From Brian Johnson himself, on deciding to hang it up...
Brian Johnson explains bowing out of AC/DC tour - CNN.com
""On March 7th, after a series of examinations by leading physicians in the field of hearing loss, I was advised that if I continue to perform at large venues, I risked total deafness," Johnson said in a statement. "While I was horrified at the reality of the news that day, I had for a time become aware that my partial hearing loss was beginning to interfere with my performance on stage."
Johnson, who joined AC/DC in 1980 after the death of original singer Bon Scott, said he was having trouble hearing the guitars and other musicians on stage.
AC/DC reschedules shows to save lead singer's hearing
"Our fans deserve my performance to be at the highest level, and if for any reason I can't deliver that level of performance I will not disappoint our fans or embarrass the other members of AC/DC," Johnson said. "I am not a quitter and I like to finish what I start, nevertheless, the doctors made it clear to me and my bandmates that I had no choice but to stop performing on stage for the remaining shows and possibly beyond." "