This article will be giving a detailed explanation of audio compression in music mixing. This tutorial will be applicable in any of the various DAWs whether; Pro tools, REAPER, Fl Studio, Ableton live, Logic, ProTools, Cubase etc. It does not matter as the process is all the same.
What is audio compression?
I once came across a very simple definition of compression as given by a hilariously wise guy on gearslutz. He said “a compressor is like an automatic volume control fairy which helps to even out your music’s volume levels”. Evening out of volume of an audio simply means lowering the loudest parts(peaks) and increasing the lowest parts. It should be noted however, that a compressor does not just start adjusting volumes at will but rather acts on some given information as set on its parameters by the user.
What audio compressors do
Music is a very dynamic art. Dynamism in this sense means the movement of the sound of music as it goes from high to low, fast paced to slow, and from steady to inconsistent. This can be best seen in the image representation of a musical waveform below.
We can see that some wave levels are higher and some are lower, while some other parts have fair consistence in levels. The spiking levels indicated by the red lines are called the “peaks”, while the fairly consistent parts marked by yellow lines is called the RMS (average range). The lowest parts of the waveform is marked with green. The difference between the highest parts and the lowest parts is called the “dynamic range”. This is illustrated with the blue arrows indicating the difference between the red lines and the green ones in the image. So, what compressors/limiters do is to even out the waveform by reducing the dynamic range in order to make the volume of a performance more consistent.
How Audio Compressors Work
Compressors do not just start adjusting volumes automatically on their own. They follow a set of given instructions as set on their parameters by the sound engineer. The basic parameters on a typical compressor are: threshold, ratio, attack, release, knee/texture, and output gain.
This control is used to set the limit of highest volume permitted to go through without being affected by the compressor. Compression threshold is the volume level at which compression is triggered. So, any sound signal trying to surpass or go over the set threshold is pushed back down (compressed) according to the given ratio and the rest.
Based on the basic mathematical knowledge, a set ratio instructs the compressor how much volume reduction is applied to a sound source when it crosses the threshold. For example; a ratio 4:1 means whenever the volume of a sound crosses the threshold by 4db, only 1db output will be allowed. Ratio can be better understood as either a multiplier effect, or divisional effect. Don’t be confused. By multiples it means in our above example – when the volume goes over the set threshold two times the given ratio which is 4db to 1db which equals 8db, then only 2db will be allowed. If it goes 3 x 4db which = 12db only 3db will be allowed. This same rule goes for whichever ratio you select ( 2:1, 4:1, 10:1,….)
Still using our above example as a case study, ratio as a divisional effect means – even when a sound source is not going over a given threshold as much as hitting the 4db mark in the ratio 4:1, that doesn’t mean the compressor won’t work still. It doesn’t stay idle until it hits 4db, then only allow 1db through, or till it doubles to 8db to let 2db through…. No! It is still working in a divisional mathematical way like fraction or decimals. Still on our ratio 4:1 example – when the sound volume is about 2db above the threshold, the compressor halves the 4db and only lets a ½db or .5 db through. When it goes 3.5db or 7.8db over, the compressor does the maths. Hope I have not ended up confusing you more than I helped. Just calm down and read over again to understand. This can be better understood as seen on the ratio controls of some plugin compressors that do not have the static ratio figures 1:1 – 2:1 – 4:1 – 6:1 – 10:1 …..etc. , rather they have a knob or slider control that goes all the way as can be seen in the example below.
This controls how fast or slow a compressor jumps in to grip down the volume peaking above the threshold. The value is often calculated in milliseconds and seconds. A fast attack will kick in immediately to compress the signal right out at the beginning of the sounds(transients), while a slow attack will let some the transients through before compressing rest of the signal body or tail.
This setting determines for how long or short (also in milliseconds and seconds) a signal is compressed before being freed from the compressor’s grasp. The relationship between attack and release cannot be overemphasized in the role of using compressors to give a sound or music some certain textures or characters like smooth, pumping, energetic, relaxed, snappy, e.t.c. More reason why compressors ate tagged dynamic processors.
After the whole gain reduction from compressing a sound, it oftentimes lead to a lower sound going out of the compressor than it came in. So the output gain control is raised to push the audio signal back to how loud it was before compression. This is best set by toggling the effect on and off while listening to difference in volume levels. Be careful not to raise the output too much as most times louder tend to sound better to the ears, and that might affect the volume balance of your mix. Except of course it works well for your particular project without marring things.
That is about all the knowledge you will need to understand and effectively use audio compressors. Though some compressors might have further fine tuning parameters like knee (how hard or soft a signal is processed), look ahead, and side chain functions, you can always learn about these additional features as you get more used to compressors. Go ahead and practice more with these tips, I bet you’ll be a pro compressor user in no time. Have fun!