Generally speaking, a modulation effect essentially doubles the signal flow (in this case, the sound of an instrument like the guitar) and alters it. It then combines the effected (or wet) signal with the original (dry) signal in order to produce the effected tone. Because guitar effects are such an intricately designed and manipulatable facet of sound design, you should try it out either with your own rig or gear resembling your rig as closely as possible. However, there are still some guidelines and concepts that you can learn about that can help you make a more informed decision about your purchase.
Phaser Effects
Phasing, also referred to as phase shifting, is where a signal is duplicated, and then the new copied signal is shifted to be out of phase with the original signal. The resulting sound creates a spacey, “whoosh” effect evokes the sound of a watery atmosphere.
While signal phasing can sometimes be an unwanted by-product of audio recording, the effect began to be used intentionally on psychedelic records in the late 1960s, notably on “Itchycoo Park” by the Small Faces. The effect was later popularized in decades to follow from guitarists such as Eddie Van Halen, Queen’s Brian May, and Incubus axeman Mike Einziger.
Flange Effects
Similar to phasing, flanging duplicates the original signal of a guitar’s output but delays one signal by a small and gradually changing period. The resulting sound is similar to a comb filter effect, one that has been characterized as “sucking air” or “the Darth Vader effect”. Whether used subtly to suggest a spacey feel or cranked all the way up to create an unnatural, synthetic-like sound, flanging is a guitar effect that doesn’t seem to be losing popularity anytime soon.
Although wildly popular as a guitar effect, flange was first popularly utilized by producer George Martin during recording sessions for the Beatles. John Lennon would often use the effect on his vocals; in fact, historian Mark Lewisohn claims that it was Lennon who first gave the technique its name. The first Beatles song to feature this effect was “Tomorrow Never Knows” on their album Revolver; almost every single song on that record had some sort of flanging effect on it.
Tremolo Effects
Tremolo produces a sound similar to flanging in that it creates a vibrating sound similar to what it would sound like if you hit a note on your guitar and then turned the volume knob up and down rapidly. Speed can be adjusted to create alternating volume fluctuation rates, while the depth controls on a tremolo pedal affects the range at which that signal is altered.
Of all the effects that fall under the modulation umbrella, tremolo is perhaps the most popular simply because it’s an effect often incorporated as a default setting in guitar amps. Most popularized in niche genres like rockabilly and surf rock, tremolo has also been used prominently in songs by artists such as Radiohead, the Rolling Stones, Creedence Clearwater Revival, and more.
Chorus Effects
Perhaps the most subtle of all the modulation effects, chorus duplicates the original output signal and alters it slightly so that the signal sounds like it’s being voiced by multiple sources or as the name implies, by a “chorus” of instruments. Chorus is a great way to thicken up the sound of a guitar part, and can be used with other effects such as distortion to expand multiple layers of tones within a signal.
Perhaps one of the most instantly recognizable uses of chorus in popular music is the beginning intro guitar riff to “Come as You Are” by Nirvana. Like tremolo, chorus effects are often built into guitar amplifiers; however, working with processors like pedals (which are solely designed to produce this effect) will almost always result in a better and more nuanced sound.