A few months ago, some Welch businesses started broadcasting a high-frequency tone outside places that they didn't want teenagers loitering. Apparently as you age, you lose the highest frequencies of your hearing. But then someone realized the reverse implications of this, and made it a ringtone. So kids can hear their phones ring, but adults can't. Brilliant.
You can listen here or here to see which you can hear. I'm not in "super freak" territory, but I could hear most all but the top tones. And although I was suspicious that it would really work - because after all I'm not a teenager - my father couldn't hear any. So I'm glad I wear those earplugs at concerts and clubs after all; it seems to be working.
(And by the way, as the blog above notes, do not listen to these over and over, they do make your ears hurt even if you can barely hear them.
So what can you hear?