An effects unit or pedal is an electronic or digital device that alters how a musical instrument or other audio source sounds. In the 2010s, most effects use solid-state electronics and/or computer chips. Some vintage effects units from the 1930s to the 1970s and modern reissues of these effects use mechanical components as well (e.g., Leslie rotating speaker, spring reverb, and tape recorder-based echo effects) or vacuum tubes. Some effects subtly "color" a sound, such as a reverb unit used on a low setting, while others transform it dramatically, such as a distortion pedal used with electric guitar, with the overdrive set to its maximum level. Musicians, audio engineers and record producers use effects units during live performances or in the studio, typically with electric guitar, electronic keyboard, electric piano or electric bass. While guitar effects are most frequently used with electric or electronic instruments, effects can also be used with acoustic instruments, drums and vocals. Rackmounted or audio console-integrated reverb effects are commonly used with vocals in live sound and sound recording. Examples of common effects units include wah-wah pedals, fuzzboxes and reverb units.
Effects are built into amplifiers (reverb and distortion are the most common in 2010s-era amps, but vintage amps and modern reissued amps may also have tremolo and vibrato); housed in table top units designed for DJs and record producers, a format which typically contains multiple effects; "stompboxes" and "rackmount units", or they are built into the instruments themselves, as in the case of the Hammond organ, which includes chorus and vibrato effects inside the instrument chassis. A stompbox or "pedal" is a small metal or plastic box placed on the floor in front of the musician and connected to the instrument and the instrument amplifier with patch cords. Pedals are usually the least expensive format. Typically, one or more on-off foot-operated switches control a device that provides only one or two effects, with many pedals having knobs for controlling the volume, tone and intensity of the effect. A rackmount device mounts on a standard 19-inch equipment rack and usually contains several types of effects.Rackmount effects typically have buttons and/or knobs on the face of the chassis for controlling the effects and a patch bay at the rear of the unit.
While there is currently no firm consensus on how to categorize effects, the following are seven common classifications: distortion/overdrive, also called "fuzz", which produce the "growling" audio clipping sounds that are a key part of electric guitar playing in electric blues, hard rock and heavy metal music; dynamics effects which affect loudness, such as volume pedals and audio compressors; filters which modify the frequency range of the instrument, such as the wah-wah pedal and the graphic equalizer; modulation effects, such as the chorus, flanger and phaser; pitch/frequency effects such as a pitch shifter, a common example of which is an octave pedal, which can shift a note down an octave (or up an octave); time-based effects, such as reverb, echo effect and looper pedals (the latter can be used by a one person band to record a riff and then solo over it); and feedback/sustaineffects, such as electric guitar feedback and the EBow, which are two different techniques for producing pipe organ-like sustain on the electric guitar. Guitarists derive their signature sound or "tone" from their choice of instrument, pickups, effects units, and guitar amp and from the different settings they use with their pickups, effects units and amp.