a piano shouldn't sound like a piano, a guitar shouldn't sound like a guitar." In their search for new sounds, the band incorporated musical instruments such as the Indian tambura and tabla, and clavichord, vibraphone and tack piano into their work for the first time. The guitar sound on the album was more robust than before, through the use of new Fender amplifiers; the choice of guitars, which included Harrison using a Gibson SG as his preferred instrument; and the introduction of Fairchild 660 limiters for recording. With no expectations of being able to re-create their new music within the confines of their live shows, the Beatles increasingly used outside contributors while making the album. This included the band's first use of a horn section, on "Got to Get You into My Life", and the first time they incorporated sound effects extensively, during a party-style overdubbing session for "Yellow Submarine". Innovations Author Mark Brend writes that, with Revolver, the Beatles advanced Meek's strategy of employing the recording studio as a musical instrument and "formalized this approach into what is now an accepted option for pop music making". A key production technique they used was automatic double tracking (ADT), which EMI technical engineer Ken Townsend invented on 6 April. This technique employed two linked tape recorders to automatically create a doubled vocal track. The standard method had been to double the vocal by singing the same piece twice onto a multitrack tape, a task Lennon particularly disliked. The Beatles were delighted with the invention, and used it extensively on Revolver. ADT soon became a standard pop production technique, and led to related developments such as the artificial chorus effect. The band's most experimental work during the sessions was channelled into the first song they attempted, "Tomorrow Never Knows". Lennon sang his vocal for the song through the twin revolving speakers inside a Leslie cabinet, which was designed for use with a Hammond organ. The effect was employed throughout the initial take of the song but only during the second half of the remake. According to author Andy Babiuk, "Tomorrow Never Knows" marked the first time that a vocal was recorded using a microphone wired into the input of a Leslie speaker. Much of the backing track for the song consists of a series of prepared tape loops, an idea that originated from McCartney, who, influenced by the work of avant-garde artists