Within an hour of completing the last overdubs on the album's songs, on 20 April 1967, the group returned to Harrison's "Only a Northern Song", the basic track of which they had taped in February. The Beatles overdubbed random sounds and instrumentation before submitting it as the first of four new songs they were contracted to supply to United Artists for inclusion in the animated film "Yellow Submarine". In author Mark Lewisohn's description, it was a "curious" session, but one that demonstrated the Beatles' "tremendous appetite for recording". During the "Sgt. Pepper" sessions, the band also recorded "Carnival of Light", a McCartney-led experimental piece created for the Million Volt Light and Sound Rave, held at the Roundhouse Theatre on 28 January and 4 February. The album was completed on 21 April with the recording of random noises and voices that were included on the run-out groove, preceded by a high-pitched tone that could be heard by dogs but was inaudible to most human ears.