Without going into too many details, you could say that the Chorus is made up of one or two delay lines – depending on whether it's mono or stereo – whose delay times can be modulated by an LFO with variable Depth and Rate. The idea behind this effect is to basically double an instrument playing unison by introducing slight tone and sync variations. But, in my opinion, limiting the use of the Chorus effect to this application during mixdown would be neglecting other very interesting possibilities it offers. In fact, this household effect in recording studios can offer much more than it seems at first glance...
But let's start at the beginning: The Chorus can work wonders on choirs and strings, making them thicker. It can also liven up ordinary synthetic sounds by providing them some personality.
Then again, considering that it has a tendency to smoothen sounds, the Chorus effect can also be very useful to rein in an instrument that is too aggressive. Along the same lines, it can also push an instrument to the back of the mix without the need of any reverb. This makes it an ideal tool whenever you want to make an instrument step back a bit when the arrangement of the song is too busy, without making the song denser with a trailing reverb tail.
On acoustic guitars, a chorus can add some density while enlarging the sensation of stereo. On the main vocals, it can be used to mask slight out-of-tune issues. And even on a bass line it can work wonders if correctly adjusted, because it can soften the sound while at the same time making it thicker in the background. A positive side effect of this is that the bass leaves more room for the punch of the kick. Interesting, isn't it?
Finally, it can also be useful to insert a subtle chorus right before a reverb whose diffused sound field is a bit "static." It's an elegant way to restore the image of a poor-quality reverb providing it an extra touch of liveliness.
Regarding the settings of a chorus, there are no rules on how to dial it in, except for one: the denser the arrangement, the more pronounced the effect ought to be for it to be effective. In other words, this means that the modulation ought to be more intense and the balance between dry and wet signals ought to tilt towards the latter. But be careful! For the effect to remain transparent, it's wise to keep the settings low enough so the tonal alterations aren't clearly perceived as such.
Tools of the trade
Here's a list of the plug-ins I personally use when I need a Chorus. As usual, there certainly are hundreds more that are just as good or even better, but you'll also surely understand that it's impossible and futile to try to list them all here.
The good news is that the two plug-ins I'm about to name are free, yes like in free beer! I'll start by mentioning the Chorus developed by French software maker Blue Cat Audio, which despite its minimalist GUI still manages to get the job perfectly well done. And then there's the more recent Multiply by Norwegians Acon Digital Media. And if you are willing to spend some money on a nice Chorus plug-in, you could always give the excellent Thorus a try.
See you next week to discuss the Flanger.