Compressors don't make things louder; they turn things down. All Compression is at its purest form, is volume reduction.
Compressors can have other uses, such as imparting colour and shaping tone, which I will show later. But for now, let's focus on their primary use.
Compression is the act of reducing the volume of a signal. In essence, it's a fast volume fader.
The difference between the loudest and quietest sections in a sound is the dynamic range. A compressor reduces the dynamic range of a signal. It does this by turning down the loudest parts, bringing them closer in volume to the quieter passages.
A compressor never makes a sound louder by increasing the gain. It only increases volume as a by-product of reducing the dynamic range. By reducing the dynamic range of a sound, we're reducing the loud peaks in comparison to the quieter passages. This allows you to then raise the average level of the sound, allowing for a higher perceived loudness before clipping occurs.
A compressor only reduces volume by turning down the sections that are too loud.
If a compressor reduces the level of a signal, by turning it down when it gets too loud — what defines 'too loud'? That would be the threshold control.
The threshold is the level. The level at which the compressor begins to reduce the volume.
The threshold is measured in decibels, abbreviated dB.
Another helpful aid you can use is phrasing the control as though it's asking a question.
The threshold control is asking you: at what decibel in the signal, should the compression begin?
That's the only choice you need to make. At what decibel level in the signal, should the compressor begin to reduce the volume?
With the threshold control, you're telling the compressor 'Hey, when the signal reaches this volume, turn it down.'
Here's a kick drum:
It's peaking at 0 decibels, or thereabouts.
Let's adjust the threshold to -4dB, which means that the volume reduction will begin when the signal goes over -4 decibels. The compressor will only apply the reduction to the part of the signal that is above -4 decibels.
So, to summarise, the threshold tells the compressor when it needs to reduce the volume, but by how much should it turn the signal down by?
This is where the ratio control comes in.
The ratio control is the amount.
The ratio determines the amount of compression applied to a signal. You can think of it as how the compressor responds to a sound. The higher the ratio, the more gain reduction you apply.
You're telling the compressor how many decibels to turn the signal down by, once it reaches the threshold.
The ratio control is asking, what amount of compression would you like applied to the signal? A light kiss? Or something more severe?
The settings on the control are mathematical ratios between two numbers.
Here's how you could think of them.
To work out the compression amount for each ratio, take the second number, off the first. So, with a ratio of 4:1, we have 3 decibels of reduction for every 4 decibels the signal rises above the threshold. As four minus one equals three.
Then to work out the output level. Divide your input signal level by the first number of a compressor ratio to determine how much signal will be output.
You could think of a 4:1 ratio, as the compressor eating 3 decibels of the signal, while leaving a single decibel of output level. The compressor is reducing the volume by 3 decibels, for every four that surpass the threshold.
These numbers are an average of how compressors will respond to a signal. Use them to guide your decisions, but don't expect the readouts to be mathematically accurate.
Examples of ratios include 4:1, 8:1, 20:1 thru to infinity:1, which is limiting. 4:1 is a medium compression, 8:1 strong, and anything above is severe — as you're reducing the signal by seven or more decibels.
The threshold and ratio controls are best used with the gain reduction meter to determine the amount of compression you're applying.
Let's keep the 4:1 ratio example, and set Neutron to that setting.
As per the explanation, there are 4 decibels of signal crossing the threshold. Two to three decibels of compression taking place and the output signal has fallen by 3 decibels, or thereabouts.
So, in summary, the threshold tells the compressor when to reduce the signal, and the ratio determines how much to reduce the signal by.
Next, let’s decide at what speed the compressor should respond to the incoming sound.
This is what the next pairing of controls are for, the attack and release.
Attack & Release
The attack and release controls are both the speed.
The attack is the beginning speed.
The release is the ending speed.
The attack is the speed in which the compressor goes from no compression to full compression.
The release is the opposite. The time it takes the compressor to go from full compression, back to the original signal.
Together they form the timing with which the compressor's gain reduction begins (attack) and ends (release).
The attack control is asking you, at what speed would you like the compressor to go from no compression to full compression? How should it react to the signal? Do you want it to be instant, compressing as soon as the sound crosses the threshold? Or do you want it to take its time, giving the transient of the signal some breathing room?
The release control is the ending speed and is asking the opposite. At what speed should the compressor go from full compression, back to the uncompressed signal? Do you want the compressor to return to the original signal instantly, or should it take its time, allowing for a more natural sound?
But enough talk, it's showtime.
Let's now have a look at how you can use these controls to shape a sound.
First, let's address the most common pitfall of compression.
Every time you load up a compressor, you need to ask yourself a bunch of questions.
Why are you reaching for a compressor? Just out of habit? If so, you need to stop it.
Ask yourself why you're using a compressor and what sound you're after.
Answering these can be hard initially, so here's one way you could approach compressing a kick drum.
The ratio and threshold controls determine the amount of compression. The amount you should apply is subjective to the material. There's no one size fits all preset. However, through experimentation, you can find the settings that sound best to your ears.
The gain reduction meter displays the amount of reduction happening, which you should keep your eye on while setting each control. But of course, use your ears first.
The attack and release have the most significant impact on the sound of a compressor.
Unless you wish to crush the transient of a drum, use a slow attack. Here's how a fast attack sounds:
Then, a slow attack:
Which sounds better?
With a fast attack, we're just reducing the level of the transient, like so:
These are the two extremes, but finding a balance is where we can get creative.
With a fast attack and around 20 decibels of gain reduction, we can see that the transient has gone, leaving us with more body than punch. You can start shaping the sound by deciding how much transient to body ratio you'd like.
Increasing the attack lets more of the transient through. You can hear this as the punch of the kick. The orange line here is representing the reduction thats starting just after the transient.
Pulling back the release lets the compressor return to the uncompressed signal quicker, bringing up the body of the kick. This gives you a 'boomy' sound. You can see that the orange line up here is displaying how the release is shaping the sound. When combined with a long attack, the compressor isn't starting to apply compression, until after the transient has passed.
Generally, this is the best approach for drums, as keeping the transient loud is what provides the punch.
For material such as vocals, pianos, guitars and other transient heavy sounds, you may wish to use a fast attack to catch those peaks and smooth out the performance.
Let’s copy over the kick drum compression settings to this piano:
It's not controlling these transients because of the slow attack. The volume reduction begins just after the initial peak here, which doesn't smooth out the performance.
Pulling back the attack to catch those transients provides for a more controlled compression style. But, it also dulls the sound, pushing the piano back in the mix. So, somewhere in-between could be a nice balance.
In this case, the release should be pulling back between hits or resetting back to no compression. Aiming for as long release as possible, while still dropping back between the peaks is a safe option.
These are but a few ways to use a compressor to shape a sound's tone, rather than only controlling the volume. It would be best if you practised compressing sounds to develop your understanding and taste further.