Just recently tryin' to understand some of this stuff, so I'll tell you straight out that all of this might be wrong (if so, I invite one of the more knowledgeable cats to make rude corrections as necessary)
BUT the way I've been understanding it, high frequency sound waves have a much higher energy content than their low freq counterparts, and so they can propagate further and do more damage to our ears. So at louder volumes, the high energy content is extra lethal to us because the energy dissipates IN OUR EAR and wrecks the little hair cells in there. Probably worth mentioning that we have a nonlinear perception of loudness (look up the 'Fletcher-Munson Loudness Curves'); at low dB-SPL our ears are most sensitive to sounds in the ~1kHz --> ~6kHz band, pretty much what Killawatt was saying. As you turn up the volume, the nonlinearity disappears but by then you're already in the 80+ dB-SPL range, which can cause hearing loss. When you're at that volume level, the bassy frequencies can wreck your ears too, but due to the aforementioned human treble sensitivity you won't notice the bass damage as much. Apparently it's more dangerous on headphones where your ears are right next to the transducer, but I can't imagine that dangerous audio content gets very well absorbed by the air in a club setting either, hahah.
was just going through this article yesterday, more geared towards headphones but some of you might find it useful:
http://gilmore2.chem.northwestern.edu/a ... ng_art.htm
Definitely up for learning more about ear plugs too, gotta preserve hearing!