What Is “Knee” In Compression?

Martin Kristiansen

My name is Martin Kristiansen and I’m the founder and chief editor of HomeStudioIdeas.com. I’ve been playing, recording and producing music for the last 10 years.

It’s easy to get overwhelmed by all the knobs and controls on a compressor. This blog post is all about one of the most frequent questions sent to me: “What is knee in compression?”. If you’re in a hurry, the Knee-setting is how fast the compression is applied to the signal once it surpasses the threshold. In my opinion, it’s also one of the most important settings in regards to how a compressor deals with the incoming signal. Continue reading for a more detailed explanation!

Other compressor parameters such as Ratio, Threshold, Attack and Release get a much higher level of spotlight. The knee-setting is often overlooked and neglected. Why? Maybe because most compressors lack a controllable knee.

Still, knee on a compressor can hugely affect the way the compressor is dealing with the incoming signal.

The knee-setting allows you to alter the characteristics of the compressor like no other settings can. It’s a really useful parameter that lets you fine-tune the compressor and get the results you want.

Let’s take an example:

If the knee is set to zero, the compression applies as soon as the signal surpasses the threshold. The signal goes from uncompressed to fully compressed instantly.

If you want a slower transition and have your compressor to compress the sound more progressively, the knee-setting is your best friend!

If the knee-settings are set to extremes combined with a fast attack, you can get some pleasing distortion which can be used for a number of purposes.

Let’s Talk About Ratio First

Ratio is a key-setting found in every compressor and it determines the intensity of reduction. Understanding ratio is a necessary first step if you want to understand the concepts of knee.

You could say that knee is the interaction between a compressor’s threshold and ratio settings. It determines how fast the ratio kicks in, after the signal reaches the threshold. 

Soft-knee Vs. Hard-knee Compression

Before we start talking about soft-knee vs. hard-knee compression, let me explain why it’s called “knee” in the first place. Take a look at the gain-reduction graphs below:

The line right after the compressor starts reducing the gain looks like a sharp angle, like the break of a knee. When the angle is steep, the compressor processes the signal very quickly. When there is a smooth curve, the compression applies gradually. That curve/angle is the knee!

Many compressors give you the option of either “Soft-knee” or “Hard-knee”. You may not be able to hear the difference if the knee is too soft, but you should be able to pick up on the aggressive change when you use hard-knee compression.

A range between the two is ideal in order to sculpt the perfect transition between compressed and uncompressed. Below is a more detailed explanation of soft-knee compression and hard-knee compression.

Using Soft-knee Compression

Soft-knee compression gives you more gradual compression results. The key advantage is that it makes the very start of gain reduction/compression less noticeable. It will keep the musical phrasing sound natural and less processed. 

If your compressor has a graphical display, you can tell that the graph looks curvier. There is no single point where the compression kicks in.

This means the compression effect it implies is more suitable for audio that has more sustain. Soft knee settings do not wait until the threshold has been crossed to apply compression.

Soft-knee compression is ideal for any instrument where you don’t want to kill the dynamics:

Using Hard-knee Compression

With hard-knee compression, the compressor will strike more aggressively. This is the default setting of every compressor, and very few allows users to tweak to soft-knee or somewhere inbetween.

The hard-knee setting tells the compressor to do its magic with full effect the moment the audio passes the threshold. The signal goes straight from uncompressed to compressed which will be much more noticeable. This is suitable for sounds with little or no sustain, like kicks or snares.

Try hard-knee compression on tracks that have more predictable volumes and transients:

Do All Compressors Have Knee?

All compressors on the market have knee, but not all of them have a controllable knee-setting. Instead, they feature a more-or-less baked-in knee that sound engineers know and rely on.

If you have a compressor that you love, you’re probably already familiar with its knee behavior without realizing it. If you’ve ever found a compressor to be too aggressive, there is a solid chance that the knee was a big part of the problem.

If you’re a beginner and just started experimenting with compressors, you probably should stick with the basics. Although if you’re an experienced producer, using a compressor with a controllable knee offers more flexibility and tweaking options.

Compressors With Controllable Knee

Even if many compressors lack dedicated controls for knee settings, you can still find a few that have. Below I’ve listed some of my go-to compressors that include controllable knee:

Knee Vs. Attack

Many producers confuse the knee-setting with the attack-setting. The Attack is working in a time-domain of milli-seconds (ms), and the Knee works in decibels.

Think of the Knee setting as the attack of the threshold. It’s telling the compressor how fast the compressor should go from no compression to full compression when the audio signal passes the threshold.

The knee is the way in which the compressor reacts to the signal. The attack-knob defines how fast the compressor reacts to the audio signal.

Other Basic Controls On A Compressor

Threshold, Ratio, Attack, and Release are the most basic controls of an audio compressor. Understanding how they work and what they do is crucial before you start experimenting with the knee-setting.

Unfortunately, I don’t remember the source, but one genius explained compressor parameters as your mother asking you to turn down the volume:

Threshold

The threshold is the level the audio signal has to rise above for it to be compressed. For instance, if you have the threshold set to -12 decibel (dB), any signal which exceeds -12 dB will get compressed. The audio signal below -12 dB will remain uncompressed. If you have it set to low, more audio will pass the threshold and the compressor will work more consistently.

Ratio

Ratio (or compression ratio) specifies the amount of compression that will be applied to the audio signal once it passes the threshold. You will find a wide range of different ratios available on different compressors.

It’s important to understand that all compression ratios are expressed in decibels. If you set the ratio to 2:1, it tells the compressor that any signal exceeding the threshold by 2 dB will be compressed down to 1 dB above the threshold. Audio signals exceeding the threshold by 8 dB will be attenuated down to 4 dB above it etc. You get the point!

Here are some guidelines when you set the ratio:

Attack Time

Attack time refers to the time it takes for the audio signal to become fully compressed after it exceeds the threshold. If you have a slow attack, it takes some time before the compression kicks in. Attack time is measured in microseconds (us) or milliseconds (ms), where a slow attack generally ranges between 10-100 ms, and a fast attack somewhere between 20-800 us.

Release Time

Release time is the opposite of Attack, and it refers to the time it takes for the signal to go from compressed to uncompressed (original sound). A faster release means the compressor lets go quickly, and the sound retains more of its original dynamics. If you set a slow release, the compressor will hold onto the sound longer.

If the release time is set too short, the compressor will cycle between active and deactivate. The result? You get this distinct pumping/breathing effect that you hear in modern EDM tracks. 

Release times will be considerably longer than attack times, as they usually range between 40-60 ms 

Wrapping Up

The knee-setting allows you to fine-tune how hard or subtle the compressor will compress the signal. Both soft-knee compression and hard-knee compression have their place in every production. It depends on what you are mixing and what your goals are.

Perhaps not as much make-or-break as the standard parameters like Attack and Release. Still, the knee-setting is something that you should pay close attention to since it typically determines how “musically” the compressor behaves.

How do you like to set your knee value for different instruments? Let me know in the comments below.