What do attack and release do on a compressor?

Attack and release in plain English

A compressor turns a sound down whenever it gets louder than the threshold, and the ratio decides how far. Those two controls set how much. Attack and release set when: how long the compressor takes to get there once the sound goes over, and how long it takes to come back once the sound drops under again.

The timing changes the sound as much as the amount does. A slow attack lets the first few milliseconds of a drum through untouched, so the hit cracks out and then sits back, while a fast one catches that spike too and the drum flattens. A short release lets each hit bounce back up straight away. A long one keeps the level held down across several hits, which smooths a part out.

The basics of threshold, ratio and makeup gain are in the main compression lesson. This page stays on the timing, and the lab above has its knee and mix controls locked so you can hear attack and release on their own.

Reading the lab

The amber lane along the bottom of the Whole loop picture is the gain reduction. It dips when the compressor turns down and rises back towards zero as it lets go, so its shape is the attack and release you've set, drawn across the whole loop. The One hit up close tab zooms in on 160 ms around the loudest hit, and the shaded strip at the front of that hit is as wide as your attack time. That makes it easy to see how much of the hit gets through before the gain comes down.

The Turning down readout above the picture shows how many decibels are coming off as the loop plays. Switch to Original at any point to hear the loop without the compressor at matched loudness.

What does attack do?

Attack is the time the compressor takes to bring the gain down once the level crosses the threshold. Most compressors let you set it anywhere from a fraction of a millisecond to around 100 ms, and on anything with a sharp start, like drums, plucked strings or a slapped bass, it changes the sound more than any other control.

The reason is the transient. Every drum hit starts with a spike a few milliseconds long, and that spike carries most of what you hear as the click of a kick or the crack of a snare. The body and ring that follow are quieter and last longer. Whether the compressor reacts before or after that spike decides whether you keep it.

Fast attack

With an attack under 1 ms, the compressor is already turning down while the transient is still rising. On the drum loop in the lab, a 0.1 ms attack takes about 10 dB off the top of the loudest hit, which brings its first spike down close to the level of the body behind it. The drums sound softer and set back, as if they were further from the microphone.

That's useful when a part has peaks that jump out too far: a bass player who digs in on some notes, say, or a snare with a harsh crack. It's also the setting for room mics and for a parallel drum bus, where you want the space around the drums brought up and don't need the hits themselves.

Slow attack

Slow the attack to 20 or 30 ms and the transient gets out before the compressor has properly reacted. In the lab at 30 ms, the first spike loses less than 1 dB while the body of the same hit is still turned down by about 6 dB. The front of the hit now stands further above the rest than it did before. That's punch, and it comes from the compressor turning the body down and leaving the start alone.

You can see it in the numbers too. The drum loop has a crest factor, the gap between the peaks and the average level, of about 19 dB on its own, and with a 10 ms attack it goes up to around 22 dB, because the peaks survive and the average level drops. A compressor normally makes the crest factor smaller; a slow attack is how it can make it bigger.

When the attack is too slow

Past a certain point the attack stops shaping the hit and the compressor simply misses it. At 100 ms, most of the drum hits in the loop are over before the gain has come down very far, so the deepest gain reduction drops from about 9 dB at a 10 ms attack to under 3 dB. On short sounds a slower attack always means less gain reduction, since the sound has gone before the compressor gets there.

Very slow attacks still have a place on sustained sounds like pads or a mix bus, where they let the compressor follow the overall level and ignore the quick details.

What does release do?

Release is the time the compressor takes to turn the gain back up once the level falls under the threshold. Typical values run from about 20 ms to well over a second. Attack shapes the front of each hit, and release shapes what happens after it, including whether the compressor has let go before the next hit arrives.

Short release

A short release, roughly 30 to 80 ms on drums, recovers in the gaps between hits. Each hit gets its own bit of gain reduction and the gain is back to normal before the next one, so the compression moves with the rhythm. On the drum loop with a 20 ms attack, a 60 ms release has the compressor working on about half the loop, with the amber lane getting back near zero between the main hits.

Pair a short release with a fast attack and each hit is clamped and let go almost at once. The quieter part of the sound, the decay of the drum and the room around it, is left almost untouched while the hit comes down, so once makeup gain brings the level back, that decay and room sound come up next to the hit. That's a big, rough drum sound, and it's the usual setting for heavily compressed room mics.

Long release

A long release holds the gain down for a while after each loud moment, and if it's longer than the gap between hits the compressor never fully lets go. On the drum loop, any release of about 200 ms or more with these settings keeps the gain reduction above 1 dB for practically the whole loop. Quiet notes that follow a loud one get turned down along with it.

That suits a vocal that needs to sit at one level through a phrase, or a bass line that should feel like one continuous part. On drums it takes the life out, because the space between the hits gets pressed down along with the hits.

Short releases on bass

Low notes are slow waves. A 55 Hz note, the open A string on a bass guitar, takes about 18 ms to complete one cycle. If the release is shorter than that, the compressor starts turning the level up and down within each cycle, following the waveform itself, and that bends the shape of the wave. You hear it as a buzz or a fuzzy edge on the bass, and a very fast attack does the same thing on the way down. Pick Bass in the lab and pull the release under 20 ms with a fast attack to hear it, then bring the release back up past 50 ms and the buzz goes away.

How attack and release work together

You set the two controls separately, but you hear them as one movement. This is what each pairing does on drums, where the difference is easiest to hear.

