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The combination of loud and heavy blues rock with psychedelic rock and acid rock formed much of the original basis for heavy metal. The variant or subgenre of psychedelic rock often known as "acid rock" was particularly influential on heavy metal; acid rock is often defined as a heavier, louder, or harder variant of psychedelic rock, or the more extreme side of the psychedelic rock genre, frequently containing a loud, improvised, and heavily distorted guitar-centered sound. Acid rock has been described as psychedelic rock at its "rawest and most intense," emphasizing the heavier qualities associated with both the positive and negative extremes of the psychedelic experience rather than only the idyllic side of psychedelia. In contrast to more idyllic or whimsical pop psychedelic rock, American acid rock garage bands such as the 13th Floor Elevators epitomized the frenetic, heavier, darker and more psychotic psychedelic rock sound known as acid rock, a sound characterized by droning guitar riffs, amplified feedback, and guitar distortion, while the 13th Floor Elevators' sound in particular featured yelping vocals and "occasionally demented" lyrics. Frank Hoffman notes that: "[Psychedelic rock] was sometimes referred to as 'acid rock'. The latter label was applied to a pounding, hard rock variant that evolved out of the mid-1960s garage-punk movement. ... When rock began turning back to softer, roots-oriented sounds in late 1968, acid-rock bands mutated into heavy metal acts."