I'd heard about this being used before, but the demonstration was interesting

The authorities in Britain and now New York City have installed sound makers that produce an annoying tone that can generally be heard only by young people as a means to keep away young loiters. On the other hand, some young people use sound at a similar range as ringtones so teachers and other adults can't hear their phone ring.

But this video demonstrates the ranges. Can you hear all of these sounds? I can hear all but the last (the one for under 24s). (But then, I'm 43.)



(I will say that in most of the YouTube videos purporting to have this stealth sound on it, I can hear it. Of course, I can hear whines from televisions and monitors when they're on, if there's no other noise. But this one I couldn't, and one other that claimed to be at the same level.)

Via Joe.My.God.