I heard about this on NPR months ago.  This guy in Britain invented a device called "the Mosquito".  Its a speaker you can install on a storefront (or anywhere really) that emits a piercing ultrasonic whine that most adults can't hear.  In fact the older the person is, the less likely they can hear it.  Young people on the other hand, can hear it, and find it (as you might imagine) extremely annoying.  So if you're a shopowner who has teens loitering around your store you can install one of these devices to drive them away.  In June NPR carried a story with a new twist on the mosquito...

Apparently some kids have found a way to turn the high pitched whine into a ringtone on their phones.  Now they can receive text messages on their phones in school, and adults can't hear them beeping.  This is all very amusing I suppose, but today was the first day I had a chance to listen to the sound.  There's a link to play an MP3 recording of it on NPR's site.

The first time I clicked it I could see the waveform on my Real Player visualizer, but I couldn't hear a thing.  So I cranked the volume on my computer and tried again.  Now I could just faintly hear an exceedingly high pitched tone.  It was very faint, and slightly annoying.  I was about to try again when my 9 year old daughter suddenly burst into the room with her face in a grimace and said "what is that awful noise!?"  I explained to her "it's a sound kids and teens can hear but grownups can't."  I turned the volume down and played it again and she immediately clapped her hands over her ears with a little gasp.  In the kitchen, the dogs started barking.

I shut it off and she explained that not only could she hear it clearly, but that it was painful to hear.  "I can feel the sound in my throat and head.  Please don't play that noise again."  So I won't.  But I can assure you that the product works.  Five seconds of it was enough to irritate a 9 year old two rooms away.  There may be more to it though.  Although my wife and I could only just barely hear it, we both got headaches shortly after listening to it.  So if you are a store owner, maybe it's not something you want to run all the time.