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Compression Explained: An Overview of Standard, Multiband, and Parallel Compression

Compression is one of the most often misunderstood and misused techniques in home studios around the world. It took me quite awhile to truly understand how compression worked and, more importantly, how to implement it properly.  Compression becomes more complex when you factor in different types of compression such as multiband and parallel. But fear not! This blog post will break down how to use compression in simple terms, and you’ll be able to use these techniques right away!

Anatomy of a Compressor

So what is compression anyway?  Compression reduces the dynamic range of a signal.  In other words, it takes the loud parts of a track and turns them down while taking the quieter parts and turning them up.  The result is a track that consistently maintains its place in the mix; it doesn’t get buried beneath the other tracks and doesn’t overwhelm them.  The key to using compression well is getting that consistency in the signal while still maintaining a sense of dynamics.  Now that you know what compression is, let’s find out how a compressor actually works.

Part of what makes compression confusing for many of us is that not all compressors are the same, even if they’re fundamentally doing the same work. When it comes to standard compression, generally speaking there are two main types of compressors:  variable threshold compressors and fixed threshold compressors.  These compressors ultimately do the same thing — reduce the dynamic range of a signal — but they go about it in different ways.  To explain this, let’s first go over what threshold is.

Threshold simply refers to the point at which a compressor starts to work.  For example, if you set the threshold of a compressor to -6db, when a signal exceeds -6db the compressor will start to, well, compress.  Variable threshold compressors are exactly what they sound like — compressors that allow you to vary the threshold.  If you are looking at a hardware or plugin compressor that has a threshold knob/setting, you know you are working with a variable threshold compressor.

As you might have guessed, not all compressors have a threshold knob/setting.  These are, of course, fixed threshold compressors.  The threshold is set a fixed point automatically and cannot be changed.  So how do you get this sort of compressor to work if you can’t set the threshold yourself?  Simple - you turn up the gain of the signal until it exceeds the compressors threshold, at which point it will begin to compress.

Now if all of that feels a bit confusing, don’t worry.  Let’s take a look at some of the other settings on a compressor and put it in simpler terms.

Threshold: We have already learned that the threshold is the point at which a compressor starts to work.  Think of the threshold like this - if you’re listening to music next to someone, the point at which they reach over at turn it down because it’s too loud is the threshold.

Ratio:  The ratio is another important feature of a compressor.  Ratios are typically listed as 1:1, 2:1, 3:1, and so on, which while that seems confusing, it’s really very simple.  If the threshold tells the compressor when to turn down the signal, the ratio tells the compressor how much to turn it down.  Let’s take the previous example of a threshold at -6db, and we’ll say that we have set the ratio to be 3:1.  For every time the signal exceeds -6db by the ratio, the compressor will only allow 1db to go through.  So if the signal exceeded the threshold by 3db, you’ll only hear it as 1db over the threshold.  If it exceeds the threshold by 6db, you’ll hear it as 2db over the threshold and so on.  The ratio simply tells the compressor “if this signal exceeds the threshold by [whatever the first number in the ratio is], only let 1db through.”

Attack:  The Attack setting refers to how fast the compressor actually works.  Keeping with our example so far, the attack refers to how quickly the compressor reacts to the signal exceeding the threshold; it determines how quickly the compressor reduces the signal by whatever ratio is set. A fast attack will compress the audio very quickly as it exceeds the threshold whereas a slow attack will allow more fo the signal to continue exceeding the threshold before reacting.

Release:  Release refers to how quickly the compressor stops working; It determines how quickly the compressor releases the signal after compressing it.  Just like with attack, a fast release will stop compressing the signal sooner whereas a slow release will “hang on” to the signal for longer.

Output:  The output (sometimes called make-up gain) is exactly what it sound like: it increases the level of the now compressed signal.  Since compression is reducing the dynamic range of a signal, the overall signal gets quieter as a result. To get the level back to where it needs to be in the context of the mix, the output or make-up gain knob adds volume to the signal.

These are the main settings that you’ll find on most compressors.  There are a few additional settings such as “knee,” which refers to how “hard” or “soft” the compression is, but by and large, these are the settings you’ll find on most hardware and plugin compressors. 

Standard Compression Overview

We have established that compression reduces the dynamic range of a signal — making quieter parts louder and louder parts quieter — so how do we use this to our advantage? Obviously how you implement compression will depend not only on what you’re compressing (vocals, drums, etc.), but it will further vary from song to song.  Fortunately, there are some things to keep in mind that can help you with your decision making when it comes to compression.

  1. The goal of compression (generally speaking) is to generate a signal that sits in the mix properly without drastic differences in volume. There are times where you may want to use compression in more aggressive or creative ways, of course, but by and large, this is the basic purpose of using a compressor.
  2. While you want a more even and uniform signal, you don’t want to lose the sense of dynamics.  When a signal no longer has any dynamics at all, this is typically considered “squashing” or “over compression.” As you’ll see further down, there are times where you may want this effect. But for standard, vanilla compression, you want to maintain a sense of dynamics in your signal.
  3. Most compressors have subtle nuances between them with regards to how they behave in relation to a signal. Some compressors are more aggressive or impart some sort of “color” onto a signal.  Other compressors can be pushed hard without coloring the signal.
  4. Especially in the case of aggressive compressors, stacking compressors can give you better results.  By stacking 2-3 compressors that are each doing smaller amounts of compression and gain reduction, you’re able to compress a signal by a larger amount without sucking he life out of it because the compressor is too aggressive.

Multiband Compression Overview

Multiband compression is an incredibly useful tool that, once understood, can change the way you mix. Multiband compressors isolate certain areas of the frequency spectrum (bands) — not unlike an EQ — and only applies compression to that area.  This means that you can apply different amounts of compression to the high end, mid range, and low end or only compress one of those areas.  It allows you to apply specific compression to certain areas of a signal without compromising the entire track. 

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This is an example of a multi band compressor plugin. As you can see, these bands isolate certain areas of the frequency spectrum and apply different amounts of compression to each of those bands.  Most multiband compressors allow you to move the bands wherever you like. Outside of that, the same principles we learned with regards to basic compression apply.

Parallel Compression

Parallel compression is another awesome tool that when understood can instantly improve your mixes.  Fortunately, parallel compression is pretty easy to understand and implement. Simply put, parallel compression is the process of blending an uncompressed or lightly compressed signal with a heavily compressed version of that same signal.  Take for instance your drum bus. You can apply some light compression to the drum bus, duplicate the bus and heavily compress it, and then blend those sounds together.  Experimenting with this process can lead to some interesting sounds and will help certain tracks carve out their place in the mix, especially with vocals and drums.

So there you have it — an overview of compression.  Hopefully we’ve helped de-mystify an incredible important and often misunderstood tool and technique in audio production. Good luck with your recording and mixing!

Gabriel Hawkins