Reverb

       "Tony Visconti rigged up a system, a creative misuse of gating that may be termed "multi-latch gating", of three microphones to capture the epic vocal, with one microphone nine inches from Bowie, one 20 feet away and one 50 feet away. Only the first was opened for the quieter vocals at the start of the song, with the first and second opening on the louder passages, and all three on the loudest parts, creating progressively more reverb and ambience the louder the vocals became.  Each microphone is muted as the next one is triggered. "Bowie's performance thus grows in intensity precisely as ever more ambience infuses his delivery until, by the final verse, he has to shout just to be heard....The more Bowie shouts just to be heard, in fact, the further back in the mix Visconti's multi-latch system pushes his vocal tracks, creating a stark metaphor for the situation of Bowie's doomed lovers." 

  http://www.soundonsound.com/sos/Oct04/articles/classictracks.htm