in "Norwegian Wood" and the swarmandal in "Strawberry Fields Forever". They also used novel electronic instruments such as the Mellotron, with which McCartney supplied the flute voices on the "Strawberry Fields Forever" intro, and the clavioline, an electronic keyboard that created the unusual oboe-like sound on "Baby, You're a Rich Man". Legacy Former Rolling Stone associate editor Robert Greenfield compared the Beatles to Picasso, as "artists who broke through the constraints of their time period to come up with something that was unique and original ... [I]n the form of popular music, no one will ever be more revolutionary, more creative and more distinctive ..." The British poet Philip Larkin described their work as "an enchanting and intoxicating hybrid of Negro rock-and-roll with their own adolescent romanticism", and "the first advance in popular music since the War". The Beatles' 1964 arrival in the US is credited with initiating the album era; the music historian Joel Whitburn says that LP sales soon "exploded and eventually outpaced the sales and releases of singles" in the music industry. They not only sparked the British Invasion of the US, they became a globally influential phenomenon as well. From the 1920s, the US had dominated popular entertainment culture throughout much of the world, via Hollywood films, jazz, the music of Broadway and Tin Pan Alley and, later, the rock and roll that first emerged in Memphis, Tennessee. The Beatles are regarded as British cultural icons, with young adults from abroad naming the band among a group of people whom they most associated with UK culture. Their musical innovations and commercial success inspired musicians worldwide. Many artists have acknowledged the Beatles' influence and enjoyed chart success with covers of their songs. On radio, their arrival marked the beginning of a new era; in 1968 the programme director of New York's WABC radio station forbade his DJs from playing any "pre-Beatles" music, marking the defining line of what would be considered oldies on American radio. They helped to redefine the album as something more than just a few hits padded out with "filler", and they were primary innovators of the modern music video. The Shea Stadium show with which they opened their 1965 North American tour attracted an estimated 55,600 people, then the largest audience in concert history; Spitz describes the event as a "major breakthrough ... a giant step toward reshaping the concert business". Emulation of