Dynamics & Compression: An Overview

Posted by James Cullen on

There are many plug ins available to us as music producers, but none is so ubiquitous as the Compressor. It's used on pretty much everything (at least it can be!) and you'll find it across the entire genre spectrum of music production. A recording engineer in a huge elaborate hollywood studio will use a compressor, and so will a bedroom producer. It's an essential tool in the music producer's virtual (or hardware) toolbox, but it's also misunderstood and often misused.

When used correctly, Compression can add depth and punch to your mix. It can drastically improve the clarity of both your overall mix and individual sounds, and when used on the individual elements of your tracks, it can help them be more defined and identifiable in the wider context of your mix. Beyond this, there are also more advanced methods of deploying and using Compressors, to take your production up to another level.

Before we dive into some specifics, let’s remind ourselves how Compressors work.

 

The controls and parameters of a Compressor.

There are standard parameters on most Compressors, though some may have their own unique ones thrown in. But here are the basics.

 

Threshold - This is the level that the Compressor kicks in. Anything below the threshold value is not affected by the Compressor, anything that falls above it is Compressed or reduced in level. This value is measured in decibels (dB).

Ratio - This control decides the amount of gain reduction applied to the audio signal which comes in above the Threshold value. Ratio is measured as X:1, where x is the amount of increase to the input level required to generate a 1dB increase in output. So a 4:1 Ratio would mean a 4dB increase in input level is required to generate a 1dB increase in output for above threshold signals.

Attack - This parameter controls how quickly the Compressor kicks in when the input signal comes in above the Threshold value. Attack is usually measured in milliseconds (ms). The higher the value, the more of any early transients will be preserved. This works by delaying the onset of the Compressor. A good way to remember this is to think of it as reaction time.

Release - This is the amount of time the Compressor takes to return to 0dB of gain reduction once the audio signal has returned to a value below that of the Threshold. Think of this as return time.

Make Up Gain - A Compressor fundamentally is a tool which reduces the gain or level of an audio signal, bring down the levels of the peaks. This level needs to be injected back in to the signal, or ‘made up’ to get things back to their original level. The overall effect this has is that the peaks remain the same volume while the quieter sounds are raised in volume.

 

So, those are the standard controls that every Compressor has. As I mentioned before, some may have controls unique to that specific plug in or hardware unit, but they’re only worth knowing about if you own that piece of kit! 

 

Experimenting with Attack and Release

When you’re using a Compressor, there can be two different scenarios in which you may find yourself.

  • You may want to alter the shape or envelope of a sound, or

 

  • You may want to adjust the dynamic range of an audio signal. (This is useful if an audio signal has lots of difference between the loudest and lowest volumes.)

Now, to hear the effects of a Compressor, let’s do an exercise. Open up Ableton (or your DAW of choice) and create an Audio track. Stick a Compressor on, and find yourself some drum samples. Drum (or single hit) samples are great for demonstrating the effect of a Compressor, because the source sound is transient in nature, meaning it has a sharp beginning and very quickly trails off. So, even with a standard or preset Compression setting for a snare drum, adjusting the controls can get very different results. Let’s try it out.

 

Fast Attack and Release

Adjusting the Attack and Release Controls on a Compressor can make drastic differences on the signal it puts out. If you set the absolute fastest Attack and Release time your Compressor will allow, it will react extremely quickly when the input signal spikes above the Threshold level. This means that the transient of the snare drum is reduced in level almost immediately. Drums are by their nature a very sharp sound wave with a fast attack, so this Attack and Release combination changes that. As soon as the hit is over, the fast Release time restores the level to normal in a matter of milliseconds so that any decay in the waveform is brought back up to its original level. 

See below.

 

If you look carefully at the Waveforms above, you'll see that with some Make Up Gain, the drum Snare sound has lost some of its initial impact, but the sustain of the drum is increased. It's subtle, but you can see how this type of control set up affects the input signal. For reference, I've screen capped my Compressor settings below.



Fast Attack and Longer Releases

As you may have been able to figure out, the results are different if you adjust either of the Attack or Release controls. Let's see what happens if we keep a fast Attack, but lengthen the release time slightly. 

What happens in this instance is that the initial transient of the drum sound is reduced, but the decay or ring is also reduced by the same amount. You can see this in the waveforms below. Where the example above has the tail of the snare increasing in level, this example shows how the Compressor has reduced the gain. 

 

Here are my Compressor settings for reference.

 

Slower attack and moderate release.

Now, with the two examples above we've used a fast attack. This means the Compressor has kicked in almost instantly, reducing that initial transient of the snare drum. But what if we use a slower attack value and see how that affects the sound?

Using a slower attack, somewhere in the region of 10 - 20ms allows the transient of the drum hit to pass through the Compressor before it turns on and reduces the gain of the rest of the signal. You'll need to experiment with different attack times for different drum sounds, as you want to make sure the entire transient passes through before any gain reduction is applied. The overall effect this type of setting on a Compressor has is that it accentuates the initial hit, it allows it to be louder relative to the decay and tail of the drum sound. It's a really good way of making yoru drums sound more defined, but take care you don't reduce the attack time too much as this can potentially take the weight out of the sound! 

