Compression is a crucial tool in any mixing engineer's kit. It reduces the dynamic range of an audio signal by attenuating the loudest transients and boosting the overall sound.
The loudest and quietest parts are now closer in volume, giving a glued together or squashed effect. Done well, this gives the track a more natural sound.
The Different Types of Compression
There are a number of different types of compression and in this article we are going to look at the five most common.
Each different type of compressor has its own unique characteristics that are worth noting so that you may apply them best to your music.
First up is VCA compression, which stands for Voltage Controlled Amplifier. This analog compressor amplifies and attenuates a signal by using a VCA, hence the name.
Known for its fast response times, it's also very ‘clean'. It doesn't ever distort the signal so its transparent nature makes it very popular on a master bus.
When you've got a busy mix with a lot of transients that need reining in, this compressor is best in class.
Because it often lacks clarity on the high end it's common to also use it in conjunction with a high pass filter.
Probably the best known VCA compressor is the SSL-G which was famously included on every channel of the legendary Solid State Logic mixing desks that dominated popular music recording studios in the '80s.
Each channel had a compressor and noise gate built in so as not to impede the creative process with unnecessary patching of outboard gear.
Thankfully the SSL-G is widely available in affordable digital format courtesy of Waves.
Tube Compression (Vari-Mu Compression)
Although many compressors use tubes in their design, such as for amplification for make-up gain, a a tube compressor specifically uses a vacuum tube to attenuate the signal. Mu is the technical term for gain, so vari-mu means variable gain compression.
The more signal that is sent to the tube, the more gain reduction occurs. The nature of this exchange is why these compressors often don't have a ratio setting.
Vari-mu compressors have a colourful sound with slow attack and release times. High transients are smoothly tamed by this gluey compression and therefore it sounds great on a mix bus or for overhead mics on a drum kit. It is also very popular with mastering engineers.
An example of a popular variable-mu compressor is the Audified U73b, an old German broadcast compressor, used initially to even out peaks and transients on radio transmissions.
This compressor gets its name due to the Field Effect Transistors it employs to recreate the tube sound of a variable-mu compressor. They are very popular in the rock genre due to their fast and punchy response.
Due to the warm colouration they offer they are often used to bring life to a drum kit. The harmonic distortion that they offer means that they're also popularly applied to guitar tracks.
They are program dependent: that is the incoming signal dictates the amount of compression. Due to this fact many FET compressors do not have a threshold function.
A notable example of a FET compressor is the classic staple of vintage compressors, the Fairchild FG-116.
An optical compressor turns the incoming signal into light and uses a light element and an optical cell to attenuate a signal.
Due to the warm-up time and afterglow effect from the light element they are known to instil unexpected characteristics onto a track. These non-linearities make it popular with bass, guitars and vocal compression.
Overall they are known for their slow attack and release times, transparent sound, and low distortion. The Universal Audio Teletronix LA-2A is one of the most commonly known opto-compressors, and has been popular since its creation in the 1960s.
A worthy digital emulator of opto-compression is the Custom Opto by Slate Digital, which is modelled after some of the well most units of their time.
Digital compressors are either computer-based recreations of their analog counterparts, or else designed to offer extremely precise and transparent compression.
Due to the fact that they are not bound by the constraints of physics or encumbered by hardware limitations, they can often offer settings you wouldn't see together.
For instance, you might see an FET compressor with a threshold function, or a vari-mu emulator with a ratio setting. In addition to this they often have EQ parameters, sidechain, and tube amplification.
Due to their versatility and high level of accuracy they can be used on anything, be it individual tracks, busses, and even mastering.
How Do I Choose The Right Compressor Style?
In order to choose the right compressor you need to consider what kind of compression settings you require.
Do you have a vocal performance with a lot of dynamic range that needs reining in? Do you require heavy compression to crush your drum bus or beef up a snare drum? Is your bass guitar in need of livening up via some colouration?
