Keith Everette Smith

Producer, Songwriter, Multi-Instrumentalist

Compressor Wars

Compressors are perhaps the most mysterious of audio processors. You either admit that you don’t know how to use them or you are always learning how to use them more efficiently and to every extent that they are capable. I guess what I’m trying to say is that compressors are deep.. WAY deep. There’s a lot to learn. Even once you understand their concept there seems to be hundreds of ways to use them.

Different ways to use a compressor (there may be more… I’m still learning too)…

  • Control dynamics
  • Add “thickness” to an audio source
  • make something “punchy”
  • make something “pump”
  • control sibilance
  • control EQ of an audio source
  • add “color to an audio source
Like I said, there are more ways than this to use a compressor and within these functions there are plently of ways to achieve the same goal.

Honestly, the sound of modern recording is the sound of compression. I’m not just talking about a dynamically squashed sound (although this can be the result of the following) but compressors thicken and add color to anything it’s applied too, especially in heavy doses. Rock music often adds compression generously, thus giving the listener the illusion that a mix is louder and thicker. Consoles like the SSL E Series console offer dynamic processing on every channel allowing mixers to use as much compression as was needed. Today, most mixes are performed inside of a DAW (digital audio workstation.) This offers the mixer even more control over dynamics.

I want to talk about the common controls that a compressor utilizes. We’ll talk about the different ways to use a compressors and possible settings for different instruments later. For now, the basics.

INPUT - controls the amount of gain that enters the compressor.

THRESHOLD - the point when the compressor kicks in. i.e. If the threshold is set to -10bd then any audio louder that -10db that enters the compressor will be compressed. Anything under the threshold will be left alone (except "soft knee” compression… see below.“

ATTACK - I like to rename this "attack speed” because I feel it accurately verbalizes what the attack setting does - Once an audio source rises over the threshold the compressor can kick in at different speeds. A fast attack would turn the compressor on very quickly (keeping harsh peaks at bay) where a slow attack would let the first transients (the first part of the audio source) pass through before the compressor reacts (this setting will let any percussive sounds keep their “punch”

RELEASE (or “release time”) - The release setting determine how fast or slow a compressor shuts off after the source has gone below the threshold. The release time may be determined by the speed of the rhythmic-ness of the instrument. A release time that is slower that the next transient peak may sound lifeless and flat.

RATIO - Without a doubt the most complicated part of the compressor. The ratio decides by what ratio is an audio source reduced. You'lll see settings like 3:1, 4:1, 6:1. A 3:1 ratio means that an audio source will be allowed to raise 3db before 1db of compression is applied. This allows there to still be dynamics in audio even when the audio is being compressed. To complete this definition. A source that has risen 6db above the threshold and has a 3:1 ratio will be reduced by 2db. In the same manner, an audio source that has risen 12by above the thresh. and has a 4:1 ratio will be reduced 3db. It is actually very simple.

OUTPUT (or “make-up gain”) - It’s true, any time you add compression you are lowering the dynamic range of at least some part of the sound. The output knob simply adds gain where the compressor has taken it away. A good way to put it would be if you turned down the volume of your TIVO but turned up the TV to make up the volume loss. OR when your ipod is too loud in playing through your car stereo and it distorts… you turn the ipod down and turn up the car stereo to make up the difference.

SOFT-KNEE COMPRESSION - Soft knee compression basically makes it so the compressor is actually working a bit below the threshold. It gradually begins working harder as source nears the threshold. You might use this setting if you want the source colored in general, not just at the peaks.

HARD -KNEE COMPRESSION - this type of compression is the “normal” way of using a compressor… the compressor begins working only above the threshold. You would use this setting when you desire for only the peaks to be reduced.

That’s it… the basics. I’ll be touching more of compression soon. There is so much to cover. If I could offer one tip… when you’re first starting to use compression, try using a LOT while you’re messing around with your gear. Like any gear, take it to the limit. It is at this point where you find out what your compressors can do and what they sound like. When you learn the sound of a compressors extremes you will learn to hear when it is applied in small doses.