Dynamic Range Compression: What Is The Knee Control?


Dynamic range compression is a popular and important effect/process in audio production. The knee control/parameter of a compressor will affect how the compressor responds to audio signals and is worth understanding.

What is the knee of a compressor? The knee of a compressor refers to the transition point around the threshold of the compressor where the output becomes attenuated versus the input. A hard knee offers a more distinct triggering of the compressor while a soft knee allows for a smoother and more gradual transition to compression.

In this article, we’ll discuss the knee control of compressors in detail to deepen our comprehension of compression and how to use it effectively in our audio projects.


A Brief Discussion On Audio Dynamic Range Compression

Let’s begin the bulk of this article by quickly going over the purpose of dynamic range compression and the basics of how it works.

What is dynamic range compression? Dynamic range compression is the process of reducing the dynamic range of an audio signal (the difference in amplitude between the highest and lowest points). Compression does so by attenuating the signal amplitude above a set threshold point.

So as the name would suggest, audio compression effectively compresses/reduces the dynamic range of an audio signal.

The dynamic range of an audio signal is the difference between the loudest part of the signal and the quietest part of the signal.

We can liken compression to an automated volume fader to help us visualize the effect/process. In this comparison, a compressor can be thought of as a volume fader that automatically reduces the volume of a signal/track as the amplitude of the signal/track surpasses a set threshold.

The fader/compressor, therefore, reduces the dynamic range by bringing the loudest parts down in volume while leaving the quieter parts the same.

Compression is one of the most common processed to apply to audio signals. Its general uses include, but are not limited to, the following:

  • Maintaining a more consistent level across the entirety of the audio signal/track
  • Preventing overloading/clipping
  • Sidechaining elements together
  • Enhancing sustain
  • Enhancing transients
  • Adding “movement” to a signal
  • Adding depth to a mix
  • Uncovering nuanced information in an audio signal
  • De-essing
  • “Gluing” a mix together (making it more cohesive)

To learn more about audio dynamic range compression as a whole, check out my in-depth article The Complete Guide To Audio Compression & Compressors.


What Is The Knee Control Of A Compressor?

To understand the knee of a compressor, we must first understand the threshold and ratio parameters of a compressor:

What is the threshold of a compressor? The threshold of a compressor is a set amplitude limit that dictates when the compressor will engage and disengage. As the input exceeds the threshold, the compressor begins compressing/attenuating the signal and as the input drops back down below the threshold, the compressor disengages.

What is the ratio of a compressor? The ratio of a compressor defines the ratio of input signal amplitude above the set threshold to the output signal amplitude above the threshold. It controls the relative amount of attenuation the compressor will apply, when engaged, to the signal.

Let’s visualize the threshold and ratio controls. We’ll consider the following graph with the input level on the x-axis and the output level on the y-axis:

We can see that, below the threshold, the output level matches the input level. This is also true of a 1:1 ratio above the threshold.

With the other ratios, we see that the compression reduces the output of the signal above the threshold level. The higher the ratio, the greater the attenuation.

This covers the basics of threshold and ratio (and what we’ll need to know for this article). To learn more, check out the following My New Microphone articles:
Dynamic Range Compression: What Is The Threshold Control?
Dynamic Range Compression: What Is The Ratio Control?

Now onto the knee control of compressors.

First, it’s important to note that not all compressors will have an adjustable knee control. In fact, it’s actually a rather uncommon parameter. That being said, if you’re reading this article, you probably have (or at least know of) a compressor that has an adjustable knee control.

As was mentioned in the initial paragraphs, a compressor’s knee affects the region near the threshold of the compressor.

With a hard knee, the compressor will act just as we’ve discussed thus far: no compression will happen unless the signal amplitude surpasses the threshold. At that point, the compressor will be fully active after the attack time. It will fully disengage as the signal drops back below the threshold after its release time.

For more info on compressor attack and release times, check out my article Dynamic Range Compression: Attack & Release Controls.

A hard knee can be visualized with the following graph:

The antonym to a hard knee is a soft knee, which is another option when a compressor’s knee is adjustable.

A soft knee is not as reliant on the strict limit of the threshold. Rather, soft knee compression will apply its compression gradually as the signal approaches and surpasses the set threshold. In other words, compression will begin before the threshold and the full ratio will be attained above the threshold.

The transition between uncompressed and compressed audio is less abrupt with a soft knee compressor. This can be made even less abrupt will longer attack and release times.

A soft knee can be visualized with the following graph:

Some compressors offer the option of switching between hard or soft knees.

Some other compressors allow users to blend between these two knee types. This control is often defined as a decibel value that determines the decibel range between the point where compression starts (below the threshold) and end (above the threshold). Other times this control could be a simple minimum-maximum or percentage control.

Note that the smaller the decibel range, the harder the knee (a true hard knee has no decibel range).


How To Set The Knee Of A Compressor

Some compressors offer different knee modes (primarily hard and soft knee settings and perhaps a linear setting) while others offer finer adjustments (via decibel, percentage or other metrics). As was mentioned, some other compressors have no knee control whatsoever.

The best, yet most trite, advice when adjusting the knee of a compressor is to use your ears. Compressor settings are highly dependent on the desired outcome of the processing.

That being said, there are some loose rules for hard and soft knee compression.

Hard knee compression is often best suited for transient signals with fast and strong peaks. Examples include:

  • Drums/Percussion
  • Rap vocals
  • Staccato strings

Soft knee compression is often best suited to instruments with softer transients. Examples include:

  • Piano
  • Singing
  • Legato strings

Of course, it’s best to make adjustments and confirm what sounds best with your ears.

For most cases, it’s best to begin with a low ratio (between 2:1 and 6:1) with fast/medium attack and medium release times. Set the threshold so that 3 to 6 dB of gain reduction is achieved and make up for this reduction with makeup gain.

For more information on compressor makeup gain, check out my article Dynamic Range Compression: What Is The Makeup Gain Control?

After that, make adjustments to find the compressor settings that work best. This included, of course, the knee (if the compressor offers control over the knee).


What are the main controls of a compressor? The main controls/parameters of a dynamic range compressor are as follows:

  • Threshold
  • Ratio
  • Attack Time
  • Release Time
  • Knee
  • Makeup Gain

What is audio data compression? Audio data compression is the process of encoding digital audio information into few bits than the original signal/file, thereby compressing/reducing the file size. Data compression can be either lossless (eliminating redundant info) or lossy (eliminating unnecessary or “less-important” info).

Popular lossless audio compression formats include:

  • FLAC (Free Lossless Audio Codec)
  • ALAC (Apple Lossless Audio Codec)
  • APE (Monkey’s Audio)
  • OFR (OptimFROG)
  • WV (WavPak)
  • TTA (True Audio)
  • WMAL (Windows Media Audio Lossless)
  • Dolby TrueHD
  • MLP (Meridian Lossless Packing)
  • MPEG-4 ALS (Audio Lossless Coding)
  • MPEG-4 SLS (Scalable Lossless Coding)
  • RealAudio Lossless

Popular lossy audio compression formats include:

  • Dolby Digital
  • Dolby Digital Plus
  • DTS Coherent
  • MPEG-1
  • MPEG-2
  • MPEG-4
  • MPEG-H
  • Vorbis
  • WMA (Windows Media Audio)

Arthur

Arthur is the owner of Fox Media Tech and author of My New Microphone. He's an audio engineer by trade and works on contract in his home country of Canada. When not blogging on MNM, he's likely hiking outdoors and blogging at Hikers' Movement (hikersmovement.com) or composing music for media. Check out his Pond5 and AudioJungle accounts.

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