms.

With a very fast attack, the compressor catches the very front of the hit and flattens it, so the snare sounds softer and further away. Slow the attack down and the first few milliseconds slip through before the compressor reacts. The hit now has a sharp front edge followed by a turned-down body, which is why it can sound punchier than with no compression at all. Faster isn't better or worse. It's a choice about the shape you want.

Our take: on drums, start with an attack around 10 to 30 ms and listen to the front of the snare while you move it. You'll hear the moment the snap comes back. On a vocal you may want it faster, to catch sudden loud syllables.

Where it breaks

Push the release very short (a few milliseconds) on the bass line below.

Lab: When release gets too fast · Listen and compare

Starts with: the shared bass line, Threshold −30 dB, Ratio 8:1, Release 80 ms.

Reader controls: Release, from 1 ms to 500 ms.

Ask: "What do you hear in the low notes as the release gets very short?"

Fallback: clips at 1 ms, 80 ms and 500 ms.

Two different things go wrong at the two ends:

Too fast: the compressor reacts within a single cycle of a low note (a 55 Hz note takes about 18 ms per cycle). It starts reshaping the wave itself, and you hear a grainy distortion. You've accidentally built a distortion effect.
Too slow on a busy part: the compressor never fully lets go between hits, so everything stays turned down. With a big kick, you may hear the rest of the mix breathe or pump up and down in time with it.

Pumping isn't always a mistake. A lot of dance music does it on purpose, which is what the ducking guide is about.

Same idea, different place: a release short enough to follow the waveform is the same idea as distortion: any gain that changes within a single cycle changes the wave's shape, and that adds harmonics.

A few more controls you'll meet

The full Maru Compressor has the controls you'll find on most compressors:

Knee: how gradually it starts acting around the threshold. A soft knee eases in; a hard knee switches on at the line.
Detector: Peak or RMS. Peak reacts to instant level; RMS reacts to the average over a short window (10 ms in this module), which tends to feel smoother on sustained sounds.
Sidechain filters: these change what the compressor listens to, not what you hear. A high-pass here stops a big kick from making the whole drum bus duck.
Mix: blends the compressed and dry sound. Low mix values are a quick way to do parallel compression: the dry hits keep their punch while the compressed copy fills in the body.
Range: a ceiling on how far it may turn down, however loud the input.
Auto makeup: sets makeup gain for you. Handy, but check it with your ears and a matched comparison.

Make it yours

Load the bass line below and compress it so every note sits at a similar level without losing the pluck at the start of each note.

Set Ratio 4:1 and lower the threshold until the loud notes show 3 to 6 dB of gain reduction.
Adjust attack until the pluck comes back.
Adjust release until the reduction recovers before the next note, with no grain on the low notes.
Turn on Match loudness and compare. Is it better, or only louder?

Then try your settings on the drum loop, and save the version you like as a drum-bus preset.

[Open this Compressor in the studio] (planned link: opens the module with this guide's bass line and settings loaded)

Quick check

Threshold −20 dB, ratio 2:1. A hit peaks at −8 dB. Roughly how much does the compressor turn it down?
Answer
It's 12 dB over; at 2:1 it comes out 6 dB over, so it's turned down by about 6 dB (before any knee smoothing).
Your snare lost its snap after compression. Which control do you reach for first, and which way?
Answer
Attack, slower. A slower attack lets the front of the hit through before the compressor reacts.
A friend says their compressed mix "obviously sounds better". What's the first thing you'd check?
Answer
Whether the two versions are at the same loudness. A louder version almost always sounds better at first.

Next steps

Use it: Gates, sidechain and ducking. You heard pumping as a problem; here's how to make it the groove.
Go deeper: Limiting and loudness. What happens when the ratio goes to infinity, and why mastering engineers worry about it.
Sideways: Listening and comparing. The level-matching trick from the start of this guide, applied to every decision you make.

Go deeper

Inside the Maru Compressor. It's a feed-forward, log-domain design: it measures the input, computes the gain reduction in decibels, smooths it with the attack and release times, and applies it. The soft knee is quadratic and can be up to 72 dB wide. A ratio of 1000:1 or more is treated as infinity (limiting). Hold delays the release only. See the Compressor module and its developer notes.

Sources and examples. Drum loop and bass line: Maru shared source set (rights to be confirmed before publishing). All lab numbers are planned settings, to be checked against the rendered audio.