How Compression Works
September 07, 2019One of the hardest things for engineers and mixers to use effectively is compression. While the basic concept is pretty simple, good compression often thrives on its transparency, which is why it can sometimes be difficult to hear and comprehend what it’s doing to your sound.
Today, I want to cover the basics of compression: what is does and how it does it.
It doesn’t matter if you’re brand new to recording or have been doing it for years. Understanding the fundamentals of compression is a key concept that anyone in a studio environment should have. After all – it’s the glue that holds everything together!
What Is Compression?
Compression, in an audio production sense, is the process of levelling out your signal for more consistent dynamics. A compressor’s primary function is to reduce the levels of loud peaks to bring them more in line with the level of the rest of the track. With those peaks under control , a mixer can more effectively control that track’s presence in a mix, making it louder or softer without stray peaks skewing their decisions.
What Are The Elements of a Compressor?
Compression relies on a handful of settings to do its job effectively. The main parameters found in most compressors are:
- Input – The amount of signal you’re feeding into the compressor
- Threshold – The level that the signal needs to pass to start acting on the signal
- Ratio – The amount of compression being applied for each decibel the signal goes over the threshold
- Knee – A setting that “ramps up” the ratio depending on how far the signal has crossed the threshold
- Attack – The time it takes for the compressor to start acting on the signal once the threshold has been crossed
- Release – The time it takes for the compressor to stop acting on the signal once the level returns under the threshold
- Output – The amount of signal you’re sending out of the compressor (sometimes referred to as Make-Up Gain )
Dialing In A Compressor
Combined, these settings provide a sort of “signal chain” for your compressor, telling it exactly what to do and how to do it. You start by feeding the compressor a good, clean signal with the Input, then set your Threshold to a level that will catch the peaks that are popping out to you (this will vary based on source and type of music ). Your Ratio will then determine how much you want to suppress those peaks; a low ratio of 2:1 will reduce the processed signal to 1 dB above the threshold for every 2 dB the incoming signal exceeds the threshold, while a high ratio like 10:1 will reduce the processed level to 1 dB above the threshold for every 10 dB the incoming signal exceeds it. The higher the ratio, the closer you get to limiting, which is where the threshold acts more like a hard ceiling that no level can pass.
The Knee option isn’t available on all compressors, but when it is, you can adjust your ratio to compress less heavily for incoming signal that barely crosses the threshold while applying the full ratio to peaks that hit the compressor at a higher amplitudes. A “hard-knee” compressor will use the ratio that you’ve set for all peaks above the threshold while a “soft-knee” eases into it for a smoother transition between compressed and uncompressed parts.
Finally, the Attack & Release settings work together to determine the timing of your compression. Fast attacks are usually used with sources that have quick, snappy peaks such as drum hits . Slower attacks can commonly be found on sources like vocals where the engineer might want to let peaks bleed through a little bit to retain dynamics.
Almost inversely, common release settings are used to draw out the tail ends of notes, making them ring a bit longer due to compression when a slower setting is selected. This is commonly seen on bass guitar parts where the compression is held to create a more present bass tone. Fast releases are most common when the engineer only cares about catching the peaks and are trying to leave the rest of the signal largely unprocessed. Several compressors offer auto-release functionality that determine the release time automatically based on the other settings and amount of processing occurring within the plugin.
Rounding out a compressor’s settings is the Output gain, where you’re able to set the level leaving your compressor plugin for the next part of the signal chain. Some plugins such as Gain Reduction Deluxe pair the output gain with saturation, allowing you to get some really unique and harmonically pleasing sounds as they’re turned up. Some engineers will use compressors like the Empirical Labs Distressor for this kind of saturation alone – running their ratio at the bare minimum (as low as 1:1 – no compression) and only using the processor for the color it adds to a signal.
Where To Use Compression In Your Mixes
While the key to dialing in a compressor relies heavily on your understanding of what the compressor does, it’s not as if anyone that understands how a compressor works can use one effectively. It takes trial and error. It demands practice and experience – you’re not going to become a compression expert overnight.
With that said, you can absolutely follow the steps taken by professional engineers and producers to keep you on the right track. By following others’ processes, you should be able to look at your plugins from a new perspective and hopefully pick up on why the pros are doing things a certain way.
One of the best ways to see how professional mixers are working is with a membership to Nail The Mix. Nail The Mix gives you access to monthly sessions from the top producers and mixers in rock and metal for you to practice with. These aren’t covers of songs – they’re the actual sessions that were used for commercially released albums.
At the end of each month, the pro that worked on the original song will walk you through their mixing process, sharing the how and the why of their mix.