Acid rock in the context of "Fuzzbox"

Distortion and overdrive are forms of audio signal processing used to alter the sound of amplified electric musical instruments, usually by increasing their gain, producing a "fuzzy", "growling", or "gritty" tone. Distortion is most commonly used with the electric guitar, but may be used with other instruments, such as electric bass, electric piano, synthesizer, and Hammond organ. Guitarists playing electric blues originally obtained an overdriven sound by turning up their vacuum tube-powered guitar amplifiers to high volumes, which caused the signal to distort. Other ways to produce distortion have been developed since the 1960s, such as distortion effect pedals. The growling tone of a distorted electric guitar is a key part of many genres, including blues and many rock music genres, notably hard rock, punk rock, hardcore punk, acid rock, grunge and heavy metal music, while the use of distorted bass has been essential in a genre of hip hop music and alternative hip hop known as "SoundCloud rap".

The effects alter the instrument sound by clipping the signal (pushing it past its maximum, which shears off the peaks and troughs of the signal waves), adding sustain and harmonic and inharmonic overtones and leading to a compressed sound that is often described as "warm" and "dirty", depending on the type and intensity of distortion used. The terms distortion and overdrive are often used interchangeably; where a distinction is made, distortion is a more extreme version of the effect than overdrive. Fuzz is a particular form of extreme distortion originally created by guitarists using faulty equipment (such as a misaligned valve (tube); see below), which has been emulated since the 1960s by a number of "fuzzbox" effects pedals.

Heavy metal (or simply metal) is a genre of rock music that developed in the late 1960s and early 1970s, largely in the United Kingdom and United States. With roots in blues rock, psychedelic rock and acid rock, heavy metal bands developed a thick, monumental sound characterized by distorted guitars, extended guitar solos, emphatic beats and loudness.

In 1968, three of the genre's most famous pioneers – British bands Led Zeppelin, Black Sabbath and Deep Purple – were founded. Though they came to attract wide audiences, they were often derided by critics. Several American bands modified heavy metal into more accessible forms during the 1970s: the raw, sleazy sound and shock rock of Alice Cooper and Kiss; the blues-rooted rock of Aerosmith; and the flashy guitar leads and party rock of Van Halen. During the mid-1970s, Judas Priest helped spur the genre's evolution by discarding much of its blues influence, while Motörhead introduced a punk rock sensibility and an increasing emphasis on speed. Beginning in the late 1970s, bands in the new wave of British heavy metal such as Iron Maiden and Saxon followed in a similar vein. By the end of the decade, heavy metal fans became known as "metalheads" or "headbangers". The lyrics of some metal genres became associated with aggression and machismo, an issue that has at times led to accusations of misogyny.

Psychedelic music (sometimes called psychedelia) is a wide range of popular music styles and genres influenced by 1960s psychedelia, a subculture of people who used psychedelic drugs such as DMT, LSD, mescaline, and psilocybin mushrooms, to experience synesthesia and altered states of consciousness. Psychedelic music may also aim to enhance the experience of using these drugs and has been found to have a significant influence on psychedelic therapy.

Psychedelia embraces visual art, movies, and literature, as well as music. Psychedelic music emerged during the 1960s among folk and rock bands in the United States and the United Kingdom, creating the subgenres of psychedelic folk, psychedelic rock, acid rock, and psychedelic pop before declining in the early 1970s. Numerous spiritual successors followed in the ensuing decades, including progressive rock, krautrock, and heavy metal. Since the 1970s, revivals have included psychedelic funk, neo-psychedelia, and stoner rock as well as psychedelic electronic music genres such as acid house, trance music, and new rave.

Psychedelic rock is a subgenre of rock music that originally emerged during the mid-1960s, inspired by psychedelic culture and primarily centered around the influence of psychoactive and hallucinogenic drugs. The music incorporated new electronic sound effects and recording techniques, extended instrumental solos, and improvisation. Many psychedelic groups differ in style with the label often applied spuriously.

Originating in the mid-1960s among British and American musicians, the sound of psychedelic rock invokes three core effects of LSD: depersonalization, dechronicization (the bending of time), and dynamization (when fixed, ordinary objects dissolve into moving, dancing structures), all of which detach the user from everyday reality. Musically, the effects may be represented via novelty studio tricks, electronic or non-Western instrumentation, disjunctive song structures, and extended instrumental segments. Some of the earlier 1960s psychedelic rock musicians were based in folk, jazz, and the blues, while others showcased an explicit Indian classical influence called "raga rock". In the 1960s, there existed two main variants of the genre: the more whimsical, surrealist British psychedelia and the harder American West Coast "acid rock". While "acid rock" is sometimes deployed interchangeably with the term "psychedelic rock", it also refers more specifically to the heavier, harder, and more extreme ends of the genre.

Blue Cheer was an American rock band that initially performed and recorded in the late 1960s and early 1970s and was sporadically active until 2009. Based in San Francisco, Blue Cheer played in a psychedelic blues rock or acid rock style. They are also credited as being some of the earliest pioneers of heavy metal, with their cover of "Summertime Blues" sometimes cited as the first in the genre. They have also been noted as influential in the development of genres as disparate as stoner rock, experimental rock, and grunge.

Iron Butterfly is an American rock band formed in San Diego, California, in 1966. They are best known for the 1968 hit "In-A-Gadda-Da-Vida", providing a dramatic sound that led the way towards the development of hard rock and heavy metal music. Although their heyday was the late 1960s, the band has been reformed with various members with varying levels of success with no new recordings since 1975. Their second album, In-A-Gadda-Da-Vida (1968), remains a best-seller, and Iron Butterfly was the first group to receive an in-house platinum album award from Atlantic Records.

According to music critic Manish Agarwal of Time Out, "Iron Butterfly blended hard rock with ordinate, acid-friendly textures." Mark Deming of AllMusic stated that they were one of the first musical groups to fuse the two styles in this way, and described their sound as a "blend of trippy musical exploration and open-ended jams with a hard, distorted attack".