What is the knee on a compressor?

The knee in plain English

A compressor leaves a sound alone until it passes the threshold, then turns it down according to the ratio. The knee decides how sudden that change is. With a hard knee there's no in-between: a sound 0.1 dB under the threshold passes untouched, and a sound 0.1 dB over it is compressed at the full ratio.

A soft knee spreads the change across a range of levels. Compression starts gently a few decibels below the threshold and only reaches the full ratio a few decibels above it. The name comes from the shape of the transfer curve, the graph of output level against input level. Its line bends at the threshold, and that bend can be a sharp corner or a rounded one, the way a leg bends at the knee.

On a lot of material you'd struggle to hear the knee at all. It earns its place on sounds whose level sits around the threshold for long stretches, such as a vocal or a bass line, where it decides whether the compressor sounds like a hand easing a fader or a switch flicking on and off.

Hard knee vs soft knee

A hard knee is a knee of 0 dB. Below the threshold the transfer curve runs at 45 degrees, meaning the output equals the input, and at the threshold it snaps straight to the slope of the ratio. Every sound that crosses gets the full treatment, which makes a hard knee firm and predictable. It's the obvious choice when you want the compressor to grab something, like the hits in a drum loop or peaks you want held under a line.

A soft knee has a width in decibels. A 12 dB knee on a −20 dB threshold starts working at −26 dB and reaches the full ratio at −14 dB, half the width either side of the threshold. Inside that band the ratio climbs smoothly from 1:1 to whatever you set, so there's no single level where the compressor kicks in. That makes it harder to hear working, which is why soft knees turn up on vocals, acoustic instruments and mix buses.

A peak above the top of the knee comes out at exactly the same level whichever knee you use, and so does anything below the bottom of it.

What does knee width in dB mean?

The number is the width of the band where compression fades in. On many compressors, and in the lab on this page, the band is centred on the threshold, so a 6 dB knee starts 3 dB below the threshold and finishes 3 dB above it. A 24 dB knee, the widest the lab goes, starts 12 dB below.

Here are the numbers for a compressor with its threshold at −20 dB and a 4:1 ratio, once with a hard knee and once with a 12 dB knee. They come from the same formula the lab's compressor runs on, before attack and release smooth anything out.

Look at the −20 dB row first. Right on the threshold the hard knee does nothing, while the soft knee is already taking 1.1 dB off. At that point, halfway through the knee, the soft knee is compressing at about 1.6:1, and it only reaches the full 4:1 at the top of the band. By −14 dB the two have met, and from there up they behave as the same compressor.

Compressors don't all count the same way. Some put the whole width above the threshold, so compression begins at the threshold and reaches the full ratio higher up, and some use a scale such as 0 to 1 instead of decibels. If the manual doesn't say, watch the gain reduction meter while you play a sound just under the threshold: a soft knee will already be moving it.

Reading the lab

The lab opens on the Transfer curve picture, with input level along the bottom and output up the side, both in dBFS. The dashed amber line marks the threshold. The shaded amber band is the knee, and it disappears completely at 0 dB. The Turning down readout above it shows how far the vocal is being turned down as it plays.

While the loop plays, a white dot rides the curve at the current input level. On the vocal you'll see it spend a lot of time inside the shaded band, and that's the reason the knee changes how the vocal sounds. Switch to the Whole loop picture and the amber strip along the bottom shows how much of the loop is being turned down. When you stop the loop, the Most turned down readout shows the most it was turned down at any moment.

Attack, release and mix are locked at middle-of-the-road settings, leaving the knee to work with the threshold and ratio. Those other controls have their own lesson in attack and release, and what is compression? covers every control together.

The knee only works near the threshold

Because the knee covers only the levels around the threshold, how much it does depends on how much of your sound lives there. The lesson in the lab starts with the clearest case: the vocal at a −14 dB threshold and 8:1 with a hard knee, where only the biggest words cross the line. Soften the knee to 12 dB and then 24 dB and more of the loop gets turned down, because words that were just under the threshold are now inside the knee and losing a decibel or two.

Now bury the threshold at −36 dB. Nearly the whole vocal is far above the knee, in the part of the curve where hard and soft are the same, and dragging the knee from 24 dB down to 0 barely changes the deepest gain reduction. With heavy compression the knee makes almost no difference.

The ratio changes how much the knee can do. At 2:1 the corner between the two slopes is shallow, so rounding it off changes little. At 8:1 or 10:1 the corner is sharp and there's far more for a soft knee to smooth out, which is where the difference is easiest to hear.

Drums are the opposite case to a vocal. A drum hit jumps from almost nothing to its peak in a millisecond or two, straight through the knee and out the far side, so the hit itself gets much the same treatment from a hard or a soft knee. What the knee changes on drums is the quieter material just under the threshold: tails, ghost notes and room sound. Pick Drums in the lab and drag the knee: on the Whole loop picture the deepest dips in the amber strip hardly move, and what does change is how much of the loop the strip covers, as the quieter sound between hits drifts in and out of the knee.

