Audio Compression Intro and Basics [Part 2] - Sidechaining

In the first post about audio compression we looked at how a compressor will squeeze the volume above a certain level to create an overall louder sound. That was using compression at it's most basic. There is much more too it however, simple compression is great to tame an instrument or a mix but it can be used much more intelligently.

Let's consider we are in a club environment, 17.5 Jagerbombs in (0.5 accounting for spillage and dribble). Not only are we chuffed that it has come to under £20 for drinks but the bass is rumbling and the kick is punching. The drink makes us want the music loud and here is where sidechain compression is the key.

When the bass and the kick fit just right.

When a dance song drops, you want the kick drum to be as loud as possible and you want the bass to be as loud as possible. Thing is if you have the kick as loud as it can be without going into the RED you are not going to have much room for anything else let alone the bass. Likewise if you have the bass maxing out the system, something as loud as a bassy club kick drum will take the volume waaaay into the RED.

"Let's just mix them together and slap on a compressor" you say, after all we now know that a compressor can reduce the volume to manageable levels and stop it distorting in the RED. The bass will be loud and every time the kick plays, everything is reduced and fine...right?

I would say it's not wrong. There is an alternative, and of course, it is sidechain compression. In the club, we want really loud bass, however when the kick plays, we need to realise it too holds a lot of bass frequencies. We don't need both bass and kick sounding for the duration of the brief kick drum so we can lower the volume of the bass to make the kick sit nicely in it. Here is a diagram of how a sidechain compressor works:

I often feel you need to know the concept already for these diagrams to make sense.

In normal compression, every time the track volume exceeds the set threshold, the compressor kicks in and reduces the volume of anything over that threshold. But we want to reduce the volume of the bass, every time we hear the kick. So we can set the compressor to work on the bass every time the kick exceeds the threshold, without compressing the kick.

For my example I'm going to use Reaper's ReaComp, which comes natively to Reaper and is a compressor capable of sidechain compression. I am also aware that some of you won't have great speakers or headphones on your person at all times so I will replace the bass with a mid-range chord pattern so you can hear the effect even if your listening device cannot reproduce sub-frequencies.

On an important side note, this effect is not limited to bass frequencies or even kick drums. It also doesn't require the kick drum to be heard in the song as it may not be sent to the master bus. a good example is in the Robin Schulz remix of "Prayer In C" at [1:42] where there is a synth string section rhythmically dipping in volume. This is where an inaudible kick is activating the compressor and dipping the synth volume accordingly in time.

The Steps

This is what we are working with, the kick drum in red and the chords coloured blue. 

 Here is the mixing view I have adopted for this stage (check out this guy's video on editing Reaper screensets to maximise efficiency) it can be found in the mixer section of reaper. You will see two faders, one for each track, not unlike a real mixing desk. You can see above the track titles, there is a green FX button from which you will add your desired effects and you can see the tracks have ReaEQ and ReaComp respectively (the EQ plugin is just to make the kick more audible on shitey speakers).                                                                                                                                                  Now for the fun bit. This is where Reaper truly excels as a DAW, the routing options. What we need to do is send the audio from the kick channel to the Chord (or bass) channel.                                                                                                                                                                                 Sending tracks can be complicated and I found it daunting at first so I do apologise. If we were to just send the kick audio through to the chords track, we would hear the kick even if we muted that channel since it is also playing through the chords channel. Normally a stereo channel has two "noises", 1 and 2, one for each ear. So if we were to send the kick sound to sections 3 and 4 of the chord track, then it wouldn't be audible since only 1 and 2 go out to the speakers. 
The next picture explains this much better!!!

So each track has a button next to FX that says, among other things, " I/O ". Clicking this opens up the track send and receive menu. If we click on the kick drum's I/O and select add new send, we can choose any existing track to send the audio to. 

There will be a menu beneath that option once the track is sending and you can pick how it sends. For example you can choose Audio: 1/2 => any option. If you select it to send to 1/2 then it will be audible in the track it sends to. We don't want this right now. So we select it to send to 3/4. The next selection asks whether to send MIDI data or not. I have not yet found a practical use for this so be sure to select to none.

We are now ready to look at our compressor.

16 words short of word count reaching 1000 words. I feel bad for not even showing the compressor we will use yet.

By clicking FX on the chords channel and selecting ReaComp, we can set up the compressor to be activated by the kick. The bit we want right now is the Detector Input. Normally it would say 
Main Input L+R (the original sounds e.g. the chords) but we want it to detect the Kick coming in on 3/4 so we select Auxiliary Input L/R which corresponds to 3/4. Now try playing the song and you will find that the threshold control "VU" will light up green in correspondence with the kick sounding and not the chords as would previously have happened. See below:

Now all you need to do is drop the threshold to below the max volume of the kick and increase the ratio.

My tips are to play around with these three controls: Threshold, Ratio and, Release until you get a bouncing sound on your chords or bassline in between the kicks sounding.