Ringtone

I can easily hear this high-pitched ringtone that some teenagers have been using. It's unbelievably annoying.

UPDATE: Here's a very useful page that lets you test the upper limits of your hearing. There are links that let you play tones from 10,000 hertz up to 25,000 hertz. (I can hear everything, although the 25,000 tone is a little fainter than some of the others.)