Recording Tips: Side Chain Compression

Now that we have covered the basics on both the compressor and the EQ, we can now start discussing a few of the cool ways to use these tools to improve our recordings. For this discussion, I’ll be discussing a very useful compressor function for those working with voice; Side chain compression.

Side chain compression is feeding one audio source into the compressor to trigger the compression of another audio source. While this can be used in many different ways, I will only focus on its basic use for voice. The side chain compressor is most frequently used for “ducking” or turning down a sound to get out of the way of another sound. You might use this if you are mixing a music bed underneath a voice over track. If set up to do so, this will allow the music to be automatically turned down when the voice track is running and swell back up once the track is silent. Some compressors have this built in and can simply be switched to side chain mode, but it is more likely that you will have to seek one of these out. If you do not have one, there are some good free or very cheap ones that can be downloaded with very little searching. Please realize that every compressor is a little different in its functionality, in this example I’m using a basic Digirack Compressor/Limiter. I will explain how to use a side chain compressor to “duck” a music bed under a voiceover track. Here are the basics:

First you will need to route your “Key Input signal” (the signal you want to be used to trigger the compressor), in this case your voice track, through a bus. Next, place the compressor on the music track to be “ducked” and set the compressor to side chain mode (also sometimes called key input mode). On the Digirack compressor clicking the button that has a key on it does this. Next you will need to let the compressor know what signal it is looking for to be triggered. Look for something that is labeled “key input signal” or similar. This will need to be set to the bus you assigned to your voice track. At this point all of the signal routing is complete and all that is needed is to tweak the settings to “duck” the music track based on the voice track. These settings will be very different depending on the tracks that you are using but I will typically use a fairly slow attack (100-150 ms) and very slow release or decay (600 ms – 1 s) to make this effect as unapparent as possible. I also use a fairly high ratio (6:1 or higher) to limit the music track when the voice track is playing and adjust the knee of the compressor (usually soft) to make the volume transitions as seamless as possible. Also, you will not need any gain compensation for this. The threshold will be dependent on the level of your recorded audio, but all of these parameters can be tweaked until it sounds right to your ears. Find what feels right to you and go with it.

Using a side chain compressor can save you a lot of time when mixing voice over on top of music and will certainly prevent the music from overpowering the voice. It can feel very confusing at first, but once you get it, my bet is you will find yourself using it on just about any project that has these two components.