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An audio compressor can be a tricky thing to understand when you're first starting out.
I will try and break it down in as simple of terms and explanations as I can for you.
(there is a video at the end of this article breaking it down as well)
At its basic description, an audio compressor is really just an automatic volume fader control.
It does more than just turn things up or down, but I'm trying to simplify it for teaching purposes here.
It can add dynamics (soft, hard, quiet loud) punch, energy, sustain, and tone, but for now I want to stick to the basics. I will be covering more on this topic as the weeks go by.
Music and all its instruments and variety of sounds is by nature very dynamic. Audio compression helps us tame these dynamics and "Glue" them together. Dynamics are very good in music, so we don't want to kill the dynamics, just tame, enhance, massage, where needed. Music should breath, not be a flat line.
I want you to have the fundamental knowledge so when someone asks you what is compression in music, you can say "I'll tell you!"
lets dive into the controls of a compressor and I'll explain.
This is the "BOSS" of the knobs in my opinion. Mr. Threshold tells all of us if there is going to be any audio compression or not.
If the Threshold is set to "0db" then aint a damn thing happening on the job today.
You have to set the threshold at what volume level (dbs) you want the compressor to kick on.
If you turn the threshold down just a small amount, then you prob will just compress the peaks of audio going over your threshold, and not compress the rest of the audio below your threshold.
If you turn the threshold WAY down you will essentially be compressing the whole audio signal, because most of the signal will have met the threshold and gone way over it.
Now This SOB was the most confusing part of a compressor for me. After doing tons of research, I see this explained with the least amount of clarity.
I also see people getting confused between what Ratio and Threshold do. Threshold tells you WHEN to compress, and The Compressor Ratio tells you HOW MUCH to compress once it's passed the threshold level. Comprende?
I'll go into more detail in a second, but the main thing you need to know is the lower the Ratio settings, 1.5:1 2:1 3:1 etc etc, the LESS compression will be taking place.
The HIGER the Ratio, 5:1 6:1 8:1, the more compression you will be using.
Here's the confusing part
I'll try to explain the best I can
So if your ratio is 3:1, then for every 3dbs an audio source (snare) goes over your set threshold, only 1db will actually be allowed to pass that threshold.
I guess Mr. Ratio is the door man. He decides how many people (dbs) get through.
So here is an equation that will help (I didn't come up with this, I'm not that smart)
So lets Say you have a snare that is hanging around -6db in volume. (no compressor)
you slap on a compressor, and set the threshold to -10db (which is more quiet than the -6db snare)
and you set the ratio at 2:1
So for every 2dbs (2:1) above -10dbs (threshold) only 1db of the snare will pass the threshold.
so your new snare signal will be at -8dbs, so a compression of 2dbs quieter than before. (Gain Reduction of 2dbs)
So just increase the Ratio number in this equation and you can see that more compression will be applied.
confusing I know!! Once you get your head wrapped around it, it becomes easier.
Just remember, low ratio less compression, high ratio more compression.
Attack and Release
So if Mr. Threshold and Mr. Ratio decide when and how much audio compression is applied, Attack and Release decide how fast or slow, and for how long the compression is applied.
This is where compression can affect the tone of the sound, not just the volume.
The faster you have the attack set, the quicker the compressor clamps down on the signal.
If its set at a slower attack, the compressor will let some of the signal pass before it starts to act. We are talking milliseconds here. SUPER small measurements.
If you have a fast release, then the compressor will let go really fast after it initially clamps down.
If you have a slow release, then it will hold on and compress a little longer before it lets go.
Here is an example of a snare that has been compressed. The first picture is uncompressed, and the second pic has a very fast attack and fast release. (The threshold and Ratio will have big effect also, but I just want to illustrate the attack and release for now)
uncompressed snare hit
compressed snare hit with fast attack and release
This compressor had a VERY fast attack and release, with a low threshold and a 3:1 ratio.
You see in the first uncompressed pic, how the initial transient of the snare is the loudest (biggest spike) and then the tail tapers down. (sustain of the snare sound)
When I set the compressor to attack that initial transient I completely squashed the sucker to where it was almost the same volume of the "tail" or "sustain" of the snare sound.
This is extremely exaggerated to illustrate the effect, but I hope you catch my drift.
So thats what you could do with a snare, squash that transient down to the level of the tail, then bring up the overall volume of the whole snare which would give you a fatter sounding snare or tone. That's just one example.
You could set the release to be a little longer holding on to that sustain or tail, then bringing the overall volume up even more.
Remember, It's ALL about using your ears and not killing the sound and the dynamics of whatever your compressing, whether it be individual tracks or a stereo mix.
What is compression in music? You're starting to understand I hope!
