On January 11, 1967, Jimi Hendrix left a productive studio session to play a show at a London club called the Bag O’Nails. The crowd was intimidating: Keith Richards, Pete Townshend, Paul McCartney, Eric Clapton. (On the way back from the bathroom, the singer Terry Reid ran into Brian Jones from The Rolling Stones, who said the front of the club was totally wet. When Reid said he didn’t see any water, Jones said it was from all the guitarists, weeping.) But the most important person in the audience was a man named Roger Mayer. Mayer was a scientist for the Royal Navy who conducted acoustical analyses on how to make ships quieter for underwater warfare, but in his free time, he made pedals and sound effects—the strangely doubled guitar at the beginning of “Purple Haze,” for example, was a Mayer invention called the Octavia. Hendrix later called Mayer the secret to his sound, and, for reasons unclear, referred to him as The Valve. Of Hendrix’s three studio albums, what stands out about Axis: Bold as Love is how much it opened up his music as sound: the swirling effects of “EXP” and “Bold as Love,” the watery textures of “Little Wing,” the screaming feedback on “If 6 Was 9.” Hendrix called it the “sky effect”: music that sounded like it was coming down from heaven. Of course, what ultimately made Hendrix’s music vital is how preoccupied it remained with Earth: the heavy blues of “Spanish Castle Magic,” the lust of “Wait Until Tomorrow,” the way “Bold as Love” takes colors and feelings and personifies them—a late-’60s conceit if there ever was one (“My yellow in this case is not so mellow”), but one that captures the dualities of his art. It’s funny to think of someone celebrated so completely for his raw performing abilities as being a technophile, but, in a way, that’s Hendrix’s legacy: Not just an architect of hard rock or a steward of the blues, but someone searching the horizon of sound for new ways to convey old, elemental feelings.