A chorus effect is an auditory effect obtained either by modulating both temporal and frequency characteristics of a discrete audio signal with a signal processing unit or by superimposing waveforms of a complex audio source.  It is usually related to audio in music, as the effect can be perceived only if the waveform has tonal qualities. The effect is described as "thick," "rich," adding subtle tonal movement, "shimmering quality" and "dreaminess" to a sound, making the impression that it is doubled in quantity.  In most cases, the processed effect is associated with vocals, electric guitars, basses and electric pianos while it can be heard naturally in choirs, synthesizers, mandolins, accordions, string and brass ensembles, acoustic twelve-string guitars and out-of-tune acoustic pianos due to microtonal differences between individual instruments or sound sources.
The original chorusing effect was made by combining two reel-to-reel tape recorders, one being slightly out of sync with the other, whose power supply was connected to an oscillator. The subtle differences between their pitch, timbre and time would create the effect. This analog recording technique (later named ADT – Automatic Double-Tracking) had been developed in 1966 by Ken Townsend, a recording engineer who worked at EMI's Abbey Road Studios during that recording period of the Beatles. Instantly doubling the source during the recording process delivered a fuller, richer quality to the sound, and reduced the time of producing a song. 
The first audio signal processing units that were able to simulate the chorus effect were made during the 1970s, most notably Boss Chorus Ensemble and Rockman's Stereo Chorus  famously made by Tom Scholz.
Setting the chorus
Guitar and bass chorus effect may be simulated with an audio signal processing unit, which is in many cases seen in a pedal format (known as chorus pedal),  as rack mount effect, built in an instrument amplifier (usually for electric and acoustic guitars or pianos ) or a software version made for computer use.
The effect is based on doubling the input signal and delaying the copied signal by a certain amount of time, ranging from 5 to about 40 milliseconds. The delayed audio is then blended with the original signal and sent to the output of the effects unit. The movement effect is created by a low-frequency oscillator that constantly alters the delay time of the chorus. As it cycles its time value, it is slightly shifting the pitch of the original signal by a small amount. 
Common chorus parameters include depth (amplitude or intensity), delay (or time), rate (or period), feedback and level (sometimes named dry/wet mix).
- Depth is the amplitude level of the delayed signal.
- Delay is the overdue time of the copied signal.
- Rate depicts the modulating frequency of the low-frequency oscillator that affects the fluctuation of the delay time.
- Feedback is the level of the wet signal that is being sent to the filter's buffer.
- The level parameter manipulates the ratio of the modulated signal and the original, unaffected one.
There are many types of chorus effects, most notably differentiated by their sonic outputs, number of voices and complexity of its LFO signal. A chorus may be mono, stereo or surround. A mono chorus delivers a modulated signal to a mono output. Stereo chorused usually affect stereo input signals that may share the same modulation rate, but the stereo delay is commonly shifted apart by 180 degrees out of phase, resulting in widening the stereo image of the effect. Surround choruses are similar to stereo versions of the effect, but they modulate each input channel independently by all parameters. Basic chorus units are usually single-voiced effects – meaning that they use a single delay and manipulate a solitary copy of the input signal. Multi-voice choruses have more than one delayed signal, creating a richer sound with more "shimmer" and movement. They may have different LFO values, or they might use a single LFO value starting of a different phase of the cycle. Single LFO choruses sometimes may sound obvious, so using multiple LFO signals benefits more subtle chorus choices, hiding wobble effect that in many cases isn't desirable. 
Phasing and flanging are considered to be effects related to choruses but vary their parameters and modulation source.  Phasers use multiple frequency filters instead of utilizing delays, thus creating comb filtering effects that vary in different rates. Flangers use shorter delay times than choruses and more delay feedback, creating a specific "zipper" noise.
Notable examples of the chorus effect
There are countless recordings that utilize the chorus effect:
The Police - "Walking on the Moon"
Nirvana - "Come as You Are"
Daft Punk - "Get Lucky"
Metallica - "Welcome Home"
Boston - "More Than a Feeling"
Guns N' Roses - "Paradise City"
Pink Floyd - "Run Like Hell"
- ↑ Center for Computer Research in Music and Acoustics (CCRMA), Stanford University "Chorus Effect"
- ↑ 1 2 TestTone.com "What is a Chorus Effect?"
- ↑ Of Buckley and Beatles "What's That Sound? Beatles Production Tricks: Part 1. 'ADT'"
- ↑ Rockman.fr "Rockman Stereo Chorus/Delay"
- ↑ GuitarToneOverload.com "How to Use Modulation Effects Part 3: the Chorus"
- ↑ TheHub.MusiciansFriend.com "Tech Tip: What is a Chorus Effect?"
- ↑ whatis.techtarget.com "Chorus"
- ↑ Gibson.com "Effects Explained: Modulation—Phasing, Flanging, and Chorus"