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How to use an audio compressor confidently

how to use and audio compressor confidently
The 2 most important controls on an audio compressor, highlighted in red. (Unisum mastering compressor)

 

Introduction :

The 2 most important controls on a compressor are not what you might think.

If you cannot hear what a compressor can or is doing it will greatly slow your progress in how to use them with confidence, after reading this you will be on your way to confidently and effectively compress your sound sources.

This explanation of audio compression should turbo charge your confidence when using compression. It cuts out the unnecessary, to focus on the vital. There is also a short video with a simple example further down. Please feel free to share online.

 

Super short version :

Make sure you match the unprocessed and processed audio levels as close as possible by ear using the make up gain and bypass controls. Otherwise you cannot accurately hear the effects of the time constants on the audio, they will be masked by level change (volume drop). I suggest not using inaccurate automatic make up gain.

TOP TIP : When testing a compressor in the following way I suggest using a compressor with a continuously variable, smooth make up gain instead of discrete steps. This gives you the necessary accuracy of control.

 

Short version:

1) Choose one and only one well specified compressor, one that ideally has peak/RMS/soft/hard knees for flexibility later. (as well as the more obvious ratio/threshold – attack/release and make up gain) Klanghelm DC8C is low cost, low CPU use and effective for mixing. (I have no affiliation with this developer in any way and do not use it for mastering.)

2) The 2 most important controls on any compressor are the make up gain and bypass.

3) I suggest not using automatic make up gain on a compressor. (they all tend to be different or inacurate, that is not at all helpful. You need to match up very accurately by ear alone.) Instead train your ears to use make up gain manually.

4) Use the make up gain knob on your compressor manually, to “make up” for the gain reduction applied. Try and make the sound being compressed close to identical in level as possible (to your ears) when you bypass the compressor i.e. the before and after. Once matched, quickly switch bypass on and off and compare. (The ear has very a short time to make good judgements, 2-3 seconds maximum)

5) The reason why point 4 is critical is if you do not you are mainly hearing level change to which the ear is highly sensitive. You MUST counter the level change in order to accurately judge the effect of the compression itself. If the action of the compressor is hidden by volume change you cannot make a good judgement.

6) A Gain Reduction meter is ONLY a guide. It is a very important informative guide that should be watched so you can tweak up or down your make up gain in response. Use it is a guide but match before (bypassed) and after (engaged) by ear alone. As you change any controls of a compressor gain reduction will change. (i.e. peak/RMS/soft/hard knee, ratio/threshold)

7) My suggestion is that make up gain should start at around 70pct of gain reduction. If your gain reduction is 1dB make up gain of around 0.7dB should be about right to the ear* (see bottom or article for my reasoning). Always let your ears decide though, this is ONLY a starting guide to speed up volume matching.

 

The longer read :

At first the short read must seem almost over simplified this is intentional, with the many controls even a basic compressor has it is easy to get lost in the many parameters on the facia/GUI. Coupled with the need to hear a reason to use a compressor. The basic controls of attack, release, ratio and threshold have been covered before many times. It is critical that you give yourself a chance to actually hear what the attack and release controls are doing by unmasking them from the drop in level. (otherwise it will be unnecessarily confusing for a potentially sustained period.)

I have seen many tutorials, videos and engineers explaining dynamic range compression in a half baked way that I wanted to give people clear, direct to the point knowledge (that is often omitted or skimmed over) to compress sound sources more confidently.

Compression is one of the most spoken about topics in audio engineering and it seems if you are not using it then you are missing out on something extremely important. It is only the case if you actually understand what a compressor is doing and why you might need to apply one.

 

Why an audio engineer may use a compressor.

1) To catch vocal peaks when recording so a mixer input / channel / audio interface input does not overload and distort on very loud parts of a vocal performance. (analogue compressor often used in mic/mixer line level signal path)

2) To shape the dynamics of a sound.. i.e. a drum such as a snare – using the time constants of a compressor allows you to shape the attack (start) of a sound and make it more or less peaky for example. Shaping transients, reducing or increase transients (peaks) in a sound. Usually called adding punch.

3) To control and hold a sound in place, if a sound is too peaky and sticking out of a mix you can tuck it back in.

4) To change the tonality (some may mentioning fattening) of a sound by adjusting its dynamics which can emphasize different parts of the sound over its duration. (time) Sounds harmonically evolve as they go through time. Consider a kick drum, we have an attack portion which might have “click” this will then be followed by the “body” of the sound, the tone of the drum will change as time goes forward and then we may have the subby “whump” of a kick drum. So by changing the level of sound over time we can influence tone.

5) To increase the overall consistency and perceived average volume of a sound over time.

 

What you should listen for and training yourself

 

As a beginner there may be great pressure to put compressors on sounds because it is the done thing, cause everyone is doing it right ? And a £200.00 plug in must make your sound much better than a £20.00 plug in compressor right ? It might and it might not it depends on what you are trying to achieve.

The truth of the matter is you need to identify a sonic problem as outlined above before you need to use compressors. Once there is an identified issue then you can use a compressor to help control an aspect of a sound/s.

