I got this link from a caver message board I visit. It talks about the fact that hearing at high frequencies drops off with age, and that high frequency tones can be used to either repel younger folks from areas they congregate or as teenage cellphone ringtones that adults can't hear. I checked my hearing using my laptop, and I can't hear above 13KHz. I'm sure it's because of the darn laptop speakers...
http://www.ochenk.com/entry.php?id=63
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