Sgt. Pepper's Lonely Hearts Club Band "Being for the Benefit of Mr. Kite!" and "A Day in the Life". Producer George Martin and engineer Geoff Emerick helped realise the group's ideas by approaching the studio as an instrument, applying orchestral overdubs, sound effects and other methods of tape manipulation. Recording was completed on 21 April 1967. The cover, depicting the Beatles posing in front of a tableau of celebrities and historical figures, was designed by the British pop artists Peter Blake and Jann Haworth. "Sgt. Pepper" is regarded by musicologists as an early concept album that advanced the use of extended form in popular music while
Revolver (Beatles album) as the Beatles'. While also highlighting the importance of the production, David Howard writes that "Revolver" was a "genre-transforming album", on which Martin and the Beatles had "obliterated recording studio conventions". Combined with the similarly "visionary" work of American producer Phil Spector, Howard continues, through "Revolver", the recording studio had become "its own instrument; record production had been elevated into art." Whereas "Sgt. Pepper" had long been identified as the Beatles' greatest album, since the 2000s "Revolver" has often surpassed it in lists of the group's best work. Sheffield cites the album's 1987 CD release, with the full complement of