Last night About2Flip commented on the Intro to Compression video:

Again Great Video!!! But what does KNEE do? I have been searching for almost a year now.

The knee function on a compressor is not as important as the ratio, attack or release functions. However, it can still be a useful tool, and it deserves a little attention. Thanks for asking, About2Flip.

Take a look at this screenshot of a compressor plugin:

knee-1

The orange vertical line is the threshold. The diagonal line to the left of the threshold represents the signal before compression is applied. The line to the right of the threshold represents the signal after it crosses the threshold. The higher the ratio, the more compression occurs, and the “flatter” this line becomes.

In case you haven’t guessed yet, the knee of this diagram is where the white line meets the orange threshold line. The white line begins to bend at this point, and it kind of looks like a bent knee.

The knee allows you to determine how curved this transition is. In the picture above, you see that the knee is set to zero, this means there is an immediate transition from no compression to 13.7:1 compression.

But what if you wanted the compression to slowly kick in? What if you wanted it to only compress the signal a little bit when it first crosses the threshold and compress it progressively more as it gets louder? That’s where the knee comes in.

Notice in this picture I’ve cranked up the knee control. As you can see, the “bend” in the line is much more curved, indicating a slow, smoothe transition into “full compression” mode.

knee-2

This is the kind of thing you’ll need to play around with, but basically, if you want the compressed sound to “sneak up” on you a little more, try increasing the knee. That way there won’t be an obvious transition from the uncompressed to the compressed sound.

I like a softer knee on things like vocals and piano. However, on more percussive, choppier tracks, like drums or strummed guitar, a low knee tends to work better.

Does that help? Do you use a “hard” or “soft” knee when compressing? Leave a comment.

  • Maxwell Ryan

    Thanks, this helps a lot. You’ve explained everything I needed to know without going overboard on anything.

  • Joel

    GREAT EXPLANATION! thanks

  • Richard Jon

    yes! thank you.

  • ike

    still doesn’t say which is hard and which is soft, turned up is hard lower is soft? or is iit soft when u turn it up bc it sounds soft when u hear the instrument creep up???

    • Soft is when there’s a smoother curve. Hard is a sharp angle.

  • jampaul

    Great job. Thank you!

  • Johanan

    Wow.. That’s the best article on compression I’ve ever read over the past two years… Simple and Crisp..!

  • Snow

    Great explanation and the pictures comes in handy to better understand what’s going on before and after any compression. We’re still using Pro-Tools at the Art Institute of Seattle, and may eventually transition into 11, but I’ve been wondering about the “knee” since we’re learning about compressors currently in class. Thanks again.

  • Ifugue

    Very interesting stuff Joe….at the moment I’m trying to get my head around the multi band compressor on Izotope Ozone 5 Advanced……….[it’s too advanced for me 🙂 ].

  • How is the Knee different from the Attack parameter? Doesn’t the attack also tell the compressor how quickly to apply the attenuation?

    Thanks,
    -Searching

    • The knee has to do with the ratio, not the attack. Attack tells the compressor wow quickly to compress the signal once it crosses the threshold. The knee essentially tells the compressor to start compressing a LITTLE bit before the signal actually reaches the threshold.

      • voodoo

        Like a mini pre-attack? Because to tell a compressor “to start compressing a LITTLE bit before the signal actually reaches the threshold”, is still in relation to time and speed… yes? no?

        Thanks so much for your patience.

        – Still searching.

        • voodoo

          Or is it like a fine tuning adjustment for the beginning of the attack?  Would applying a knee be like creating soft clipping (knee applied) as opposed to hard clipping (no knee applied) of the wave file?  So that one is now manipulating the harmonics of the waveform being sent through the compressor?

          • voodoo

            … Is the application of the knee also used to help to hide the fact that compression is being applied? 

            (So many questions…  but I feel my little light bulb flickering over this!)

            – Thanks so much!!!

            • Great questions. I didn’t explain it super well in that last comment. Let me try again.
              Knee has nothing to do with attack or timing. If the attack is set to 10 ms, it will always be 10 ms.
              The knee has to do with the ratio. So let’s say the ratio of the compressor is 4:1, and you’ve got the knee function enabled. Let’s say the threshold is set to -12 dB. When the signal reaches a level of -12 dB, the compressor will be operating at a ratio of 4:1. But BEFORE the signal gets to -12 dB, The ratio will slowly increase.
              So, it might do something like this:

              When the signal’s at -16 dB, the ratio is 2:1
              When the signal’s at -14 dB, the ratio is 3:1
              When the signal’s at -12 dB, the ratio is 4:1

              If the knee feature was NOT being used, the compressor wouldn’t be compressing the signal AT ALL until it reaches the threshold. With the knee enabled, the compressor starts compressing as the signal APPROACHES the threshold.
              The result is a smoother-sounding, less obvious compression. It gets harder to tell when the compressor is engaged and when it’s not.
              Hope that helps.

  • soedesh

    what are the basic settings for vocal compressions, can i compress midi files also where vcan i download a good compressor i use cubase 5.1
    greetings

  • stephen

    In the second picture does the db reading next to the the “knee” mean that when the sound gets 25.2 db over the threshold it will be compressed at a ratio of 13:7:1?? Ive noticed some compression only have an on/off switch for the knee and im just trying to get my head around what exactly the db reading next to it is for.

    • You might be reading into it a bit too far. I don’t think knowing EXACTLY what it stands for will make much of a difference in how you use it. 🙂

      I think it just means over how many dB the “curve” goes. Hard knee = no curve. “Soft” knee = some curve. How MUCH curve? That depends on the setting or the plugin.

  • Clarence

    Thanks man, this really helped me to understand how to help the transition of my compressed work. Thanks for being the “Good Samaritan” for the ones that aren’t as knowledgeable.

  • Jay

    Joe you are the best man you a really are thank you so much for everything and I am so for the home studio engineers out there.

  • Matt Needham

    Thank you!

  • sofia graells

    great, thank you Joe for a clear explanation

  • About2Flip

    Thanks for explaining!!! You are Great Person for doing this type of site.