Chorus is aptly named so as it attempts to emulate the effect you would hear in a vocal choir, or any other setting where two instruments are playing a part in unison. The only caveat here is that the effect typically works in the pitch domain. Some definitions of chorus will include differences in the time domain (IE delay around 30ms) but chorus guitar pedals typically don’t work in that way.
To understand this effect, we need to understand beat frequencies. Beat frequencies occur as a third “note” when two pitches which are slightly out-of-tune are played against each other. For example, a note is played at 60hz and another note is played at 61hz. The resulting beat frequency will be at 1hz or one cycle per second. This is perceived as a wavy-ness in the sound. This creates the unique choral effect we associate with vocal choirs. For the purposes of guitar, we only experience one more “voice” as opposed to multiple doubling voices in a vocal choir (e.g. 4 tenor singers). The guitar’s signal is processed to modulate in pitch at a specified rate and amount and then mixed back in with the original signal.
If you’ve read my other articles on effects, you might realize by now that a common theme is that less is more. It’s not a rule so much as a guideline to keep you out of trouble. You can get away with a little more when it comes to the chorus effect but it will start to give it a surreal and metallic quality if you mix in too much of the processed signal.
This parameter controls the amount that the original pitch is modulated in pitch. In other words, think about it like you’re controlling how out-of-tune two voices are to each other.
This controls the rate at which the pitch (set by the depth parameter) modulates at. In other words, you can think of it like being able to control the speed of a vibrato by a vocalist in a choir. Typically, depth and speed will have an inverse relationship: the more depth you use the less the speed will be needed to have an audible difference.
Some interesting things can happen when we use extreme settings with this effect. The most notable user of extreme chorus settings is John Scofield. He attempts to recreate the warbling sound of a rotary (Leslie) speaker that is typically used by electric organ players. This involves a high rate/speed setting and a mid to high level of depth. Another cool use is to set the mix so that it is 100% wet, so you only hear the affected signal and none of the original guitar sound. This reminds me of the sound of tape warping where the speed of the tape reel affects the pitch of the music. The possibilities are endless!