Knee compression is one of the trickiest things to understand about the functioning of compressors. It can make or break the dynamic range of a track but once you understand how it works, makes a dramatic difference in the sound of your mixes and masters. In this article we'll explain this crucial compression setting so, by the end of it, you'll understand how to use it and get the perfect settings for any scenario.
What Is The Knee on The Compressor?
Knee refers to how the compressor transitions into gain reduction from the non-compressed signal to the compressed signal. The knee can be either hard or soft. A hard knee offers aggressive compression and a soft knee offers gradual compression.
Usually, the knee will have a numerical value in a compressor in decibels; however, we refer to it as soft, hard, or somewhere in between, because it's way simpler to explain that way.
That being said, understanding knee compression is easier if you think of it as a second threshold for the ratio.
The knee setting has a decibel value that goes below the compressor's threshold level, so the higher the value, the softer the knee will be.
However, with a soft knee compression occurs gradually, so as the knee is set to a decibel value of 10dB, then compression will start 10dB below the threshold, so it kicks in at -13dB and then transition from 1.1:1 up until it crosses the threshold line and reaches the ratio.
This affects the way your compressor behaves because an audio signal might get more attenuation depending on how you set the knee value.
To better understand how can this parameter be critical for your mixes, we need to discuss the difference between a hard knee and a soft knee.
What's The Difference Between A Hard & Soft Knee?
In short, a hard knee compression will reduce gain by the whole ratio value as soon as the incoming signal crosses the threshold. A soft knee will compress before the signal reaches the threshold by gradually increasing the ratio up to its predetermined level.
The best way to illustrate the difference between each other is to see what happens to the waveform after processing. In the following image, we'll observe what happens to a kick drum after compression is applied.
You can see how the hard knee compression works in a way that the waveform is cut clean, whereas the soft knee rounds it up.
In a real situation, you'd probably want to choose according to the genre and vibe of the track, but generally speaking, a hard knee is better for grit and a soft one for a cleaner sound.
What Does Soft Knee and Hard Knee Sound Like?
Hard knee sounds more aggressive, it's better to preserve transients and make something sound punchier. Soft knee compression sounds rounder due to the gradual compression, which is better on bass, acoustic guitars, pianos, and similar signals to make them fuller and rounder.
It's a pretty subtle control if you don't know what to hear for, but it's all about the attack.
To give a practical example of how these two knee settings can affect your signal, let's see how the knee setting entirely changes the way the compressor regulates dynamics.
We can hear how the bass performance is not excessively dynamic, but the bass is clearly masking our kick, making it feel thin.
However, a hard knee might work to enhance the bass' dynamics, depending on the case
If you use soft knee settings it'd provoke a considerably longer sustain, while it controls the peaks, allowing a much smoother blend with a kick by staying out of its way.
Also, it's worth paying attention to how the soft knee compression achieves more gain reduction taking down the loudest parts, but brings the sustain in the quieter parts, leaving a more uniform performance.
Tips For Dialling The Knee Compressor Setting In
There are a ton of different situations where knee compression can save the day. Here, we'll cover a few different common situations and explain which knee settings to use and why.
Case 1: To create the sense of cohesiveness
Here we have a ska rock mix. I'ts a live recording, so it's important to nail the right settings because they'll have a direct and audible impact on the overall mix and if done well, it can contribute a lot making the mix more cohesive. For instance, if I'm working on this mix:
My thought process would be first noticing that the arrangement is meant to be contained, which I know because the drummer is not using many cymbals, but just a closed hi-hat while hitting the kick and snare pretty quietly. The bass is riding along only playing eight notes while following the pulses of the kick drum.
In this situation, I'd use a hard knee compressor, like the DBX 160, because first, I want to preserve the transients as much as possible, and second, it serves to the production of the song creating even more hype for the upcoming chorus.
Here's the result:
Notice how the hard knee, along with the fast attack and release times round up the transients of the kick and snare drum but preserves the attack thanks to the clipping distortion when gain reduction occurs.
Case 2: When you're mastering and need more kick and snare
Knowing how to set the knee setting of your compressor is also good in mastering situations, like in this case, where we have a mix that needs more punch for the main beat, for which we'll use a hard knee exclusively in the mid image to make the processing more focused:
As you can hear, it's punchier and it also worked to make the track feel like it's moving. However, drums are all over the place now, so my next move is to use a soft knee compressor, in this case a Vari-Mu compressor, to add glue and keep the output signal under control.
Notice how combining hard and soft knees can help you achieve a fuller and punchier sound. Also, hear how it has a bounce to it after this combo, which is helping the hype of the song, particularly on the rise at the end of the sample.
Case 3: To get that nice, upfront vocal sound
Lastly, when mixing vocals, depending on the genre and general vibe, it's good to use different knee settings to nail the perfect vocal compression. In these two examples, we have a rock song and a rap song, and we need to compress the vocals.
In rock music production, vocals are usually significantly more compressed than other genres because the distorted guitars tend to fight a lot with vocals. In this case, we want to preserve the energy and aggression, so it's better to use hard knees in these cases because it'd also produce some clipping distortion and give it more character, besides staying upfront.
However, in rap and trap music vocals aren't usually as heavily compressed in most cases. In fact, one of the most common compression chain used on vocals is the 1176 followed by an LA-2A, which are both soft knee compressors.
See how the soft knee settings are also cleaner because the gradual change between attack and release times favors the higher ratio settings.
Knee Compression Techniques For Mixing
Tip 1: Hard knee to increase loudness and clarity
Whether it is a mastering situation or an instrument that's not cutting through the mix, you can benefit from the way hard knee compression works and use it to make the audio signal be percieved as louder.