Both identifying a problem to be resolved and the solution of using a compressor needs your ears to be trained to discern the problem and resolution of the problem. 2 of the more obvious problems might be :

1) A snare is not punchy or snappy enough

2) A sound is inconsistent and needs to be more consistent in level to sit better in the mix

The best way to learn compression at first is to create extreme situations, play a drum loop and put a compressor on 6:1 ratio and an attack time of 2ms and release of 80ms then pull down the threshold so you get 10dB of gain reduction. Then turn up the make up gain to 7dB and then bypass and check the before and after levels of the drums are as close as you can possibly make them by ear. (The closer you can match the level of unprocessed and processed audio the easier your judgements will be) With an extreme example and a lot of gain reduction (say 10dB) you can tweak attack and release for the desired effect then raise the threshold by 50pct for example. Then tweak the make up gain so unprocessed sound matches the processed level (by ear) and you can get the desired results quite easily.

Once you have matched up pre and post processing levels on a compressor you can start to listen to how the attack and release are acting on the sound. You can start to focus on the tone of the sound, the punch, the snappyness of the leading transients, the consistency of the sound over time and how it sits and sounds in the mix with other sounds.

 

Our ears are very sensitive to pitch and volume :

 

The reason this is critical is the ear is hypersensitive to pitch and level. In this case level, as compression reduces the level of sound. The initial drop in level will swamp your ability to hear the time constants acting on the material. By pushing up the make up you compensate for the gain reduction. As you change any parameters keep an eye on the gain reduction meter amount and ensure you compensate and match processed and unprocessed signals. (bypassed/engaged – before/after) This is the ONLY way you are going to clearly hear the sonic effect (action) of the compressor. You MUST counter the level change otherwise you hear …… you guessed it… mainly level change (level drop) as opposed to the time constants effects on the source.

 

That really is it, that is the tip. Easy, yes.. but vital. It will unmask the level drop so you can hear what the compression is actually doing.

 

I suggest using one and only one compressor to train with, as all compressors sound a little different from each other. With different compressors the internals may have different internal reactions, interdependencies and idiosyncrasies. Try another compressor once you become confident with the first one.

The difficulty is keeping on top of the make up gain tweaks quickly (almost intuitively after a while, whilst tweaking parameters) and getting your eye/ to knob/ to ear coordination up to speed. This takes time so try regular daily sessions over a week on different source material in a project. When learning something in audio you have to listen to multiple things simultaneously, this is a skill that takes time to develop. At first you are moving controls and having to listen to sonic changes, it is a lot to take in so do short training sessions. Don’t become frustrated, rest and come back the next day. Rinse and repeat and you will get better.

I have included a small video in this tutorial just to give you an alternative more visual guide, rather extreme to demonstrate the point. Klanghelm DC8C is a useful tool that is fine for routine mix compression tasks. It is a rough and ready video, unscripted with a mobile phone. DC8C is a good basic mixing compressor to use. In mastering I use arguably higher quality compressors of many different types and characters. I use around 12 different compressors in mastering, both analogue and digital for very specific purposes depending on the sonic qualities I wish to impart. And of course they are extremely precisely set up for sonic enhancement.

For this video we will stay with a basic, cost effective yet good workhorse compressor, useful for all routine mixing tasks.

 

 

 

 

 

 

Conclusion:

A compressor causes a volume drop. This masks the action of the compressor time constants. So you are unable to hear if it is better or worse with compression. This needs pre/post volume matching by ear, to judge.

You generally need a pre determined reason to use a compressor unless when learning.

When you use one it is vital to bypass and match processed and non processed sound by ear in order to effectively and accurately judge what the action of the time constants are doing. Bypass and engage quickly once matched, the ear has a short time to make accurate judgements (2-3 seconds)

It takes ear training and practice to get your ears / eyes (gain reduction) / make up gain coordination nicely linked up together. Using manual make up gain helps trains your ears.

And finally don’t get too hung up on using classic or boutique compressors for the training stages. Any well functioned compressor is good enough to start with. Typically, classic, vintage or advanced and unusual compressors have an – audio pass through tone – or may have idiosyncrasies which are not standard. These may confuse a little when learning. A basic, fully functioned plug in compressor should work fine including a DAW built in compressor. These tonal or stereo image effects are secondary effects (maybe from emulated transformers/valves or other components). They can be put to good use as your experience of compressors deepens.

*The reason for this 70pct suggestion is most gain reduction meters in plug ins tend to be peak reading and by using 70pct (0.7 the gain reduction value) you are approximating the RMS value (average which is closer to the ears response, as our ears do not judge volume using peak level) As an additional note, the way gain reduction is displayed is only usually a guide, on each compressor the movement of the meters may not precisely relate to the time constants chosen. (Unhelpfully the gain reduction meter may have its own ballistics, ballistics is the term used to describe metering response speed.)

The reason to use a continuous make up gain control is it gives the detailed control required, as some “classic” compressors have 1 or even 3dB discrete steps which is not accurate enough for the technique explained above.

You will become very quick at matching before and after and eventually (but do not attempt this too soon) you can be slightly less rigorous with the instant matching of before and after using make up gain, you can do it faster and more approximately as a ‘sanity check’. With long term experience and many 100’s of correct applications of a compressor there will come auditory confidence. You will get so good at it you will hear through the volume drop. Then you use the make up purely as a control to balance the volume of the ‘after’ signal into the mix directly.

In short always match up pre and post processing using your ears as they really are the best way to judge level.

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safeandsound123 at google mail.com (at being @ no gaps)

Copyright 2023 (published 29/04/23) Barry Gardner – Please feel free to like and share.