In our first installment on audio processing tools, we’ll tackle one of the most basic – yet seemingly arcane – tools in a producer’s arsenal.

The compressor.

Correctly applied compression is one of the most useful techniques for bringing a track to life, adding impact and character to everything from bass and drums to an entire mix.

The underlying principles are actually pretty easy to understand, once you familiarize yourself with the various parameters and terminology.

So let’s do that.

How compressors work

Compressors affect the most basic aspect of sound: Volume.

In studio parlance, we use the term “dynamics,” which is just a fancy word for how a sound’s volume changes over time, like the immediate crack of a snare or punch of a kick.

Another example might be a chord progression that varies slightly in level as the sound evolves.

By applying a compressor to these signals, we can modify their dynamics to bring them in line with our production objectives.

The essential concept in understanding this process is that a compressor lowers the volume of a signal when that signal’s volume exceeds the level determined by the compressor’s threshold setting.

The amount and rate of that volume reduction is determined by various other parameters with names like ratio, attack, and release.

Finally, many compressors offer different options for modifying the character and performance of the process so that it can be fine-tuned for a wider variety of production applications.

With names like peak, RMS, soft knee, hard knee, lookahead and sidechain, it’s easy to see why so many producers just turn the knobs till things sound “good.”

But actually knowing what each knob does is the domain of the pros, so here’s an overview to get you started.

Compression parameters

Threshold

This is the key parameter that governs the overall behavior of the compressor.

Threshold determines the volume at which compression kicks in.

Simply put, when a signal’s level exceeds the set threshold, the compressor activates and begins lowering the volume.

Ratio

Next in importance is the ratio parameter.

This determines how much the compressor will lower the volume once the threshold is exceeded.

For example, if a compressor’s ratio is set to 6:1, then when the threshold is exceeded by six decibels, only one decibel will pass above the threshold.

Extreme settings, like 10:1, allow only one decibel to pass for every ten.

Note that for many producers, any setting above 10:1 is considered “limiting.”

That is, at the highest settings, a compressor turns into a limiter, which serves the same essential function, but in a much more extreme manner.

Attack

As with synthesizer envelopes, the attack parameter determines how quickly a characteristic changes.

In a compressor, the attack parameter controls how quickly the processor lowers the volume after the threshold is exceeded.

Here’s a practical use: If you’re using a compressor on a drum and you want some of that drum’s original attack to cut through, set a slower attack so the compressor doesn’t clamp down too quickly.

Release

Once the signal passes back below the threshold, the release parameter determines how long it will take for the compressor to stop manipulating the volume.

At this point, you should take a few moments and experiment with all of the above parameters, paying close attention to the attack and release amounts with high ratio settings.

If you’re not careful with these parameters, is all too easy to take a wonderful, lively sound and squish it into a lifeless mess, so spend some time learning how these parameters affect a wide variety of sounds.

Use drums. Use bass. Use vocals, pads, or any other instrument that’s part of your style.

Once you have a basic grasp of your compressor’s sound, the next round of parameters will make a lot more sense.

Peak/RMS

Many compressors include a Peak/RMS switch.

This affects the behavior of the threshold control.

In peak mode, the threshold reacts quickly and strongly to sudden volume changes, which can be quite handy for drums and percussion.

RMS mode causes the threshold parameter to adapt to the overall average of the volume shifts, which is useful for more consistent signals like voice, pads and even full mixes.

Knee

Another common parameter that shapes the sound of a compressor is the colorfully titled “knee” parameter, which is so named because it changes the shape of the threshold response – looking a tad like a knee.

The screenshot on the right shows Live 7’s compressor with a softened knee setting.

Hard knee compression is the usual default, but soft knee mode is a much more pleasing sound for many producers.

Here’s why.

When soft knee mode is active, the compressor gradually kicks in as the signal approaches the threshold rather than simply engaging instantly as the threshold is crossed.

Again, there are valuable uses for hard knee mode, like drums and compression.

But when you hear producers talk about “transparency” there’s a good chance they’re referring to a soft knee mode – often in conjunction with the RMS mode mentioned previously.

Lookahead

Lookahead is a parameter that’s found exclusively in compressor plug-ins, since implementing it in hardware would defy the laws of physics.

What lookahead does is analyze the sampled audio before it arrives at the compressor, thus giving the compressor a “heads up” that a loud signal is on its way.

Like an audio crystal ball, of sorts.

Ableton Live’s Compressor device includes a lookahead function that can be set to 1 millisecond, 10 milliseconds, or “off.”

For what its worth, I just leave it in 1 millisecond mode and forget about it.

Maybe I’m just lazy

Sidechain

And now, the moment many of you have been waiting for… Sidechaining.

Sidechaining is one of the coolest features on any compressor, since it allows a second signal to control the compressor’s behavior as it processes the primary signal.

What the heck?

Here’s the most common use for sidechaining: kick and bass.

Since many kick drums share the same frequency range as the bass line, it’s quite common for the two elements to get in each other’s way.

By using your compressor’s sidechain function, you can use the volume of the kick drum signal to lower the volume of your bass sound every time the kick drum hits, thus creating room for both instruments in a push-pull manner.

In doing so, you’ll immediately be rewarded with a sound that you’ve heard on countless tracks to great effect.

We highly recommend spending time experimenting with this specific technique, so to get you started, check out the screenshot to the right.

And don’t just stop with bass, either. Try using a kick drum as a sidechain input on sustaining sounds like pads, as this will give those tracks that “bounce” that defines tons of trance and progressive house classics.

Now go compress something.