User blog comment:Lemursrule/BLEACH episode 297 review: BANKAI!/@comment-1590714-20101118060423/@comment-1351982-20101118061134

it did break the sound barrier when Gin released him Bankai. All you could hear were the buildings coming apart. Plus I believe it is stated(can't remember where though) that Gin can control the contraction speed of his Zanpaktou.