If your compressor has a "Knee" knob, don't be scared.
You have hard knee, and soft knee.
When the compressor kicks on, a Knee will tell how hard or how gentle to clamp down on the source.
A hard Knee will clamp down hard and may be more noticeable of an effect.
A soft knee will gently ease into the compression, hopefully causing a less audible compressing sound once it kicks on.
So if you're trying to gently compress a vocal for instance, and you want it to stay natural sounding, you could set it at a softer knee setting to ease into it.
Like always use your ears.
Thats pretty much it!
Make up Gain
FREE Download: Compression Cheat Sheet
The last setting I want to address is the "Gain" knob, or "Make Up Gain".
So once you've compressed your signal it will most likely be turned down by however many Dbs the settings applied.
So now you simply need to turn up the volume back to it's original state before being compressed.
"Make up" the gain (volume) lost.
The difference between uncompressed and compressed is typically a more balanced performance of the audio source.
Be careful not to ruin the performance with over compression.
The main lesson here is USE YOUR EARS.
Common Audio Compressor Mistakes
Too fast of an attack on your drums and percussion.
You don't want to completely kill your drum transients. The initial hits from the snare or Kick for example.
Drums are meant to be dynamic and have energy. Above I mentioned how to get a fatter snare, but you have to be careful when setting a faster attack. You could potentially kill the natural energy of your track.
Use your ears and play around with the attack to get it just right. start a a medium attack and adjust to the fast side and back to the slow side of the attack and see what it feels and sounds like. Then pick what you like best.
2. Thinking one compressor should handle all the work.
There is a term called "serial compression". Meaning you stack 2 or more compressors on a track and split the work load so that one compressor isn't handling all the heavy lifting.
If one compressor has to do all the work you can have negative unnatural effects on the sound source.
I like to use 2-3 compressors on a vocal track for example.
I'll also use a tape saturation plugin before the compressors to get natural tape compression and taming of the transients like old analog tape would do.
So my vocal chain would first get a tap emulation plugin, dial some tape saturation in, then one 1176 compressor to handle about 3 dbs of gain reduction, and then another 1176 to handle another 3dbs of gain reduction. Both set with a medium/slow attack and fast release.
This all helps smooth out the vocal performance and sounds more natural to my ears instead of having one 1176 do all of that.
3. Setting the attack time too fast on lower frequency instruments.
Low end frequency instruments, like the bass, need more time for the waveform to develop (mainly a problem on hardware, but be aware of this on plugins as well).
If the audio compressor attack is too fast, it will start to affect the wave form before it's fully passed through the compressor and the compressor will cut it off too quickly resulting in a distorted audio result.
If you notice this, try setting the attack a little slower to see if this remedies the problem.
There are more advance uses for audio compression like parallel compression, side chain compressing, and multi band compressing, So keep reading to learn more about these topics with links to video examples.
If you want a FREE compression cheat sheet just click the green button below!
FREE Download: Compression Cheat Sheet
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Common Types of Audio Compression
Parallel compression allows you to get the best of 2 worlds.
For example on a vocal. It's easy to overdue compression on a vocal and squash the natural feel and performance of a well recorded vocal.
What parallel compression does is allow you to keep that natural feel, but also blend in a highly compressed version to had fullness and weight to this original performance and still keep the natural dynamics of the vocal.
You set up a separate track with a copy of the same vocal and heavily compress it. Way more than you normally would.
Then pull that fader down and then slowly bring it back up blending it with the original track until it feel just right.
Some plugins allow you to do it inside the plugin on the original track, saving you the effort of setting up a duplicate track.
You can do parallel compression on anything inside your mix.
2. Multiband Compression.
Multiband compression is compression on individual, or multiple bands of the frequency range and can be set up with their own unique threshold, ratio, attack, and release on each band.
This allows for some great tonal shaping of your individual tracks or the whole mix.
I use this technique on vocals, bass guitar, and my master fader for the whole mix. I love it!
3. Sidechain Compression
Sidechain compression allows you to compress or turn down an audio source, but have it triggered by another audio source.
Say you want your bass guitar to quickly turn down a few dbs every time the kick hits to make room for the kick to stand out in the mix.
You can set up a compressor on the bass guitar, set the "key" for the compressor as the kick drum track, and then set the controls on that compressor to only affect the volume of the bass guitar when the kick triggers it. If you set it up just right it sounds great and natural!
You can get a pumping affect as well which is used in dance and electronic music a lot.
4 most common types of Audio Compressors
Most audio compressor plugins have emulations of the hardware compressors that I will list here. Different types of compressors will have different affects on your sounds. You can learn what these sound like and use them in your mixing tool bag for the desired result you are after.
Tube Compressors - Known for their slower attack and slower release times.