This is completely anecdotal...
As you know, kids like to put high frequency (16 kHz) ring tones on their cell phones so that adults (ie. authority figures) with compromised hearing cannot hear the phone ring.
Yesterday, my son asked "why is it that I can hear the first ring loudly, but by the third or fourth ring I cannot hear it anymore?"
I didn't have an answer, of course. But if you'd like me to investigate the phenomenon further with him, I can do that. This isn't horribly scientific, of course, but maybe useful.