What is Parallel Compression and how do you use it?

If you are new to music production you may well have heard the term ‘parallel compression’ used in tutorials, but may be slightly unsure of what this means or how it is used. This blog aims to explain what parallel compression is and its differences from normal compression, how it can be used effectively in music production, and finally how it can be set up inside your DAW.

What is Parallel Compression?

The term ‘parallel compression’ was first coined by Bob Katz, and it describes the technique of mixing uncompressed audio with a highly compressed version of the same audio in order to decrease the dynamic range of audio. It is thought that it was first used commercially in 1965, found in the circuitry of the Dolby A noise reduction system (shown below) before being picked up by producers in New York where it earned its other name – New York compression.

Differences from normal compression and How it can be used

Parallel compression can be used on anything, however the important question is; do you know what you’re using it for? Robjohns (2013) in a Sound On Sound article about parallel compression writes “The idea of parallel compression was conceived to achieve much the same goal as upwards compression. In other words, its aim is to leave the delicate loud transients intact while raising the level of low-level signals”. This goal of leaving transients intact makes parallel compression a prime candidate for instances such as compressing drums or voice, Reilly (2017) explains “Using parallel compression on drums can help keep all the dynamics of the original source material but add volume, depth and power”. These examples of where parallel compression is commonly employed suggest that the fundamental effect of using it is added energy, stability and power while preserving the dynamic quality of the louder sections of a piece of audio. This indicates that the material it would be most effective on are pieces of audio which contain important transients, but also have intricate and low-level information in between which may otherwise get lost.

When compared to normal compression, parallel compression uses the same technique of volume manipulation, however it does it to achieve a different goal. One drawback of standard (downwards) compression is that it can often “wind up removing not only dynamic range on the macro level, but also the instantaneous transients in our music” (Shimazu, 2021), however this is less of an issue with parallel compression because when the dry signal is at it’s loudest, the compressor (if set up correctly with a high ratio, and threshold allowing for at least 20dB gain reduction) has an almost negligible output (20dB less than the dry sound), whereas when the dry signal is quieter, the compressor makes up far more of the overall volume, giving the impression of more ‘perceived’ loudness.

If you are not sure which type of compression to use, think about the goal you have. If there are harsh transients which are not wanted and need to be tamed (such as with an aggressively played acoustic guitar which is meant to accompany a voice) standard downwards compression would be far more useful. Parallel compression in this instance would make the guitar impose more on the vocal, taking away from its accompaniment role. Rather than being seen as a replacement for compression, parallel compression should be viewed simply as another tool in a producer’s arsenal.

Setting up parallel compression

Danny Echevarria (2021), describes three different ways to set up parallel compression in a DAW, the first being the simple simple technique of just duplicating the audio track intended for parallel compression, and then compressing the duplicate while leaving the original dry.

Above is a drum track I have put into Logic Pro X, with the top being the uncompressed audio, and a duplicate of the audio on the track below, but with a heavy compressor added in the signal chain. The volume of the compressed audio can be brought up to mix with the uncompressed audio to achieve a ‘fuller’ sound.

The second method Echevarria gives is to create an aux track, placing the compressor there, and feeding some of the dry signal to it via a bus (as shown on the left).

This way, the dry signal from the drums is again untouched, however a portion of that signal is also sent to the aux track to be compressed. Control of how much is compressed is determined by how much signal is sent to the aux channel. With this method however, it is important to remember to make the send ‘pre-fader’ as “If you use a post-fader send, changing the level of the source’s channel fader will also affect its level at the compressor’s input—causing your compression to change!” (Hahn, 2019).

The final way to use parallel compression in a DAW is to use the dry/wet dial on a compressor (if it has one). The Logic stock compressors all possess this handy feature, making it very simple to leave some of the audio passing through them untouched.

The dial is located at the bottom right of the interface, just next to the output gain, and can be adjusted to taste.

There are notable advantages to using the second method over the other two. Firstly it would be significantly less strain on a computer’s CPU than the first method if you want to parallel compress more than one element in a session, as it only involves one compressor being used instead of one per channel. Secondly, as it is on an aux track and multiple channels can therefore be fed to it, parallel compressing (for example) a drum kit with drums recorded onto different channels is much easier. This is because they could all be bussed to the aux track, and would therefore have exactly the same compression without changing many compressors individually. It allows you to treat multiple elements as one whole, which may lead to a more cohesive overall sound. However, if the compression is only going to be applied to one track (for example just one voice), it may be more simple to use the mix function found on a compressor.

References:

ECHEVARRIA, D. (2021). 6 Favorite Ways to Use Parallel Compression in a Mix. [Online]. Available at: https://theproaudiofiles.com/parallel-compression/ . [Accessed: 8th December 2021].

HAHN, M. (2019). Parallel Compression 101: How to Give Your Mix Punchier Dynamics. [Online]. Available at: https://blog.landr.com/parallel-compression-bigger-mix-2/ . [Accessed: 8th December 2021].

REILLY, B. (2017). What Is Parallel Compression And How To Use It. [Online]. Available at: https://vintageking.com/blog/2017/10/what-is-parallel-compression/ . [Accessed: 8th December 2021].

ROBJOHNS, H. (2013). Parallel compression. pp. 2. [Online]. Available at: https://www.soundonsound.com/techniques/parallel-compression . [Accessed: 8th December 2021].

SHIMAZU, H. (2021). How to use parallel compression in your music. [Online]. Available at: https://splice.com/blog/how-to-use-parallel-compression/ . [Accessed: 8th December 2021].