How to use Compressor

What's a compressor? What are all the knobs? Ahhhh, the insanity!

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Going a little bit nuts when looking at those complicated compression interfaces?I don’t blame you.The first time I set eyes on a compressor I noped out of there pretty darn fast. I didn’t understand what it was, how it should be used or when to use it. I just picked up that to make my music sound “full”, I needed to use a compressor. That might sound vague right now, but in a few short paragraphs I’ll be explaining exactly what compression is and what all those different knobs mean.

What is compression and why would you use it?

To put it in one simple sentence: a compressor is an automated volume control. It’s most basic function is to listen to an audio track as it’s playing, and lower the volume once it passes a certain threshold.It reduces peaks in volume.The result of some compression on a track looks a little bit like the image below. Notice how the compressed track’s volume (the peaks) is much more aligned compared to before.Okay, that’s compression in a nutshell.

So what do all the knobs do?

Well, there’s about 5 knobs that I would consider to be the most important to understand the basics of compression.Let’s explore these in turn. It’s a little bit theoretical, but don’t worry – once you start playing with them it all starts to make sense.

Threshold

The most important knob on a compressor is the threshold. It tells the machine “if the input signal passes *this* amount of db, then activate!”.For example, if you put it at -5db, then it will lower the signal every time it passes -5db — nothing more, nothing less.The threshold knob often has two modes: peak and RMS.

Peak mode means it’s going to detect spikes in volume, which is good if you have to control quick volume bursts.
RMS means Root Mean Square and it’s simply a way to detect the mean volume of a track. It’s more useful if you want to control longer periods of time.

For now, to really hear what a compressor can do, I do suggest that you stick to peak mode. Once you understand what that does, feel free to experiment.

Ratio

Ratio, then, tells the compressor how much it should lower the signal once the threshold has been crossed.It’s expressed as.. Well.. A ratio… and if you encounter something like 2:1, it means that the output of a signal that crossed the threshold will be halved compared to the input signal. 3:1 means it’s going to be one third of the input, and so on.With threshold and ratio alone, you can do a whole lot of things already. In fact, for basic dynamic processing, it’s probably enough. But there’s more buttons that you should definitely be aware of, and they allow us to do some pretty sweet things.Attack and release for example…

Attack

Attack tells the compressor how long it should wait before engaging in volume reduction, once the threshold has been passed.For example, let’s say you set the attack time to 10ms. Then, you have a snare drum hitting hard – and it’s crossing the threshold you set.With an attack time of 10ms, it means that the automated fader that is the compressor won’t engage until after 10ms.

Release

Release is also expressed in time and it tells the compressor: this is how long you need to keep the volume fader down for.So for example a release time of 100ms would mean that the compressor will keep the volume down for 100ms, and then put the volume back up to wait for the next peak it encounters.

Why would these two parameters (attack & release) be useful?

Well, suppose you want to make a snare that sounds very long and stretched in time snap a little bit more. It would require you to only reduce the volume of the tail of the snare sound and not of the initial hit. With attack and release, that’s entirely possible: just set the attack time to just long enough that the initial hit of the snare passes through the compressor without that part being lowered in volume, and set the release just long enough to cover the tail of the snare drum.

Gain

Finally, we have the gain (or output, gain compensation,…) knob.This does exactly what it’s name implies. After you’ve compressed the peaks of a track, you’ll have more headroom to work with on that specific track. Using gain compensation, you’ll be able to increase the volume once again.If you’ve done everything correctly, you’ll have effectively made the overall volume of the track less dynamic – allowing you to make the track louder in the mix, as there are less intense peaks to cause clipping.

Now you try it!

I now want you to take a snare drum track, and try messing around with these parameters:

Lower the threshold in peak mode & see how the compressor decreases volume each time the snare hits
Increase the ratio to something silly, like infinity:1. Listen to how much that squashes the signal when a peak hits!
Play around with a shorter & longer attack or release time & really listen to how that affects the snare hits
Finally, if you’ve compressed it all – raise the gain. You’ll notice you can get the snare track sounding a lot louder than it was before the compression

Do I need a compressor on this track?

One thing I get ask time and time again is when to actually use a compressor. A lot of people understand the basic functionalities of a compressor and know what turning the knobs does or doesn’t do… but they’re unsure of when to actually put a compressor on their track.

The answer to this one is simple: your song will give you the hints you need. You just need to learn how to pick up on them.

So here are the steps you need to follow to diagnose whether or not you’ll need a compressor on a track. And for the sake of an example, let’s consider a vocal track.

Start with getting a general balance of the mix.
Listen to the song while focusing intently on the vocals.
Listen to the volume dynamics of the vocals and pay attention to any sudden peaks or drops in volume: are certain words or phrases noticeably louder than others?
Or if you’ve got your hand on the vocal fader, are you overcome with an urge to constantly lower and raise the fader during the song?

If the answer to any of those questions is yes, then congratulations: you successfully diagnosed your track with a dynamics issue – that needs to be addressed with a compressor!Follow the steps outlined to diagnose if a track needs compression. Simple enough!

Finding the right compressor for the job

Still feeling a little bit confused?You’ve started exploring the different options you have for controlling dynamics with compression. And I bet you’ve encountered a plethora of different options. And now you’re left wondering: “Which compressor is best for me…?”

First of all you can check out our roundup of the best compressor plugins in our books.

But the answer to your quesiton really depends more on the situation and the exact reason you’re thinking of using a compressor.Different compressors have different characteristics, functionalities and either have a very surgical way of operating – or indeed a distinct sound. Looking into the specifics is outside of the scope of this article, but here’s an actionable tip that will at least make sure you understand the differences in the compressors you already have at your disposal:

Check out what your DAW has to offer in terms of compressors. You’re bound to find a few.
I want you to try each and every one out on a snare drum track, with exactly the same settings.
Listen carefully to how each compressor influences the sound, by soloing out the different tracks.

No compressor is the same. Check out videos on the different types of compressors you’ll encounter, and try out the different ones you have in your DAW already!