What the numbers on the knob mean

On many compressors, including the one in this lab, the attack time is how long the gain takes to cover about 63% of the way to its target. The gain moves in a curve that starts fast and slows as it gets close, so it's still moving after the attack time has passed. Say a hit needs 10 dB of gain reduction and the attack is 10 ms. After 10 ms the compressor has taken off about 6.3 dB, after 20 ms about 8.6 dB and after 30 ms about 9.5 dB, and it's within a fraction of a decibel after five attack times.

Release works the same way in reverse. From 10 dB of gain reduction with a 100 ms release, the gain is back to 3.7 dB down after 100 ms, 1.4 dB after 200 ms and 0.5 dB after 300 ms. Getting within 1 dB of normal takes about 2.3 release times, so a 100 ms release needs about 230 ms to recover from a 10 dB hit.

Manufacturers don't all measure it the same way, so 10 ms on one compressor can behave differently from 10 ms on another. The Universal Audio 1176 has an attack range of 20 to 800 microseconds and a release range of 50 ms to 1.1 seconds, with its knobs reversed so that fully clockwise is fastest. The SSL bus compressor has stepped attack times of 0.1, 0.3, 1, 3, 10 and 30 ms, and release times of 0.1, 0.3, 0.6 and 1.2 seconds plus Auto. The Teletronix LA-2A has no timing controls at all: its optical cell gives an attack of about 10 ms and a release in two stages, quick for the first half and slower after that, taking longer the harder it has been working.

What the compressor measures changes things too. A peak detector reacts to every sample, while an RMS detector averages the level first, so it responds more slowly to short spikes even at the same attack setting. The peak vs RMS compression lesson goes through the difference.

Setting release to the tempo

Because the release decides whether the gain recovers before the next hit, the tempo of the track gives you a useful upper limit. One beat lasts 60,000 ÷ BPM milliseconds. At 120 BPM, the tempo of the loops in the lab, a quarter note is 500 ms, an eighth note 250 ms and a sixteenth 125 ms.

Put that together with the recovery maths above. If a kick on every eighth note gets 10 dB of gain reduction and you want the gain back within 1 dB before the next one, the release needs to be about 250 ÷ 2.3, which is roughly 110 ms. Any longer and the gain reduction builds up from one hit to the next. That's fine if you want the part held down, and a problem if you wanted it to bounce.

When the release is slow enough that you can hear the level swell back up after each hit, the track breathes in and out with the beat. That's pumping. On a mix bus it's usually a mistake, but a lot of dance music does it on purpose, mostly with sidechain compression so the kick drives the pumping of the other parts.

Auto release

Many compressors have an auto or program-dependent release. It lets go quickly after a short peak and slowly after a long, loud passage, so a vocal's quick consonants recover fast while a held note doesn't jump back up the moment it dips under the threshold. The Auto setting on the SSL bus compressor, the Auto release button in FabFilter Pro-C 2 and the Auto option on Ableton Live's Compressor all work along these lines, though each has its own exact behaviour. Auto release is a sensible place to start on buses and vocals. Switch it off on drums when you want the release locked to the groove.

Attack and release starting points

These are places to begin, and they match the ranges in the compression lesson. Set the threshold for 3 to 6 dB of gain reduction first, then adjust the timing by ear.

How to set attack and release, step by step

1. Find the threshold with middle settingsA 10 ms attack and a 100 ms release are neutral enough to find a threshold that gives 3 to 6 dB of gain reduction on the loudest moments.
2. Turn the attack to its slowestThen bring it down gradually while you listen to the very start of each note. Stop when the peaks feel controlled, and if the sound starts to lose its edge, go back up a little.
3. Set the release by watching the meterShorten it until the gain reduction meter gets close to zero between hits or phrases. If it never gets back, the release is too long for the tempo.
4. Listen for distortion and pumpingOn bass and full mixes, check the release isn't so short that it buzzes. On anything with a regular beat, listen for the level swelling back up after each hit.
5. Compare at matched loudnessMatch the bypassed level with makeup gain, or use the Original and Compressed switch in the lab, and keep the settings only if you can say what got better.

Common attack and release mistakes

An attack so fast the drums lose their punch

This is the most common one. If the attack is under a few milliseconds when you start lowering the threshold on drums, the transients get flattened straight away. If a drum sounds smaller with the compressor on, slow the attack to 20 ms or more before you touch anything else.

A release that never lets go

When the gain reduction meter sits at −4 dB and never moves, the compressor has stopped reacting to the music and is just turning everything down. Shorten the release until the meter moves with the rhythm, or raise the threshold so the compressor has gaps to recover in.

Release too short on low sounds

A 20 ms release that sounds fine on a hi-hat will distort a bass or a kick with a long sub tail. Anything with a lot of energy under 100 Hz needs a release of at least 50 ms, and often longer.

Copying the numbers from a chart

Tables like the one above are starting points, and the same number behaves differently on different compressors. A kick at 90 BPM and a kick at 170 BPM need different releases. Set the timing while the track plays, watching the meter and listening to the front of each note.

Using a compressor for punch when a transient shaper would do

If all you want is more attack on a drum and its level is already even, a transient shaper turns up the front of each hit directly, with no threshold to set. A slow-attack compressor gets there by turning the rest of the hit down, which changes the level as well.

Train your ears

The quiz under the lab hides the settings and asks you to name them, with the levels matched so loudness gives nothing away. Some rounds ask whether the attack is fast (0.3 ms) or slow (30 ms), and the others ask the same about the release (60 ms or 600 ms). In the attack rounds, listen only to the very start of each hit: if it clicks, the attack is slow. In the release rounds, listen to the quieter moments straight after a loud one, which stay pressed down when the release is slow. Run it on the drums first, then switch to Vocal and try again, since the attack is much harder to hear on a sound without sharp transients.

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