When it comes to mixing music, compression is one of the most powerful elements that you have at your disposal. It can be the difference between an okay mix and a great mix. If you plan on creating and mixing music, knowing how to properly use a compressor is a crucial skill that will make it easier to navigate the vast field of mixing.
In this article we’re going to go over what a compressor is, the basics of a compressor, how to use it, and when to use it.
What is Compression?
Compression is an audio processing tool that allows us to control the dynamic range (the difference between the loudest and the quietest moments in an audio signal) by reducing its level once it rises above a specified threshold.
The result of compression is a consistent piece of audio where both the loudest parts and the quietest parts can be heard clearly. For example, a guitar player might start off playing softly and then later in the song play more aggressively as he gets more excited. If the performance is left unaffected, the result would be a guitar performance with a great deal of dynamic range that would painfully stick out in the mix. Compression is used to combat this and produce a performance that stays at a consistent level.
Compressor Basics with Fruity Limiter
It can be pretty intimidating when you first load up a compressor and you’re bombarded by the numerous knobs and controls, it certainly was for me. First, let’s look at the basic controls found in most compressors, how they function, and what they actually do.
For the illustration I’ll be using Fruity Limiter because of its great visual feedback and simple interface.
First you will have to turn Fruity Limiter from the limit mode to the compressor mode. Do this by clicking on the word ‘COMP’ located on the bottom left of the plug-in. (Highlighted in red below.)
Now it will look like this.
In a compressor, the threshold sets the signal level where your compressor will start working. By setting the threshold you are telling the compressor to start working on a signal once it gets past a certain level. If I set the threshold to -18 dB, it means that the compressor will only act on the part of the audio signal that is over -18 dB.
The ratio determines how much gain reduction the compressor applies when the signal goes beyond the threshold. It is usually written as a ratio, for example, 2:1, 4:1, 8:1, 20:1. 2:1 is termed as gentle compression while 20:1 is very aggressive, but both have their specific uses.
At a 1:1 ratio, there is no compression.
At a 2:1 ratio, for every 2 dB of input signal over the threshold, the compressed output is 1 dB.
For example, let’s say we have the ratio set to 2:1 and the threshold at -20 db. A signal comes in at -10 dB which is over the threshold by 10 dB. Remember, the compressor only acts on the part of the signal that is over the threshold, so it will only act on the 10 dB. With a 2:1 ratio acting on the 10 dB, it will be compressed down to 5 dB.
Alternatively, let’s say the ratio was 4:1, the threshold still at -20 dB, and the signal still coming in at -10 dB. The compressor on a 4:1 ratio would act on the part of the signal over the threshold which is 10 dB on a 4:1 ratio and compress it down to 2.5 dB.
To understand the ratio better you can also write it as a fraction to understand its effects on the signal. Turn the ratio into a fraction by using the first number as the denominator (bottom number) and the second number as the numerator (top number). For a 2:1 ratio, its fraction is ½ meaning that a 2:1 ratio will halve the volume of the signal.
The reduction of the gain in the audio signal by the compressor is called attenuation.
Lower ratios (2:1, 4:1) apply low to medium amounts of compression and sound more natural.
Higher ratios (8:1, 10:1, 20:1) apply aggressive compression. A compressor with a ratio of 20:1 or higher is referred to as a limiter. This is because they attenuate most of the signal that goes past the threshold. Brickwall limiters have a ratio of ꝏ:1 (infinite to one) and do not allow any part of the audio signal to go over the threshold. Limiters are great for mastering and keeping in check signals that are very dynamic.
Attack determines how fast the compressor starts to work once the signal exceeds the threshold. It is usually written in milliseconds (ms). The longer the attack time, the more time the compressor takes before acting on the input signal once it goes over the threshold. The shorter the attack time, the faster the compressor acts on the input signal once it goes over the threshold.
When setting attack times, you should ask yourself if you want the compressor to react immediately or wait for some milliseconds before it acts on the signal.
A fast attack is from 8 milliseconds and below. Fast attack times are good for taming the initial transients that are poking out, creating a more balanced performance, and achieving a smoother sound.
Slow attack times are between 25 and 100 milliseconds. These react slowly hence retain the initial impact (transients) of the sound giving it a punchier feel. Due to this they work well on drums and transient heavy elements as they allow the transients to pass through.
The release determines how quickly a compressor lets go, or how fast compression stops once the signal drops below the threshold. To get the best release time ask yourself once the compressor grabs on, should it let go immediately or gradually?
A fast release is 100 ms or less. With these times, the compressor will stop compressing quickly after it’s started. Fast release times are more aggressive and create a gritty sound which can be the sound you’re looking for, but be careful as they can create artifacts or a pumping effect.
Medium release is between 100 ms and 400 ms.
A slow release is anything past 400 ms. Slow release times are great for smoothening out the dynamics of a mix or performance. They can also be used to send an instrument back in the mix and make it sound further away from the listener.
The knee setting in Fruity Limiter sets the transition between none and full compression. A soft knee sets the compression ratio to increase gradually while a hard knee sets the ratio to increase rapidly.
A soft knee.
A hard knee.
Note: When the knee knob is turned to the left side it acts as an expander, meaning instead of reducing the gain of the signal over the threshold, it will actually increase it. This is called upwards compression.
This is also referred to as the make-up gain. When you compress a signal, it reduces its volume. Make up gain allows us to bring up the gain of the compressed signal and match it to the gain of the original uncompressed signal so that we can properly judge the results of the compression. This is important because our ears trick us into believing that louder is better which can lead us to make biased decisions.
The sustain sets the duration over which the input signal is compressed. This controls the compressor sustain time and stops it from releasing too early. It is mostly found in FL Studio’s stock compressors.
FL Studio comes with a number of compressors, e.g., Fruity Limiter, Fruity Compressor, Fruity Multiband Compressor, Maximus and Transient Processor for dynamic control. To really understand how to use these compressors, practice works best. Don’t be afraid to load one up on your mixer track and experiment with different settings until it sounds just right. It is also important to note that compressors are not the same. Some may have fewer controls, some may have a lot, some may sound transparent and clear while others may have some coloring to them, but all have their uses. Try as many as you can until you find one that fits your needs. As always, have fun experimenting!