When Should You Compress Audio?

by ReverbLxnd in Mixing

This article looks at when and how pros use use audio compression on individual instruments and the mix bus.

Today we’re going to look at how you should use audio compression on individual instruments and the mix bus, like a pro.

What is Audio Compression?

We introduced and looked at what is audio compression before. Check that out if you haven’t already because this article builds onto the concepts introduced there.

In summary…

A compressor makes music or an instrument sound louder without increasing its amplitude. Amplitude is volume.

A compressor turns down the loudest parts of an audio signal to make it more even.

If I have a bass player who is playing some notes loud and some soft, I’m going to compress the bass so that it sits more evenly on the mix.

If I don’t compress, it’s going to sound like it’s coming in and out.

One of the things about the low end in your mix is you don’t want it to be dropping in and out.

When to Compress?

Ratio and Threshold

Ratios and threshold determine when the compressor kicks in and is working.

2:1 ratio is usually where you begin engaging compression.

A 2:1 ratio means that when a signal coming into the compressor exceeds the threshold by 2 dB, it will be attenuated (or turned down) by 1 dB. If the threshold was exceeded by 8 dB, it will be attenuated by 4 dB.

1:1 is no compression at all. That’s just the sound straight through.

You compress 2:1, 4:1, 6:1, 8:1, 10:1… up to 20:1. Over 20:1 is basically limiting.

Limiting is when the sound goes up and hits a brick wall so it doesn’t go any louder. It flat-tops.

If you look at the waves of a sound that have been highly compressed, they are flat at the top.

That’s one of the reasons you don’t typically compress distorted electric guitars or synths that have square saves.

2:1 ratio is mild compression. Around 3:1 and 4:1 is moderate compression. You start getting into medium compression around 6:1 and strong compression around 10:1.

Typically, pro mixers use mild compression on drums, moderate compression on the drums and the mix bus, and medium to strong compression with drums rooms.

We’ll talk more about this in a minute.

Attack

The attack time is how fast your compressor grabs the sound.

A slow attack time lets more of the transients get through. The transient is the initial strike.

A snare drum, for instance, has a very quick transient and then the sound decays fast. A piano and acoustic guitars also have quick transients.

The waves of a quick transient look very identical. There is this huge spike and then a quick decay afterward.

This transient is what you want to control with compression.

If you heavily compress the attack, again, that will flat-top the transients.

If you have a fast attack time on something that has a quick transient, you’re going to hear the compression (more snap).

You may want that snap. A lot of the time, you want that snare to attack.

If you have a slow attack time on something with a quick transient, you get more punch.

When to Use a Fast Attack

Generally, you want to use a fast attack time on the bass.

If you have a bass player that plays inconsistently, you’re going to want to have a fast attack time because those transients will be inconsistent.

You can see the inconsistent spikes in your DAW.

You want to use a fast attack on an inconsistent acoustic guitar.

That’s basically, to level out that sound and make it much more consistent and punchy.

If you quick transients on synth sounds, a fast attack will make the synths jump out of the mix.

A lot of times when tracking vocals, you might want several compressors. This is on the tracking stage.

Let me explain…

One compressor on the vocals is for slowing down the transients.

You never want the vocals to be jumping in and out of the mix all over. But might not want your vocals to be compressed at all.

This compressor will have a really slow setting, like a 2:1 ratio.

Another compressor will be set with a higher ratio behind it with a fast attack time.

The second compressor will grab only sudden peaks. This is so you don’t have to go back into the mix all the time to compress this.

You’ll know what has sharp transients by looking at the waveforms.

On the drums, you want a slow attack time because you want those transients to come through.

That is what gives your drums aggressiveness and punch–slow attack times.

Release

The release time is how fast the compressor lets go of the sound.

When to Use Long Release

A great time you use long release time on your compressor is when you have a bass sound that dies off too fast.

Compressing a bass like this will turn the waveform from one that tapers off to one with a fat backend.

That will make the note have less decay and more amplitude. It becomes more sustained.

You use the compressor to hold off the backend of the note and keep it steady so that your low end doesn’t dissipate.

Your attack and release times are crucial on the mix bus.

I mentioned before that most pro mixers will set the mix bus at a 4:1 ratio.

If you are using an SSL Bus Compressor (which is pretty much the standard), the attack time will vary from 1-10 secs. As for the release time, most mixers use auto release.

This goes for both the hardware units and the software emulation of these units.

The attack time will vary from 1-10 secs depending on what you’re mixing, what the tempo is, and how punchy you want to make it.

If you have slower attack times, you’ll get more punch.

If you want the compressor to glue everything together, you go for faster attack times.

In conclusion

Now that you have a pretty good understanding of when to use a compressor, check out some of our other articles about compressors on our mixing blog.

As always, be sure to drop your email in the subscription form below if you haven’t already. There’s more awesome mixing stuff coming that you don’t want to miss.

That’s it for this article.

ReverbLxnd

I've been a musician and brought in my stuff for mixing and mastering, I've been my own producer where I wrote, recorded, mixed and sold my own stuff. Now, I'm *mostly* an audio engineer, where I only record and mix for clients. I'm currently based in Berlin, Germany, where I operate ReverbLand out of. Got a question? DM me on Instagram or Twitter @reverblxnd everywhere, or shoot me an email reverblxnd@reverbland.com. I'd love to hear from you.

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