Ozzy, maybe Geezer, seems to be wailing through bong bubbles here. Sometimes mistaken for a vocoder, it's a signal-processing trick called ring modulation, patented by the Daleks of Dr. Who, and later used by Can. “Planet Caravan” is so quiet—or stoned—you think the album’s over. Then Iron Man shows up with his Talkbox and stomps a mudhole in the village.
#46. Black Sabbath "Planet Caravan"