U2 have released 13 studio albums and are one of the world's best-selling music artists of all time, having sold more than 170 million records worldwide. In addition, the Edge has described U2 as a fundamentally live band. Bono has nurtured his falsetto operatic voice and has exhibited a notable lyrical bent towards social, political, and personal subject matter while maintaining a grandiose scale in his songwriting. The Edge has consistently used a rhythmic echo and a signature delay to craft his distinctive guitar work, coupled with an Irish-influenced drone played against his syncopated melodies that ultimately yields a well-defined ambient, chiming sound. This approach is rooted partly in the early influence of record producer Steve Lillywhite at a time when the band was not known for musical proficiency. Since their inception, U2 have developed and maintained a distinctly recognisable sound, with emphasis on melodic instrumentals and expressive, larger-than-life vocals. Their lyrics, often embellished with spiritual imagery, focus on personal themes and sociopolitical concerns. Throughout the group's musical pursuits, they have maintained a sound built on melodic instrumentals. U2's early sound was rooted in post-punk but eventually grew to incorporate influences from many genres of popular music. Formed in 1976, the group consists of Bono (lead vocals and guitar), the Edge (guitar, keyboards, and backing vocals), Adam Clayton (bass guitar), and Larry Mullen Jr.