What is the “Knee” on a compressor?

Last Updated on February 27, 2020 by Facundo

The first time I took a look at a compressor I got overwhelmed…

So many knobs and controls that I didn’t know what they all were for.

The one that confused me the most was the “knee” control, since all the others seemed a bit more obvious.

In this post I will go into what the “knee” control does on a compressor.

What is the knee on the Compressor

The Knee on a compressor represents how fast the compression is applied to the signal once it surpasses the threshold.

It allows you to determine how curved the transition from uncompressed- to compressed sound will be.

If the knee is set to zero, then the transition is immediate from no compression to the compression you dialed in.

If you want a slower transition, where the compression slowly kicks in, or if you want to compress the signal slightly when it crosses the threshold but compress it progressively more as the signal gets louder, then this is where the knee comes in.

To better understand this, let me explain what each component of a compressor does, especially the ones related to the knee.

Are you looking for FREE compression VST plugins? Here’s a list of the best ones I could find.

What is the threshold?

Hard Knee

When taking a look at the image of the compressor above, the vertical line represents the threshold.

This would be the level required from the compressor to start clamping down on the signal, which you can configure to your liking.

If the signal is too low or the threshold is set too high, then the signal will be allowed to pass through the compressor unchanged.

The diagonal line to the left of the threshold represents the signal before compression is applied, while the line to the right represents the compressed signal.

What does the Ratio do?

The ratio specifies the amount of attenuation that will be applied to the signal.

If the ratio is set to 1:1, then no compression will be applied.

A compression ratio of 2:1 means that for every 2dB that the signal exceeds the threshold it will be attenuated to one over the threshold.

This means that if the signal is exceeding the threshold by 10dB, it will be attenuated to 5dB over it.

The higher the ratio the more the sound will be compressed. A limiter works just like a compressor but without allowing the signal to exceed the threshold.

So, as you can see, both the threshold and the ratio are important when compressing and when thinking of applying a soft or hard knee.

If you plan on applying soft compression, then maybe the knee isn’t as important since the transition will be hardly noticeable.

But when using a higher ratio, say 8:1 or 10:1, increasing the knee can be very helpful since the transition won’t be as obvious.

If you want the sound to sort of “sneak up” on you, then increase the knee. This will create a slow transition but still allow for full compression afterwards.

Related: Do you know the difference between Reverb and Delay?

Soft Knee and Hard Knee

So, by now you probably know what both these terms mean.

A soft knee is when you make the transition smooth, even if in the end the compression is really high.

There is a “bend” in the line making the compression gradual until it reaches the maximum ratio dialed in.

A Hard knee is the opposite.

As soon as the signal reaches the threshold it gets fully compressed. This will be much more noticeable.

When taking a look at the first image you can see that it’s a hard knee, there is no curve, the signal goes straight from uncompressed to compressed.

Soft Knee

In this image you can clearly see the transition, this would be a soft knee.

When to use one or the other?

Both have their place when mixing songs, it all depends on what instruments you are mixing and what effect you’re trying to achieve.

I like using soft knees on stuff like vocals, fingerpicked acoustic or classical guitar, piano, etc. Anything where you don’t want to kill the dynamics.

On the other hand, hard knees work great on percussive instruments such as drums, especially on the kick-, snare-drums and toms.

Using a hard knee on acoustic guitar when it’s strummed really hard is also a good idea, since you want to keep a consistent volume throughout whole track.

Of course, these aren’t strict rules, they are just guidelines. It all depends on what you feel the song needs.

Some other features on a compressor are:

Attack

The attack is the time it takes for the signal to become fully compressed after exceeding the threshold level. 

The faster you set the attack, the faster the signal gets compressed.

This is usually measured in ms (milliseconds).

In some cases, if the attack time is set too fast, it can create distortion in the lower frequencies by modifying the waveform.

Release

Release is exactly the opposite as the attack, it refers to the time it takes the signal to go from compressed to uncompressed, or to its original state.

Release times are considerably longer than attack times, ranging anywhere from 40ms to 3-5 seconds.

Conclusion

Using soft and hard knees depends entirely on the instrument you’re applying the compression to, and also on what the song needs.

Make sure to play around with these settings to get a feel for how they work and when you should use one or the other.

I hope tis was helpful, have a fantastic day!

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