In Summary

Knowing when or when not to reach for the compressor is just as important as knowing how to dial in the best settings. Ultimately having the end result in mind, and knowing the tools available will help guide the engineer on when to use a compressor and how to avoid mistakes.

Going Deeper

It’s easy to forget how incredibly dynamic real life sounds are, and the difference between the very quietest and the very loudest sounds are measured with numbers in the millions. Capturing and mixing these into a narrow ‘window’ that itself covers anything from a whisper to a snare drum is a tall order indeed. At some point the engineer has to de-emphasize the gulf between quiet and loud; the compressor gives automatic control over level for those times when the engineer is busy elsewhere.

Here’s our roundup of ways to avoid some well-trodden mistakes that can happen with the use of compressors.

Mistake 1 - Forgetting The Faders

Compressors have become such an accepted part of the mix that it is the absence of them that will raise eyebrows. For some engineers, however, fader ‘rides’ (where the level is adjusted in real time either into the recording or during the mix) are second nature. Although less relied upon now in most styles, classical engineers continue to use manual compression with the fader in preference to automatic gain control, aka, The Compressor.

Regardless of genre, use the faders cannot be entirely replaced by the compressor in the mix. For the ‘macrodynamic’ changes on a musical timescale, the fader is hard to beat on sources such as vocals. When combined with a ‘microdynamic’ treatment from a compressor downstream, the results can be a lot smoother than those from an overworked compressor alone.

Mistake 2 - Not Having A Clear Aim For Compression

Compressors For Control

While the compressor isn’t the only tool in the engineer’s arsenal for straight-ahead control of levels to sit sounds, it is the obvious choice when that control needs to be automatic. Certainly for transient or otherwise unpredictable and/or complex material, the compressor will always be able to respond quicker and more accurately than the engineer at the wrong end of end of ten hours in the chair! Being able to dial in time constants and the amount and manner in which gain reduction is applied is a major aid to the engineer with lots of tracks and little time. Starting with audio plugin or channel presets with the thresholds initially dialled away from the audio are the engineer’s friend.

Compressors For Sound

Possibly as a result of art mimicking science, the compressed sound has become a mix flavour of its own. The very dynamic sound of uncompressed clean guitars and pianos can almost sound almost expanded by comparison, and other sounds such as ‘squeezed’ snares and breathy vocals have a density that artists and listeners have become used to. Any character compression can serve the dual purpose of providing memorable, larger-than-life sounds as well as predictable peaks that are easier to place.

Mistake 3 - Getting Caught Up In The Details

Regardless of tool or technique, the first question to ask is whether or not the audio is too dynamic in the first place. Once the Big Picture has been established, any compressor tweaks that cannot be heard in the context of the mix might not be the best use of the engineer’s time. Newer mixers could be forgiven for thinking that techniques such as parallel processing or external sidechain triggering are a requirement of every mix; the smart money is on deciding fundamentally how much gain reduction needs to happen using simple controls.

Concentrating on basic technique using the ‘Big Four’ controls of Ratio, Threshold, Attack, and Release for faster results can often serve the music just as well as mousing around on fancy routing tricks. It will also please the client more than ten minutes spent watching the engineer tweaking their next soon-to-be-unused folder-filler.

Taking this further, one-knob compressors on some mixers and interfaces exist for a reason. These have been developed by those in the know to make mixes better.

Mistake 4 - Not Knowing The Big Four

Ratio and Threshold

Where the fundamental job in hand is gain reduction, Ratio and Threshold govern not only how much of this is applied, but also where it is happening. Mix bus compression with (for example) 4-6dB of gain reduction might see a greater part of its dynamic range undergoing compression than the snare drum with the same amount of reduction.

Ratio and Threshold go hand-in-hand when setting up the amount of compression, and taking the mix versus snare example, the former can be with lower ratio and threshold versus a higher ratio and threshold on the snare. Both have the same amount of gain reduction but with very different results. Knowing how these interact is the key to the deploying the right amount in the right place.

