Whereas Bono exhibited a full-throated vocal delivery on the group's previous releases, for "Achtung Baby" he extended his range into a lower register and used what Fast described as "breathy and subdued colors". On many tracks, including "The Fly" and "Zoo Station", he sang as a character; one technique used is what Fast called "double voice", in which the vocals are doubled but sung in two different octaves. This octave differentiation was sometimes done with vocals simultaneously, while at other times, it distinguishes voices between the verses and choruses. According to Fast, the technique introduces "a contrasting lyrical idea and vocal character to deliver it", leading to both literal and ironic interpretations of Bono's vocals. For several tracks, his vocals were treated with processing. These techniques were used to give his voice a different emotional feel and distinguish it from his past vocals.