A soft knee takes off more at the same threshold

Go back to the table. At every level inside the knee, the soft knee turns the sound down a bit more than the hard one, and nowhere does it turn it down less. So switching to a soft knee without touching anything else always adds gain reduction, and the compressed sound gets slightly quieter.

The biggest difference is right at the threshold, where a soft knee takes off an extra (1 − 1 ÷ ratio) × width ÷ 8 dB. For a 12 dB knee at 4:1 that's 1.1 dB; for a 24 dB knee at 8:1 it's 2.6 dB.

This catches people out when they compare knees, because the version with more gain reduction also has a different level. After you soften the knee, raise the threshold until the gain reduction meter reads about what it did before. In the lab, the vocal with a hard knee at −14 dB and with a 24 dB knee at −10 dB average about the same gain reduction, but the soft knee takes more than a decibel less off the loudest words. You've swapped some grip on the peaks for gentler work on everything near the threshold.

Knee settings for vocals, drums, bass and buses

Use these as starting points. The right knee depends on the threshold and ratio around it, so set those first and come to the knee last.

Peak control is the exception to all of this. When you want a line that peaks don't cross, use a hard knee and a high ratio, or a limiter, which is covered in limiting.

Knee vs attack

If you soften the knee to make a compressor gentler on drums and the snap is still gone, the knee was never the problem. The knee and the attack control different things. The knee is about level: how loud a sound has to be before it gets the full ratio. Attack is about time, meaning how quickly the compressor reacts once it has something to compress. A soft knee with a 0.5 ms attack will still flatten every transient, and a hard knee with a 30 ms attack lets them through.

They overlap in one way. With a soft knee the gain starts coming down below the threshold, so as a note swells, the compression is spread across more of its rise. On slow-building sounds like a bowed string or a pad fading in, that can feel similar to a slower attack. On anything with a sharp front, attack is the control you want.

Knee controls on plug-ins and hardware

Plenty of compressors have no knee control at all. Neither the Universal Audio 1176 nor the SSL bus compressor has one, and their knee is whatever their circuit does. Optical compressors such as the Teletronix LA-2A turn down gradually because of the light-sensitive cell inside, so they behave like a soft knee without offering one. The opto vs FET vs VCA vs vari-mu lesson goes through how those circuits differ.

dbx calls its soft-knee circuit OverEasy, and the dbx 160X has an OverEasy button that switches it in, so on that unit the knee is a choice between two settings. In a DAW, Ableton Live's Compressor and Logic Pro's Compressor both have a Knee control, and in Logic a lower value gives a harder knee. Plug-ins that copy a piece of hardware usually copy its fixed knee too, so a missing knee control is normally deliberate.

How to set the knee on a compressor

1. Start with a hard kneeSet the threshold and ratio with the knee at 0 dB, so you can hear exactly where compression begins.
2. Find out where the sound sitsWatch the input meter, or the dot on a transfer curve if your compressor has one. If the level spends most of its time within a few decibels of the threshold, the knee will matter. If it's mostly far above, it won't.
3. Widen the knee in stepsTry 6 dB, then 12. Listen to the quieter notes and the ends of phrases, where a hard knee is easiest to hear switching on.
4. Raise the threshold to win the balance backA wider knee adds gain reduction, so nudge the threshold up until the meter reads what it did with the hard knee.
5. Compare at matched loudnessSet makeup gain so the bypassed and compressed versions sound equally loud, then switch between knee settings. Keep whichever you'd choose without looking at the screen.

Common knee mistakes

Hearing a quieter sound as a gentler one

Softening the knee makes the compressed sound a little quieter, and quieter often comes across as smoother or duller. Match the level before you decide which knee you like.

Expecting the knee to save your transients

If the front of a drum hit is getting flattened, the attack is too fast. No knee setting will bring that snap back.

A wide knee under a low threshold

A 24 dB knee on a −24 dB threshold starts compressing at −36 dB, which on most sounds takes in breaths, room tone and reverb tails as well. If the gain reduction meter never returns to zero, check where the knee starts as well as where the threshold is.

Fussing over the knee on heavy compression

When the threshold sits far below most of the sound, the knee covers levels the sound rarely visits. Spend that time on the threshold and ratio, which threshold and ratio covers in detail.

Training your ears to hear the knee

The quiz under the lab plays the sound through an 8:1 compressor with either a hard knee or the softest one the lab has, 24 dB, and asks which you're hearing. The levels are matched, so the only clue is how the compression behaves. The hard knee grabs the loudest moments as soon as they cross the threshold and you can hear them being pulled down, while the soft knee starts working well below the threshold and reaches full strength gradually.

To practise without the quiz, load step 1 of the lesson, press play and drag the knee between 0 and 24 dB without looking at the screen. Listen to the quieter words and the ends of phrases. Once you can tell which end the knee is at from those alone, try the same thing at 3:1, where it's much harder.

Questions people ask

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