What Is Audio Compression And Why You Should Use It

Most of today's music use some form of compression to improve overall sound quality. Many users apply compression while recording or mixing without even knowing exactly what compression is and why it is used.

Audio compression is the reduction of the dynamic range of an audio track by attenuating the loudest parts and raising the volume of the quietest parts of the audio signal. This is often used to smooth out instruments that are too loud or too soft or fall outside the recording equipment's dynamic range.  

Although Compression is a common feature in every major DAW Software Suite, great care should be taken when using it, as it can't be undone once it has been applied to a piece of audio. That is why it is so important to understand exactly why compression is used and when to use it.  

When And Why To Use Compression

Compression can be used during various stages of your audio production for different reasons and to construct a specific outcome or add a specific characteristic.

To better understand this "broad definition" we need to take a more detailed look at each.

Making A Recording Fit

It can be used during the recording stage when the dynamic range of the instrument is much wider than that of the recording device. A recording of a loud drum performance is a good example of where compression can be used to reduce the dynamic range of a much louder instrument and prevent unwanted distortions like clipping within the recording.

Making Tracks Play Nicely Together In Same Dynamic Range

Compression is also very useful to allow recordings from different devices to "play nice together". Sometimes different instruments or vocals are recorded on different devices which have to be added together in a single mix.

(Without compression, you will end up with one ugly mess of different tracks trying to co-exist, with certain instruments sounding overwhelmingly loud next to another one that was recorded so softly that it is completely inaudible.) 

​Using compression will allow you to set a certain threshold that will force each of the individual sound tracks to fall within the same dynamic range, sounding balanced and in-tune together on the same mix. 

Adding A Distinct Flavor

Traditional analogue compressors also used to add their own unique characteristics and a specific flavor to a mix. Many digital plugins emulate these characteristics to add the same "flavor" to a digital soundtrack. 

Sometimes referred to as the "Big Four", there are basically 4 types of compressors that all have their own way of operating and compressing sound, resulting in the output sound to have a unique character. We take a brief look at each one.

Tube Compression: Also called valve compression, it is generally considered to be the oldest form of compression. Its unique ability to produce smooth and silky sounds, with a distinct vintage flavor, makes it a very sought after sound.

Optical Compression: This type of compression uses a light source that produces different intensities of light that gets transmitted to an optical cell that reacts to the amount or strength of the light. This type of compression is very popular for vocal and mix bus compression among many sound engineers.

VCA Compression. VCA (Voltaged Controlled Amplifier) makes use of an integrated circuit that allows it a very precise control over compression. Since they are less "colored" and more precise, making them less prone to distortion, they can be used for a variety of different applications.

FET Compression: FET (Field Effect Compressors) use transistors to very accurately emulate the valve/tube sound. This makes them popular for vocals, but especially suitable for drum compression.

Obviously, apart from being very scarce and expensive nowadays, analogue compressors have been largely replaced by digital compression. Still, these popular types of compression are still very much sought after. As a result, many plugins are available for DAW software that very successfully emulate the sounds these analogue compressors are able to produce.  

How Compression Is Used

To be able to get the desired result from your compression, most digital compressors give you a variety of setting you can adjust to fine-tune and output the audio mix to your taste. (This will be determined to a large degree by the type of sound you are compressing.)

We take a look at the 6 most common settings that can be adjusted to affect the way in which compression is applied:

Threshold:

This is the trigger for compression to be applied. Here,  you as the user set the loudness level at which compression kicks in. For example, you may set the threshold at -15 dB. This means that all sound levels under this threshold will remain unaffected, but all sound levels above -15 dB will be compressed.  

Input Gain: 

Since compression brings down the overall loudness of your sound signal, you can set the input gain (also called make-up gain) to increase the overall volume of the signal to match the rest of your audio mix.

Ratio:

You can set the amount of compression that takes place by setting the ratio. For example, setting a ratio of 5:1 means that once a sound signal exceeds 5 dB, the output signal will be reduced to 1 dB. (And a ratio of 7:1 will reduce any sound signal exceeding 7 dB to 1 dB etc.)

Attack

You can set the amount of time (from the moment the threshold for compression to be applied is exceeded) before compression kicks in. This is usually a very small amount of time, normally measured in milliseconds. 

Through trial and error, but mostly experience will you be able to judge how to accurately and quickly choose the correct attack setting.  

Release

You can also set the amount of time before compression is disabled (after the sound signal has dropped below the threshold set by the user). As with attack, this is also a very small amount of time, and also normally measured in milliseconds.

Knee

Not to be confused with Attack or Release, Knee refers to the way in which compression is applied (and not the set time before it is applied or disabled).

There are basically 2 types of knee that are used, and the type of knee is normally determined by the type of sound that is compressed.

Soft Knee produces a very gently and gradual compression, making it almost inaudible. Vocals and acoustic guitar are a typical examples of when a soft knee will be used. 

Hard Knee produces a much more sudden and audible compression. Drums and bass are typical examples of when a hard knee will be used. 

Conclusion

As you can see, compression is a very important and powerful function of audio recording and mixing. It has a big effect on the final mix and output.

It is quite a complex process with many variables to take into consideration, and is therefore a task that should not be taken lightly, especially for the novice user. A fair amount of experience is required for a proper and accurate compression of any recording or mix. 

Don't be afraid to experiment though. It's the only way you will really learn. (Just keep an original copy stored away safely!)

As always, feel free to leave me any comments or suggestions you may have, and I will respond and try and get to them as soon as I can.

Remember to join my  Mailing List  to be informed whenever a new article is released, and share new developments and helpful hints & tips.

Catch you in the next article and happy recording!

Wessel