If you don't want any colouration or harmonic distortion applied to the compressed signal then a VCA compressor or an opto compressor are your best bet.
There are times where transparency are needed, such as in mixing or mastering, where you just want to tame transients without adding any additional colour or warmth,.
If you want something with a slow attack and release time that will compress your signal without being overly noticeable then opto and vari-mu compressors are best for this job.
If you wish to inject some life into an underwhelming bass, or add harmonic distortion and flavour when mastering a track then the transformers in an FET compressor or a vari-mu compressor will aid you.
(Note: transparency is a personal choice. Therefore when mastering a track you must decide whether to use a transparent or colourful compressor).
Fast attack/release time
If you have many fast transients, such as on an upbeat drum track or a crowded mix then you'll require something with the power to act fast and decisively.
For this you'll require an FET or VCA compressor.
Getting The Right Compression Settings For Your Mix
Compression is a vital part of the mixing process and therefore understanding the different parameters is key to unlocking its full potential.
Train your ear to listen out for what each function does so you can dial it in tastefully to enhance your track without colouring it too much.
The threshold dictates at what dB your compressor begins to kick in. You are dictating how much of the signal to attenuate. A lower threshold will compress a larger portion of the signal, whereas a high threshold will only trim the most aggressive peaks and transients.
The ratio is how much of the signal that passes the threshold gets compressed. For instance a ratio of 1:1 means an uncompressed signal, whereas 2:1 means that if 4dB passes the threshold then compression will be applied to 2dB. If 8dB passes the threshold then 4dB is compressed, and so on.
Vocal compression settings are recommended to be between 4:1 and 8:1, with a higher ratio for rock and pop vocals. A lower ratio if the mix is quite sparse and there's room for more dynamics.
Often described as how smoothly a compressor operates, this parameter isn't always available to dial in. Sometimes the compression settings include a pre-determined knee.
Essentially, a hard knee means the ratio is applied the instant the signal passes the threshold. A soft knee applies a little compression beforehand to ease the signal into the gain reduction.
Attack and Release
Attack controls how fast the compression takes place. One common way of describing it is to picture a hand reaching for the volume dial any time the signal passes the threshold. The attack is the speed at which this occurs in milliseconds.
The release time on the other hand is the time it takes for the hand to return the volume to where it was.
If applying compression to a mix bus then you will want a slow attack time and a fast release time. That way the peaks and transients are gently pushed down and for not too long, giving it a glued together feel that doesn't sound unnatural.
The definition of stereo is when you have two speakers and each speaker has its own dedicated channel – left and right. The act of panning is simply making something louder in one of these channels to give the illusion of it originating from that direction and thus a stereo field.
For instance, when a tom is panned to the left you are simply reducing its volume in the right channel while boosting it in the left.
Now, if you are compressing the hell out of the tom it's going to end up the same volume in both channels. Your ears will no longer see it originating from the left, but actually dead centre. You'll often hear this referred to as “drifting to the centre”.
Stereo linking is when a stereo compressor or dual mono compressors are applied to both channels at once. This reduces the amount of drift that's occurring by attenuating both channels equally.
Compressors should be stereo linked if there is any instance of loud, 0ff-centre sounds where a non-linked stereo compressor might only attenuate one side of the stereo field and cause the stereo image to shift. Often you will have the option to dial it in so the effect is gently applied.
Advanced Compression Settings
Ok, now that we've mastered the basics let's take a look at some novel ways in which you can use compression to enhance your mix. Although this section is labelled ‘advanced', you'll find they are quite easy to understand.
Parallel Compression (Great Compression Setting for Drums)
Parallel compression or ‘New York compression' as it's often known is actually quite a straightforward technique.
This involves setting up a send for your drum mix to go to that's quite heavily compressed and dialling it in so that you get a blend of both dry and wet signals.
The aim is to retain the dynamics of your dry mix, while adding in some flavour from the wet mix. This works very well on glueing together a drum mix or on lead vocals, where you preserve your dynamics while giving it more body and presence.