Attack And Release

Attack is the time taken for the compressor to kick in and Release is the time taken to ‘recover’ from gain reduction. As well as being time controls these two can also affect the amount and character of compression. Better known is Attack’s ability to either catch or let through transient peaks on things such as drum hits for different effects. However, less known is the effect of compressor distortion relating to compressor release. Where release times are similar to the cycle length audio, the result can be grittiness or distortion as the release phase of the comp ‘tracks’ and changes the audio’s waveshape. This is more common on bass frequencies, and simply increasing the release time by a few milliseconds can avoid this.

Mistake 5 - Stamping On The Audio

Despite the need to make music audible to those on the move, the listener can have too much of a good thing. Although a well-deployed compressed sound can be attractive, music is not just on or off and compression can, for the most part be aiming to represent the dynamics of the music. Even if there is a lot going on under the hood, compressors can be used to help weave the impression of dynamic movement on the surface. Listening and A-B’ing following ear breaks are still one of the best ways to judge this.

Overcompression can be cumulative. Most understand why compressing drum close mics into a compressed drum subgroup into a compressed mix bus may require caution. Judging the amount compression (and where it’s coming from) is far easier with fewer stages of it going on. Staying on the drum example, when mixed high these can often pump the entire mix’s compression with unintended results. Using some high threshold, high ratio compression on the subgroup to take the tops down a bit can preserve the dynamics of the song as a whole, helping the mix to breathe as intended.

Mistake 6 - Forgetting Make-Up Gain

One of the great artistic advantages of compression is the increase in perceived loudness that it can lend to instruments and mixes. While this can help the engineer to sit tracks better it can also lend some extra density and cohesion to entire mixes. The benefit cannot be achieved however without the appropriate use of Makeup Gain, aka Output Gain, or just Gain.

Makeup is not always needed if compression is being used for level control, however it goes that the compressed output needs to be turned back up to deliver the same subjective level following a lot of compression used to achieve a particular sound. Knowing what makeup is there for, and listening and adjusting accordingly will avoid upsets in balance or overall level.

Some compressors have offered auto-makeup for decades, with varying success. Results may still need refinement with the channel fader.

Mistake 7 - Not Knowing The Compressor’s Limitations

Some things have very little dynamic range such as certain synth sounds or heavily overdriven guitars. Attempting to compress sounds such as these will achieve little, and knowing what the comp is actually for will avoid this. Certainly the compressor cannot be used to design the envelope of an already flatlined sound.

Different compressor designs can be more suitable than others in a given role. Expecting that vintage opto-comp to catch drum transients will be less successful than using a modern VCA design. Compression that approaches technical perfection might not inject the charm or gloopiness of that obscure diode bridge comp that you’ve been meaning to try. Sometimes a tool’s limitations on one thing will make it a better choice for something else. Choosing a model after listening to candidates will avoid arbitrary choices that might not suit the mix.

Going back to fader rides, many compressors can behave unpredictably when presented with overly dynamic audio. Riding levels manually into the comp is a classic technique that is easily set up in the DAW; indeed automated moves into compression could provide the most transparent results. An alternative can be to set up two comps in series to avoid overshoots into a single one. The first can play ‘punchbag’ with higher threshold and ratio to absorb the biggest hits, with a second there to provide the main layer of refinement.

Final Glue?

As with any process, one of the biggest mistakes an audio engineer can make is not properly listening to the audio they're compressing. While the rich visual feedback from some tools can be useful, it cannot do the listening for the engineer (yet)! Zooming out and concentrating on factors such as when to compress and how to deploy it as simply as possible will free up time and headspace that can be used elsewhere. Certainly, the listener is not going to hear many of the differences in approach that engineers deliberate over. Although one good question for engineers to ask themselves is how to compress, the answer means nothing without the “Why?” that comes before it.

Luckily all that is needed is a pair of ears and tools that almost any engineer has access to.

Main photo by Cinescope Creative on Unsplash

LA-2A photo by José Pinto on Unsplash