In 1963, the teenage Brian May and his father custom-built his signature guitar Red Special, which was purposely designed to feedback. Sonic experimentation figured heavily in Queen's songs. A distinctive characteristic of Queen's music are the vocal harmonies which are usually composed of the voices of May, Mercury, and Taylor best heard on the studio albums A Night at the Opera and A Day at the Races. Some of the ground work for the development of this sound can be attributed to their former producer Roy Thomas Baker, and their engineer Mike Stone. Besides vocal harmonies, Queen were also known for multi-tracking voices to imitate the sound of a large choir through overdubs. For instance, according to Brian May, there are over 180 vocal overdubs in "Bohemian Rhapsody". The band's vocal structures have been compared with the Beach Boys, but May stated they were not "much of an influence".