Here is a brief example of parallel compression to give you an idea of how it sounds:
This type of compression involves having a chain of two or more compressors on a channel in order to perform different compression tasks that one alone could not do.
For instance, you may want to apply 10dB in gain reduction. By using two compressors attenuating 5dB each you get the same level of gain reduction with much more transparent results.
In mastering it's common to have multiple compressors to tame different types of transients. Your first compressor might be fast-acting with a high threshold in order to rein in the most fleeting transients such as hi-hats and cymbals.
This creates a nice glued together signal for the next compressor which might be slower and have a lower threshold for the guitar and vocal transients.
This stands for ‘Attack, Release, Ration and Threshold', the concept first appeared in Mixing With Your Mind by Michael Paul Stavrou. The idea is to start off with very extreme settings that are almost unpalatable, and then dialling them back to within an acceptable range.
The theory is that by going too far with the settings and pulling them back, your ear can more easily identify the sweet spot.
The Compression Settings Instrument Quick List
Here we're going to touch on some ballpark settings for your various instruments in order to get you started.
Vocal Compression Settings
Let's kick things off with a vocal compression cheat sheet. It's always good to start with a ratio of between 4:1 and 8:1 for lead vocals, especially on the higher end for rock vocals.
Anything lower should be reserved for quite sparse productions where all of the dynamics of the vocal performance can be appreciated.
Then identify the quietest part of your vocal range and set the threshold to that. For instance, if the lowest volume is -10dB then adjust the threshold accordingly to set the floor of the range.
Adjust your make-up gain (also known as ‘output gain') to match the level of attenuation being applied by the compressor. If your compressor is reducing the gain by 4dB then boost it by the same amount.
Compression Settings for Bass
A ratio between 3:1 and 5:1 is preferred with bass, with soft-knee compression if the option is available. Allow for a relatively fast attack time, perhaps 2ms so as to allow the initial hit of the note to come through.
For a natural sound you should apply a fast release time of around 100ms, unless you want to go for a squashed sound – then a faster release time is ok.
Compression Settings for Guitar
Set the ratio at 2:1-4:1 so you don't completely crush the signal. The attack and release parameters should both be fast – around 20-40ms and 80-100ms respectively.
The SSL-G Native Bus Compressor works well for guitars due to its fast attack and transparent sound.
Compression Settings for Drums
A good starting point is with the fastest attack time and slowest release time, adjusting them until the drums start to sound natural. You should aim for a gain reduction of around 3-6dB.
Keep an eye on the needle and ensure that it's moving in time with the beat so that the release isn't holding on too long and smothering the next drum hit.
Use multiple compressors in order to first trim the faster transients with a fast attack time and then slower transients with a slower attack time on the second compressor.
Common Mistakes Using Compression
We have gone into quite a lot of detail on the art of compression. To finish things off, let's take a look at some familiar issues.
- Relying too much on compression
Compression is the glue that holds things together, and can provide body and make a mix punchy. But sometimes all you really need is a tiny bit of volume automation.
- Attack and release times that are too fast
If your attack times are too fast they won't allow the initial pluck of the bass or impact of a drum to shine through, leaving the result sounding very flat.
And in the same vein, short release parameters won't let longer transients such as a long vocal or the twang of a guitar ring out.
Make sure that you play around with slower release times and let the compression work more naturally.
- Only using one compressor
Using one compressor severely limits your mix's potential as it's putting undue strain on a job that should be shared. Using multiple compressors provides a much more transparent sound.
- Failing to understand your need for compression
The reason for using compression is to either tame transients and dynamics, or to boost the lowest parts of the signal and bring life to a mix.
If you're not sure why you are applying compression then it's best to hold off unless you've got a reason why.
- Having the compressor in the wrong part of the chain
Placing a compressor after the reverb means you are now compressing the tail of the reverb. Consider whether you want this to happen.
Additionally, compressing before EQ – is this what you are after or will it harm your mix in some way?