It seem's to be the high frequency stuff (hissing airlines) which cause the most damage, ot at least, cause damage earlier.
I think it's due to the shortness of the hair cells responsible for detecting sound in this frequency range, (bit like a guitar string- more likely to snap when shortened.)
A couple of good references at
http://gpnotebook.com/simplepage.cfm?ID=-1294991353 as a starting point.