Above is the waveforms from this example, you can see how the peak of the drum hit is louder than the rest of the sound. Below is my Compressor settings once again for reference.

 

Auto Attack and Auto Release

In your journey through the various studios and DAWs you've worked with, you may have come across Compressors which have an 'Auto' option for the Attack and Release controls. You can see it below the Ratio / Attack / Sustain Controls in Ableton's Compressor pictured above. But what does this do?

Since adjusting the values of the Attack and Release can have such miniscule effects on the overall sound, doesn't it seem strange to set these to Auto? Well, this is where a Compressor's versatility really shines.

An Auto setting monitors the input signal level, and attack characteristics of the sound coming into the Compressor, and adjusts and modifies the timing settings to get the most out of the sound while applying the least obtrusive sounding processing. Essentially, it focuses on a performance which may be dynamic; the attack values or the transients of the sound may be constantly changing. Obviously you will still need to adjust the threshold and ratio, but the Attack and Release are covered.

This can be especially useful when applied to something such as an acoustic guitar part or a slap bass line for example, the attack and decay chatacyeristics of audio signals such as this are constantly changing, so you want to make sure you're adding that Compression without adjusting the character of the sound. There's nothing worse than hearing a performance have the life sucked out of it by over compression! 

 

Peak vs RMS

Many Compressors allow you the choice between Peak and RMS mode, where RMS is the average loudness of a sound, and Peak is the value of peaks of the soundwave regardless of their duration, which is important. Ableton's Compressor also contains an Expand function, meaning any signal above the Threshold is increased in volume instead of reduced!

These settings determine how the Compressor does its work on the incoming signal. In Peak mode, a compressor will be monitoring the absolute peak level of an input signal, regardless of whether this is a single hit or a sustained phrase. So a single click from something like a clave or a rimshot would trigger it.

However, if we want to get a bit nerdy, that's not actually how human ears perceive loudness. The length of a sound also plays a part in how our brain measures its loudness, so a longer duration sound appears louder than shorter ones, even if their peak level in actual decibels (dB) were the same.

So, this RMS setting makes the Compressor work similar to our ears, and takes an average or the incoming signal's loudness, meaning a more even gain reduction in line with human hearing will be applied. 

Generally speaking, Peak mode is best for short-duration percussive sounds like drums and percussion, while RMS or Average mode is better to stick on vocals or acoustic guitars. Basically non-percussive stuff!

 

What is a Multi Band Compressor?

Now that we've gone over a few of the basic functions of a Compressor and how it relates to Dynamics, as well as covering some of the different ways you can use it, I wanted to take a moment to explore what Multiband Compression is. You may have noticed in Ableton the effect is called Multiband Dynamics, and you may or may not have used it before. Let's take a look at what Multiband Dynamics or Compression is, and how you can use it! 

Multiband Compression was developed to tackle one of the big issues with a conventional Compressor applying its effect across the entire frequency spectrum of the incoming signal. If there was a quieter sound such as a hi hat that plays at the same time as a loud low end kick drum, you may find that using a traditional Compressor may also duck the hi hat's signal, and it would get lost in the mix.

Can you guess what Multiband Dynamics does yet?

That's right! Multiband dynamics allows you to apply different levels of Compression across different frequency spectrums, allowing you to fine tune your dynamic processing to make sure every part of the sound shines as it's supposed to.

The problem with regular Compressors can be very pronounced when applied to an entire master bus, for example, when a single loud event in the track could trigger the Compressor to pull down the entire mix. 

So, a Multiband compressor solves this problem by splitting the incoming signal into several frequency bands, in the case of Ableton's Multiband Dynamics, we have three bands. Each of these has its own controls for Threshold, Attack, Release, Output and more. This means you can Compress different parts of the frequency spectrum at different rates and levels. 

While these are often reserved for mixing and mastering, this can be a great tool for some specific jobs in making your music. Multiband dynamics is really great for bringing out the low end on 808 Subs, or making sure that your mix is balanced. You could stick a Multiband Dynamics plug in across your entire master bus and fine tune the various bands to make sure you're not losing any power across any areas of the mix. 

While approaching Compression in this way can seem daunting - it's confusing enough figuring out one Compressor, nevermind three separate bands - there are some great uses for this plug in, so make sure you experiment next time you feel you might need to pump up a certain element (or even reduce the volume of something) in your track. Most multiband compressors allow you to Solo each frequency band so you can get super specific and hear exactly what's happening to each section.

 

 

So, we've had a look at some in depth Compression topics, including the specific controls and types of Compression, as well as what Multiband dynamics are. In part 2 of this overview, we will look at some more in depth Dynamic processing. Now that we know what Compressors are and what they can do, it's time to think of some ways we can apply them in our music making.

 

Thanks for checking in with us here at Top Music Arts, and as always, make sure you check out the rest of our site for tonnes of useful production resources including Ableton Remakes and more! 


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