Threshold is a user-set variable that essentially determines when the compressor’s circuit will be engaged based on dynamic range. Think of it as a doorway that allows in a little or a lot of audio information. Low thresholds will allow in large amounts of audio material while high thresholds let in very little – it all depends on your setting.

The knee allows the compressor to ease into action as the audio material gets closer and closer to the threshold’s settings. There are generally two types of knee: hard and soft. Hard knees offer stiffer resistance to audio material approaching the compression threshold and causes the compressor to “snap” into action similar to when your doctor tests your reflexes by tapping just below your kneecap with that triangular shaped rubber mallet (reflex hammer) to trigger a knee-jerk reflex action. Compression can be much more noticeable using hard knee compressors.

Compressors that have a soft knee design are much more forgiving as audio material gets closer to the compression threshold. It offers smoother, less noticeable gain reduction by easing the compressor into action as is characteristc on more vintage tube-based compressors.

Most analog compressors do not offer a choice or control of a knee. This is mostly due to the overall design of the compression circuit itself. For those who are curious, API’s 2500 stereo bus compressor is one such unit that offers full user control over all five parameters.

Allow me to explain this again metaphorically as if you are at a major event (i.e. the Grammys) and you’ve just won! You’ve condensed a lifetime of ‘I’d like to thank’ into 15 seconds and you’ve been regally whisked offstage by an anonymous beauty in a ball gown.

The next thing you are being instructed to do is to leave backstage and go through the giant ballroom (where a band is playing and people are partying) and go to the press room for photo ops and post-win interviews. You walk down the long hallway towards the ballroom door where a burly 6’8” doorman stands behind velvet ropes.

The doorman is the Threshold and the velvet ropes are the Knee. In order for you to get into the ballroom, the doorman has to let you in. As you arrive at the door he takes his hands off the ropes, crosses his arms and coldly stares at you and says “What are doing here? This is for Grammy winners only.” Ouch! He doesn’t recognize you. He only listens to Siberian reggae bands. High Threshold. So high that you can’t pass though.

However, if as you approach he quickly unfastens the ropes (soft Knee) says “I’m a huge fan” and moves aside without you having to break stride. You have crossed the threshold and are now inside the compression ballroom. This is a well set Threshold – only allowing in those who are authorized.

Or perhaps as you arrive at the ballroom entrance, you notice that the doorman has left, the ropes have long been knocked over (soft knee) and anyone can enter. Low threshold, soft knee, and lots of compression in the ballroom. This can be a good thing too! Think of the drum sound on Led Zeppelin’s “When The Levee Breaks”.