Hard knees create clipping distortion because peak reduction is more abrupt and aggressive. Hence, you can use a hard clipper to preserve the sound of your transients, while still keeping your dynamic range under control.
Be mindful that this will create distortion on your track, which might not be what you're looking for.
For this example, we'll use an EDM track and apply clipping distortion using fab Filter's Pro C2 with the “clean” algorithm.
What I do, is set the threshold on Pro C to trim down these peaks as much as possible without destroying the dynamic range of the track.
Once the sweetspot for the threshold is found, adjust attack and release times to be very fast so that compression doesn't cause pumping or digital saturation.
Set your ratio to the highest and your knee all the way to hard.
At this point, go into the sidechain options and isolate the midrange, and finally, adjust the gain to match the uncompressed source.
Notice how it suddenly feels more open, exciting, and overall fuller without using any EQ or extra processing.
Tip 2: Hard knee compression as a distortion
In this sample, we're going to take it from the ska rock sample we used earlier and notice how the bass is tight and works perfectly with the kickdrum, still, the bass isn't providing as much low-end as the part requires to make the chorus explode when the drummer hits the cymbals.
In cases like this, I really like a warm VCA analog compressor, to squish the end out of those lows and produce a nice clipping distortion. In this case, I used Waves' emulation of the DBX160.
Notice how the bass now makes everything feels more cohesive and fuller because the bass is more even. It also helps the part to be darker, which also serves the song making the chorus section more powerful.
Tip 3: Hard and soft knee compressors for dynamic stereo expansion
Another really cool, but more advanced trick to take advantage of a compressor's knee is using a combination of hard and soft knee compression to dynamically expand the stereo image and enhance its sense of movement.
I would typically apply this in my drums or in my two-buss if I'm mixing and every once in a while I do it in mastering situations.
In my particular case, I like to use Arturia's Comp VCA-65 for the mids, which is an emulation of another DBX compressor, the 165A, and Slate Digital's FG MU, but you can do this using Pro C2 or any combination of any VCA compressor with any optical, FET, or Vari-Mu compressor that allow M/S processing.
The VCA will compress the mid image with a hard knee, controlling the pointy transients but preserving their natural sound, whilst the FET, optical, or vari-mu will compress the sides with a softer knee, which will bring forward the quieter details on the side image, creating a dynamic stereo expansion.
Listen to the result:
Hear how the guitars now have more presence and seems like the whole mix instantly becomes more interesting.
Tip 4: Multiband compression
Lastly, we could apply all of these techniques using multiband compression. If you have Fab-Filter's ProMB, you could break the spectrum into six different bands. Make sure to set the slopes to 12dB/oct to avoid phase cancellations and because the compressor behaves more musically.
Settings will always depend on the song, but usually a soft knee setting with a higher ratio works great to round up the low-end with compression, while a hard knee setting with a mild or lower ratio works best for low mids and the midrange.
Also, try different combinations of downward compression and upward expansion as well as mid/side processing to achieve more clarity and depth. Hear how this processing affects this EDM mix.
A Brief Overview of Compression Settings
Here's a quick overview of all the compressor controls that you'll find in most compressors:
- Attack & Release – Determine how long the compression is going to last. Attack time sets how long before the compressor kicks and release time tells how long it goes away.
- Makeup Gain – Allows gain compensation to regulate after processing.
- Ratio – Indicates how much gain reduction will occur when the signal hits the threshold
- Threshold – Sets the level on which the compressor will engage
- Knee – Determine how the compressor will transition between the non-compressed and compressed states of the signal.
- Hold – Delays the release of the compression for a specific time
- Range – Determines the maximum amount of decibels the compressor will reduce when a signal is fully compressed
My Compressor Doesn't Have A Knee Setting – Does It Matter?
It doesn't matter. Many compressors don't, it's particularly common in analog emulations. However, although some compressors don't have a knee setting, all compressors have knees.
For instance, VCA compressors, limiters, and clippers have hard knees. FET, Opticals, and Vari-Mu compressors have soft knees. You can use different combinations of these to get different effects, sounds, and characters.
I Can't Hear What The Knee is Doing To My Sound. How Do I Know It's Working?
You can always set the threshold to an extreme position to exaggerate the effectand actually listen to what it's doing. This is also a great way to get to know and deeply understand how compressors work.
A very good trick to hear the knee is listening for the snap of the snare or the percussion. Soft knees tend to sound rounder and duller, whilst hard knees are more punchy and snappy.
What is Soft knee Best On?
Soft knee is better for low frequencies, rounder sounds, level regulation and sound bigger. They're good for dynamic range compression, so acoustic instruments, vocals, pads, and glue are home for this compression curve.
For example, for a guitar solo, you'd use an LA2A to level it up and lift up the smaller quieter details of the performance, which happens as a result of the gradual curve to the compression ratio.
What Is Hard Knee Best On?
Hard knee is better for drums and highly transient-focused content, to add excitement to frequencies, add harmonics and subtle dynamic range compression, they're the best for peak control, and particularly good to increase the perceived loudness of a track.
For example, you'd use a hard knee curve to reduce tall peaks in a mastering situation without disturbing the tone or energy of the mix. You can also use it to tighten up a bass or kick drum, and make vocals sound more aggressive.
Javier is a mixing and mastering engineer who specializes in Rock and Hip Hop, and writes and produces his own music. He is additionally a TV, Film, and advertising audio editor who has been working freelance for 7 years. He loves sound design and is an avid expert in his field, having written hundreds of articles for other publications online about music production-related topics. To put it simply – Javier knows his stuff.