thats not true that its always irreversable, ears hair cells do grow back
not saying that it cant be irreversable, but not always, not even typically
"There are two ways hearing can be damaged by loud noises," Auer says. "Noise can stress the stereocilia bundle so much that the tip links break. However they usually grow back in 24 hours — this is the rock-concert effect, where hearing loss is temporary. But loud noises can also shear off whole bundles of stereocilia. In mammals these can't regenerate, and the loss is permanent."
http://www.lbl.gov/Science-Articles/...ution-gap.html