This would be very useful for a lot of things if it works.  I'm sure the police would love this.  They could "see" who was listening on a phone call.
 
 
http://www.newscientist.com/article/mg20227035.200-do-our-ears-have-builtin-passwords.html
 
What sparked the interest of Beeby and his colleagues is the fact that the power and frequency distribution in the OAEs provoked by specific series of clicks seem to be highly distinctive, driven by the internal shape of the person's ear. "Anecdotally, audiologists say they can tell different people apart - men, women, even people of different ethnic origins - by the profile of the widely varying types of emissions the clicks evoke," he says.