used 72 Clair Brothers S4 Series II speakers, in positions of stage left and right, rear fill stage left and right, and left and right sidefill. For the European arena shows, the number of S4 Series II speakers was reduced to 56, as rear fill and sidefill audio were not required. Clair Brothers' P4 "Piston" cabinets were also used for nearfield/in-fill audio, with two clusters of six speakers each at stage left and right. Bass was provided by six Servo Drive Bass Tech 7 subwoofers. The sound was mixed by sound engineer Joe O'Herlihy and assistant Robbie Adams with an ATI Paragon console and a Clair Brothers CBA console, aided by an inventory of effects intended to replicate the ones used in the studio during the recording of Achtung Baby. The stage monitor system was mixed underneath the stage with six consoles: two Harrison SM5s (with a 16-channel extender), a Yamaha DMP7, a Soundcraft 200B, and two Ramsa WS-840s for drummer Larry Mullen Jr. The consoles provided capabilities for around 200 audio channels. To avoid audio feedback during B-stage performances, O'Herlihy said, "We 'ring' the system out using a separate EQ". On stage, the monitor speakers consisted of Clair Brothers' 12AM single and double wedge units, with ML18 and MM4T units for sidefill. Steve McCale served as the monitor engineer for Bono, the Edge, and Clayton, and controlled the joystick panning, while Dave Skaff was Mullen's monitor engineer. In-ear monitors were provided by Future Sonics. The production equipment was transported on 11 trucks supplied by Upstaging Trucking. The stage required 13–14 hours to build and 3–4 hours to disassemble. The crew of 75 people travelled on six buses, while the band flew in a chartered plane. North American stadium leg To redesign the stage for the 1992 North American stadium leg—dubbed "Outside Broadcast"—Williams collaborated with stage designers Mark Fisher and Jonathan Park, both of whom had worked on the Steel Wheels Tour stage for the Rolling Stones. The main stage was expanded to be , and the catwalk leading to the B-stage was lengthened to approximately , nearly four times as long as the arena version. The spires of the stage, intended to resemble radio masts, reached as high as , requiring aircraft warning lights approved by the Federal Aviation Administration to be placed on top of them. The stage's appearance was compared to the techno-future cityscapes from Blade Runner