"Avant-garde metal" is interchangeable with "experimental metal" and "avant-metal", and may also refer to a separate genre of "atmospheric metal" or "post-metal", which was named in reference to post-rock. Avant-garde metal is related to progressive metal, but avant-garde metal often has more experimentation, while progressive metal usually has a tighter focus on traditional metal instrumentation and higher levels of technical complexity. Avant-garde metal also uses unusual sounds, breaks conventions, and often includes new elements. The lyrics and visual presentation of the genre are eclectic as well. According to Jeff Wagner in Mean Deviation, electronic percussion and drum machines see widespread use by avant-garde metal bands, along with female vocals and operatic elements, all of which he attributes to the influence of the band Celtic Frost. The Canadian group Voivod also influenced future bands in the genre, pioneering technique such as robotic vocal effects, unusual time signatures, and fractured, dissonant, unorthodox guitar sounds.