Audio compressor basics

Compression is a very important aspect of audio production. You need to have an idea of what the audio compressor does and what all those buttons do. Everybody uses it differently and everyone has a method. These methods are created after mastering the tool that compression is.

You can't have a specific method to compression if you don't know how to instinctively use it. You can't only know what one button does and ignore the others. That's no method.

Therefore, let me introduce you to all the typical buttons an audio compressor has. What do they do? How do they interact with each other. When you get a grasp on which button to push, knob to turn or slider to move, you have an easier time getting that sound you want. 

Audio compressor

Basically, compressors compress the audio signal you want to process. Easy right? Well, that doesn't really tell you anything. Compressors even out the audio signal at a certain level, called the threshold and compress the level above at a certain ratio you determine. We push the loud levels lower and therefore have a less dynamic signal.

Now, let's get into all the buttons and sliders and whatnot. As an example, I'm using Logic's Audio Compressor. Any compressor you have will almost certainly have the same buttons.

Let's start with the most important parts first. If you have a incredibly basic audio compressor on your hands, chances are you only have these two parameters to work with. Threshold and Ratio. 

Threshold

The threshold determines at what level the compressor starts acting. Say you have a signal that has peaks at around -1dB on the meters of your fader. If you have your threshold at -10, the compressor will start working when your audio starts going over the -10dB threshold. 

Now, if you have a weak signal that never goes over -15dB and you have your compressor on -10dB there's no compression going to take place. Maybe, if you have a swanky cool compressor it will give you a nice color to your sound, but as for actual compression, nada. 

The signal doesn't reach the threshold, and therefore none of the other parameters of the compressor are going to start working. But once that signal goes over your threshold it will get compressed. How much will it get compressed? Well that brings us to our next button.

  

Ratio

The ratio is where you determine how much compression you are going to apply to a signal that goes over your threshold. For every signal that goes over the threshold, it gets compressed according a certain ratio.

Example: For a compressor with a threshold at -10dB and a 3:1 ratio, a nice starting point for vocals. If you have a semi-constant level of the vocal at -1dB it will become compressed so that it only reaches -7dB.

Why?

Because after going over the threshold the vocal reaches its peak 9dB after -10dB, or at -1dB. We take those 9dB and divide them by three, since the ratio is at 3:1. Out of that we get 3dB which we add to the threshold at -10dB. A compression of 6dBs reaching its peak at -7dB. Let's illustrate this with a simple formula:

In this formula you can see the basics of calculating the output of a compressor.

If we take the example above and apply it to this formula, we get this:

So you see, that if we have a higher ratio, we compress the signal more resulting in less signal coming out of the output. 

Say we have an example of a loud kick drum that's peaking at +4dB but we have a threshold at -20dB and a ratio of 8:1. That's a lot of compression but serves to illustrate a point.

We have a dynamic range of 24 dBs, from -20dB to +4dB. We are compressing everything that goes over -20dB by a ratio of 8:1.

Let's plug those numbers into the equation:

The highest peaks of the kick drum that are reaching +4dB before are now only reaching -17dB! That 24dB dynamic range we had from -20dB to +4dB has been reduced to 3dB. Talk about over-compression!

After studying these formulas and basics behind the relationship between threshold and ratio, I think we can move on to the next phase of our compressor journey.  

The limiter

A limiter does things a little differently when it comes to the ratio and how it reacts to sounds that go over the threshold. Instead of compressing the peaks that go over the threshold, a limiter simply cuts them off. Which can actually sound better sometimes!

The knee

The knee on the audio compressor has a relationship with the ratio. It applies compression gradually increasing the ratio until a certain point. See the link below for a detailed explanation.

Attack & Release

Every plugin seems to have an attack and release of some sort. And they often don't even mean the same thing. Let's dive into how the attack and release on the audio compressor work.

Attack

The attack, measured in milliseconds, is how fast the compressor starts acting on a signal. With a fast attack the compressor starts working right away on the audio, often dulling the sound of the transient. 

What's a transient? Transients are the first seconds, or attack of the envelope of a signal. Huh? The  first peaks of a signal are called the transients ok? Drums have fast and loud transients !Whack! !Whack! But a cello might have a slower transient, or attack.

By using a fast attack you make the audio compressor chomps down right away on a signal, but with a slower attack time the initial attack, transient or punch gets through before the rest of the signal is compressed. 

In practice:

• For a punchier kick drum have a slower attack so you get the untreated sound of the beater. But if you want a thumpier and more rounded kick drum, have the attack at a fast setting.
• If you have a bad bass part and you get a lot of uneven notes jumping out at you, having a fast attack setting can help dull out the unexpected pops from the bass player. You can even put the ratio into limiting by having it at 10:1 or higher if you are dealing with a really troublesome part.

Release

In contrast, release is the parameter that determines, in milliseconds, how long the audio compressor will continue acting on a signal once it goes under the threshold again. If the release is too fast you run the risk of the compressor letting go too early, but if you have the release too slow you can get a pumping effect. This pumping effect is a clear sign of over-compression because the compressor never stops compressing. It is too slow too react, even after the signal has gone under the threshold. 

Thus the compressor compresses the next signal too, even though it is under the set threshold. This can work on some instruments with a slow transient response if you want an even compression..

In practice:

• Try to make your snare drum breathe by making the release go in time with the track. You can watch the time of the release in the gain reduction meter.
  
• A quick release on the kick drum is good since the signal of a kick drum is so short. That way you can be sure that it doesn't continue into the next kick drum hit.
  

Now that we've covered the basics of the attack and release, let's make our way to the most important tool you have when working these aforementioned parameters.

Gain reduction meter

This is the most important visual meter you have when compressing. It shows you, in dB how much you are really compressing. With it you are able to gauge the effect you are making, both by making sure that your signal is reaching the threshold and also seeing how fast the attack and release are working.

If you don't have the threshold low enough you won't see it working at all, since the audio compressor isn't compressing your signal.

If you have your release too slow you can be sure to see how the meter never really goes down. By taking a visual cue from the GR meter you can tweak the release in time with the track.

Obviously it's good practice to use your ears when compressing, but being able to see the amount of reduction and compression visually is a very effective way in quickly finding the correct sound you are looking for. If you only want to control the peaks, it's easier to see on the GR meter when it is only compressing the peaks instead of trying to listen to the actual audio signal.

Makeup-Gain

Lastly, makeup gain is that last parameter we need to worry about. Since an audio compressor turns down the volume of certain parts of our audio signal, we need a gain knob to increase the average volume to where it was before we started compressing. If we are always compressing around 3dBs we will need to turn the gain up 3dB in order to make up the loss in volume by compressing.

Reference: Gibson, Bill.(2007). Instrument & Vocal Recording. Hal Leonard Books.

Buss compression

 

You can crank up the sound of your drums by using buss compression in your tracks. 

By using parallel compression underneath the dynamic drum tracks you can create a larger than life sound.

 

 

 

 

 

Multiband compression

Multiband compression is used for many purposes, and can be a handy tool for mastering your tracks. By using multiband compression you can define the specific frequency areas you want to compress, using different compression values on separate areas of the frequency spectrum. 

Conclusion

Now that we've gone through the knobs you can find on your typical audio compressor you are all set to start compressing. If you need further tips on compressing, or want to share your cool compression tips, please share it with the rest of us here below. Everybody is always looking to enhance their understanding of compression.

Source: http://www.audio-production-tips.com