What is Knee in Compression: 3 Best Practices Of It!

What is Knee in Compression


Understanding the knee setting on a compressor

In this article, we’ll learn to use the knee setting on a compressor. Alongside the other settings that manage the way in which your compressor works, such as the threshold, ratio, attack, and release, the knee is a superb setting to fine-tune the way your compressor applies compression to audio.

Some compressors let you change between ‘hard knee’ and ‘soft knee’ settings. Others let you dial in how hard or soft the knee setting relies on a decibel level. Some compressors don’t let you alter their knee in any respect. However, we feel that the knee setting on a compressor is a very useful setting. That’s as a result of the knee setting letting you alter an attribute of compression which the other settings don’t.

Both hard and soft knee compression have their perks; two examples: if you wish to squash a signal’s transients fast, you’ll need hard knee compression. If you wish to use a compressor to softly glue a mix collectively by tightening up transients, you’ll need a soft-knee compressor.

Lastly, when you have a compressor, just like the Dyn3 Compressor/limiter which comes free with Pro Tools, have a look at the image of the knee. It truly seems like a human knee!

what is knee in compression

What Is Knee About?

In case you haven’t read the article on the ratio we’ve got, and also you’re new to compression, now is likely to be a great time to go read that one out. Understanding the ratio is a vital first step for understanding the knee. In a way, the knee is about the interplay between a compressor’s threshold and ratio settings.

Although the knee is commonly assigned a numerical value in compressor controls, we have a tendency to explain it extra qualitatively: soft, hard, or somewhere in between. Hard knee compression works the best way that compression appears to work on paper: when the audio crosses the threshold, gain reduction occurs on the predetermined ratio. Beneath that threshold, no reduction happens.

With soft-knee compression, the ratio nonetheless determines the depth of gain reduction, and compression nonetheless kicks in around the threshold. However, as a substitute for that reduction occurring abruptly when the signal crosses the threshold, the compressor introduces reduction gradually — easing in from a lower ratio, beginning at a degree below the precise threshold.

Using Hard Knee

On normal compressor models, a Hard Knee setting would be the default choice and in such circumstances, it won’t be ‘switchable’ to Soft Knee.

On better high-quality compression items although (resembling Fab Filter Pro-C2), the knee setting won’t only be switchable, however, you can too exactly outline and fine-tune its value, for optimum response to the incoming signal.

Clearly, the hard knee setting has its personal benefits, particularly when coping with incoming audio materials that have little-to-no sustain. Suppose snare, or kick as an example!

What you may try is to set the high ratio and the hardest knee setting. Discover in what methods the compression changes the sound of your materials.

Then back off just a little, in case your compressor has that option, and it is possible for you to search out simply the right ‘sweet spot’ that can process your incoming audio in a better means.

There aren’t any hard and quick guidelines about it. Use your ears and attempt to sense the differences the hard knee setting is making!

Using Soft Knee

Whereas Hard Knee designs start working only the second the input signal level exceeds the compression threshold, soft knee settings solely apply minor quantities of gain reduction to input audio that’s under the threshold ranges.

The key benefit of soft-knee compression is that it makes the very beginning of gain reduction much less noticeable thus conserving the musical phrasing sounding extra natural.

The explanation that soft-knee compression is so engaging for audio engineers and studio guys alike is the truth that they’re not the default setting (and even a choice) in most compressors, with some uncommon exceptions.

Whereas Hard Knee compression is extra suitable for ‘attacky’ sounds, filled with transients, a Soft Knee setting in your compressor provides you with a more gradual compression, which is ideal for conditions when you want an extra clear outcome!

knee setting on a compressor

Tips on using Knee in Compression


Percussive components like drums, however, may give us an ideal instance of how and why to make use of a Hard Knee setting in your compressor. As per usual, first arrange the compressor’s primary options (Threshold, Ratio, Attack, Release), after which think about this development from clear to the compressed signal.

Within the overwhelming majority of circumstances, drum components will call for a hard knee. It’s because drum hits have quick peaks, very brief transients, and by their very nature, it’s logical that we should always use hard knees in these cases.


As soon as we set up the great-sounding compressor on the vocal, the knee will probably be within the ‘Hard Knee’ place, by default.

That merely implies that when the vocals of our imaginary song attain the threshold ranges, the compression will merely kick in immediately, the exact second the audio passes the threshold.

Then you may mess around with knee settings (switching to Medium or Soft Knee, or by altering decibel values that may lean extra in the direction of the higher decibel values – softer knee), and generally, with vocals, you’ll begin to discover how the rounder curve provides us more gradual compression.

This implies the compressor will begin working slightly under the threshold as well. It should pick up just a little bit of the quieter signal, as there is no such thing as an exact, strictly outlined level where the compressor kicks in.


In case your compressor has the potential of defining the ‘hardness’ and ‘softness’ parameters of the knee by selecting a decibel value, take this to your benefit. On this occasion, smaller values (dB) provide you with a steeper curve, and hard knee, whereas higher values (dB) provide you with a rounder curve and soft knee.

A soft knee on a synth part will give us not so steep transition from non-compressed to compressed signal. Gradual change will permit the general compression to sound a lot cleaner and more transparent.

api 2500 knee setting

Some details on using Knee setting

  • Not all the compressors have the switchable Knee setting.
  • There are two primary varieties of Knee settings: Hard Knee and Soft Knee (there are some exceptions, like Medium Knee as well).
  • Some extra advanced compressors can have the Knee values expressed in decibels, with smaller values denoting Hard Knee, and higher values denoting Soft Knee.
  • Soft knee values will end in an extra homogenous and refined sound of your mix components, whereas Hard Knee values will make the sound of your compression extra obvious.
  • Experiment with varied Knee and Attack combos for fascinating mix results and sounds.
  • Examples of the mix parts Soft Knee is usually most well-liked: Vocals, Guitars, Bass Guitar, Piano, Strings, Synthesized Pads, and different non-percussive elements.
  • Examples of mixed components Hard Knee is usually most well-liked: Drum Elements (Snare drum, Kick Drum, Toms…), all different types of Percussion.
  • Keep in mind that there aren’t any hard and quick guidelines relating to defining the knee value. It should all the time rely on your materials, and what you’re attempting to achieve with compression. There positively are cases while you may have to make an otherwise non-percussive component sound extra aggressive if that’s its operate in a mix. Piano-only music is usually a good instance of this! In this case, reasonably than flattening the influence of the piano sound that carries the music, utilizing soft-knee compression, you may wish to emphasize it with Hard Knee as a substitute.

three choices of knee


Once we’re dialing in compression, the knee is usually a bit much less make-or-break than, say, attack and release instances. That stated, it may well typically be the factor that determines how “musically” a compressor behaves, and for that purpose, it’s worth paying attention to.

Next time you end up reaching for a compressor, simply bear in mind to ask yourself whether or not “smooth” is the quality you’re after. You probably have a transparent sense of what you’re going for, it isn’t hard to search out the best knee setting for the job.

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Nikoloski is the founder and main content writer and editor of Mixing Tips. With his experience in audio engineering, mixing, and mastering for over 15 years, will provide hands-on experience and expertise in all the matters covered on this website.

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