Most often, we use compressors to tame the dynamics of a recording. Like all recording tools, though, compressors have less-obvious uses for shaping sound.
A compressor raises the level of quieter elements in a signal. This tends to “fatten” the recorded sound, which can add a lot of character, especially on drums and vocals. However, the effect is most pronounced when the signal is heavily compressed, and over-compression usually kills the dynamics of a performance.
What is parallel compression?
Parallel compression offers a good compromise. In its most common implementation, parallel compression refers to mixing a dry signal with a heavily compressed copy of itself. The dynamics in the dry signal are preserved while the compressed signal adds body and character to the overall sound. It works for any instrument, but on drums and vocals in particular, the added character can really bring a track to life.
This article from Sound On Sound lays out an approach to parellel compression in Logic. (Note, though, that the article incorrectly calls the technique “sidechain compression.” Sidechain compression is an unrelated procedure used to implement ducking, better described here: Sidechain compression in Logic.)
SoS’s method involves duplicating the track on which parallel compression will be performed. I prefer to use a bus or effects send which allows me to pass many tracks through the same compressor, handy for fattening related tracks together. But both approaches produce the same end result, so if your DAW or recording platform doesn’t offer busses or sends, you can still use parallel compression.
Hear it in action
Here’s a short drum passage that I’ll use to illustrate this technique in action:
The dry signal comes from 5 mics: Kick, snare, floor tom, and 2 overheads.
In this first example, I routed the kick and snare drum tracks to a second mix bus, the parallel compressor (“IIComp” as I call it – see the screen shot below,) and mixed the result with the dry signal heard above:
The effect is obvious. Note how the kick drum and snare drum have more body and punch while still sounding dynamic. (In practice, I wouldn’t mix the compressed signal quite so loud. I overdid it here to better illustrate the effect.)
It’s also obvious that the compressor on the parallel bus is really crushing the signal. For this example, and the one below, I used a Waves C1 compressor with a fast attack, a ratio of nearly 14:1, and a hard knee, as shown here:
I took the screen shot during one of the snare hits. Notice that there’s almost 20db of gain reduction. Really crushed!
This is what the compressed signal sounds like by itself:
While I most commonly apply this technique to the kick and snare only, parallel compression can also be used on the overhead microphones:
The effect is more subtle, obvious mostly on the cymbal crashes. However, depending on the sound you need, this less pronounced effect might be perfect.
Finally, here’s a screen shot to help visualize the signal chain I used:
The kick and snare drum tracks are routed to the primary drum bus (not shown) which carries the dry signal, and to the “IIComp” bus which contains the Waves C1 compressor shown above. Note, too, that the IIComp bus has its level reduced by almost 16db. It’s not necessary to mix the dry and compressed signals at the same level. In fact, doing so usually defeats the purpose of using parallel compression as the compressed signal overpowers the dry signal and smears its dynamics.
Other approaches to parallel compression
The examples above show the most common implementation. However, there are as many ways to set up parallel compression as there are ways to configure a compressor. Different attack and release times create different effects, as do different ratios. In my examples, I used a fast attack and slow release, which removes all the transients from the signal. However, with a shorter release time, you can create a “pumping” effect which often adds a distinctive sound.
Further, different compressors have different characters. The free Blockfish compressor, for example, is great for crushing a signal beyond recognition. And when mixed at low levels under a dry track, this “distressed” signal usually adds complimentary qualities.
As with all recording techniques, of course, experimentation is the key to finding sounds that work for you.
UPDATE: See Andy’s comment below about using this technique in a DAW that lacks latency compensation
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