Compression is a highly talked about topic in the world of mixing. Unfortunately, it is an often-misunderstood tool in the engineer’s toolbox.
In this article, I aim to answer the question “what is a compressor?” and perhaps more importantly – how do I use compression in a mix?
What is a compressor?
A compressor is a device (or plugin) that reduces a sound’s dynamic range (the difference in volume between loud and soft).
A visual representation of dynamic range
When set appropriately, a compressor will react to loud peaks of a sound and work to reduce them in volume, thus making them seem quieter. As a result, the soft parts of a sound that do not trigger the compressor will appear louder than they did without compression. This is because the difference in volume between the loud and soft parts of a signal has been reduced, making the signal appear more even in volume overall. Adding gain after compression will emphasize this even further.
A compressed VS uncompressed version of the same signal.
Notice how the quiet parts appear louder on the compressed signal in comparison to the uncompressed one
Why should I use a compressor?
Because compression is discussed so often, it is important to understand why you should reach for a compressor in your mix.
A compressor can be used to solve a number of problems within a mix, such as:
- To make a sound more consistent
- To prevent a sound from clipping
- To bring out a nuance in the performance
- To help a sound cut through in a dense mix
- To add character to a sound (compressors can be very distinct)
- To manipulate the attack and sustain on a sound
- As a special effect (“just because”)
What are the main controls on a compressor?
Threshold – the point in volume at which the compressor begins to work to compressor the material. Once the signal passes the threshold volume, the compressor begins to engage it based on the attack setting.
Ratio – determines how much attenuation will take place once the threshold has been breached by the signal. For example, a 2:1 Ratio means for every 2db that goes into the compressor passed the threshold, only 1db will come out, effectively cutting the signal’s volume in half.
Attack – controls how fast the compressor will react to the material after exceeding the threshold. Often a slow attack will result in less gain reduction taking place, whereas a fast attack will be more aggressive and typically result in more noticeable attenuation.
Release – controls how quickly the compressor will “let go” of the material, allowing it to return to an uncompressed state. A fast release may create a “pumping” feeling depending on the source material.
Knee – determines how hard or gradual the compression appears. Most compressors are default set to “Hard Knee” which results in the compressor reducing volume linearly. Utilizing a “soft knee” compressor or compression settings will result in a gradual slope into gain reduction. “Soft knee” compression often sounds more transparent.
Makeup Gain – allows you to add or remove gain on the source signal after compression has taken place. Often, since we are reducing volume with the compressor, this function is used to add that lost volume back to the signal.
Dry/Wet – some compressors feature a dry/wet functionality. Dry refers to the unprocessed (uncompressed) signal, and wet refers to the compressed (affected) signal. This functionality allows you to blend the dry, unaffected signal and wet, compressed signal together, displaying the best of both worlds.
Native Instruments Solid Bus Compressor – an emulation of the popular SSL Console Compressor
What alternatives are there to compression?
If you are using compression in order to control volume and attenuate peaks, you could alternatively use specific volume automation and strategic gain staging to achieve the same effect. This would have to be done manually, which will require significantly more work than simply setting and using a compressor.
Also – it is common to use parallel compression in a mix. This is when you send a signal to a bus (also referred to as an aux or return track depending on your DAW) and compress it separately from the dry signal. By blending the volume between the dry signal and compressed signal, you are able to achieve the best of both worlds by preserving the dynamics from the dry signal, while achieving the consistency and density of the compressed signal.
Do I have to use compression in a mix?
To a certain extent- no. Compression should be used for a specific reason – such as one of the few mentioned above. I personally use compression very sparingly and find myself using it more on instrument subgroups and masters rather than on individual channels. I also utilize parallel compression often in order to achieve the best of both worlds as outlined above.
If you don’t have a good reason to use compression within your mix –don’t force it. Be smart and strategic while mixing to save yourself time and to avoid frustration.
Despite being misunderstood and misused, compression is a powerful tool that can be used to transform and improve your mix when utilized correctly. Reach for one in your mix when trying to solve problems, and leave it alone or pull it back when it sucks the life out of your records.
I’ll be back with more mixing tips on compression in the near future.