Parallel compression is one of the oldest and most widely used mixing techniques in the history of recorded music. It dates back to 1965 when Dolby A noise reduction engineers included in the system’s internal circuitry parallel buses with one containing heavy compression. The basic idea of parallel processing is to duplicate the track you want to enhance, apply heavy compression to the new track and then blend it with the original. An alternate method is to send the track (or tracks), to a bus send and then into two aux tracks with the compression only applied to one. Both methods should result in a combined sound that has retained much of the dynamics of the original signal, but has been beefed up by the addition of the compressed track.
Whilst this sounds easy and seems to make sense, the theory as to why the technique actually works so well is a little more interesting, as (Ishaki, 2011 p.319), explains: “The idea of parallel compression is quite simple- instead of bringing high levels down, we bring the quiet levels up. The ear finds this more natural, and the artifacts triggered by the low-level signals make the compression less evident. In addition, parallel compression retains dynamics much better than downward compression as transients are not brought down – if anything their bottom is beefed up”. Our ears are much more sensitive to reductions in volume than they are to increases, so this upward compression is less noticeable to us acoustically and this lets us drive the compressor further than if we were using downward compression. The diagrams below show the differences in downward or ‘normal’ compression (where the louder transients above the threshold are squashed), and upward compression (where the softer transients are increased in volume up to the threshold and the louder ones are left alone).
This parallel compression technique can be applied to anything that you think could benefit from upward compression, however it is traditionally used on drums, vocals and bass. Particularly when applied to drums, the general consensus is to set the attack setting as fast as possible without actually destroying the transients and the release time is set to fast – medium to eliminate the ‘pumping effect’ or distortion of the lower frequencies. For a mixing trick that has been around for over 50 years, it is natural to look at what has been done before in others’ successes and just emulate their technique, but in my limited experimentation with this form of compression, I have found that you really have to trust your ears and not always the maths! What works well in one mix with one style of music, will rarely always work with another, so it is good to get your hands on the attack, release, ratio, knee and threshold controls, close your eyes and really listen to how the adjustments of these settings are affecting the overall track and mix. This also applies to mixing the heavily compressed track (wet), with the original (dry), track. I have always found that the compressed track (wet), will always sit underneath the dry drum track but I am sure to come across a situation where it may be centre stage!
I have experimented quite a bit and continue to with compression in general, there is a heck of a lot to learn and understand. Parallel compression is definitely a great way to beef up certain elements of your tune and help them find their place and punch through a crowded mix.
References
Feature Image courtesy of https://talkinmusic.com/musicproduction/use-parallel-compression-to-enhance-sounds/ Retrieved on 28/1/17
Izhaki, Roey. Mixing Audio : Concepts, Practices and Tools, Elsevier Science & Technology, 2011.
Compression diagram courtesy of https://www.soundonsound.com/techniques/parallel-compression Retrieved 28/11/17
Abundant Audio 