Take this Free hearing loss high frequency soundboard test based on flash. See if you can hear the moskito ringtone.
It's pretty cool to hear something loud and clear that other people can't hear at all, even though they have seemingly good hearing.
The author speculates the high frequency tones can be used to ward off teens in certain shopping areas, or even be used in cell phones so that only the teens can hear it ringing.
None of these suggestions seem very likely at all. For example, let's say 90% of people over 50 can't hear pitches over 19000. 70% of teens can hear the frequency. You'll be losing 10% in sales... is that worth the 70% of the teens you run off?
That logic is somewhat questionable, since some businesses will say, yes. The clear answer to why this wouldn't be used is in the user commentary on the blog. Many people reported not being able to hear the tones, but that the tones caused discomfort, anywhere from headaches to feeling like getting an anxiety attack.
To ward off youths with it would require a certain intensity. Doing so, even though the older customers can't hear it, may cause discomfort, leading visitors wanting to go home or left with an inexplicable unease about the store in the future. Given all the ipod'ing and illegal downloading youths do now days it wouldn't be surprising that they're unphased by the tones.
It would be good to study what kind of effect lower than audible frequencies have on people. Perhaps a harmonic. It could be it has a soothing effect, just as too high a pitch seems to cause headaches and discomfort.
Could inaudible pitches cause hearing loss. Since 99% of the population can't hear pitches above 25,000 Hz, what if there was a blaring 30,000 Hz at max intensity. For example a super computer or some kind of machine. I'd imagine it's a no since the sensitive vibrations wouldnt even register.
Recently it's shown that kicking out unescorted teens after 6pm or so is better for business. Many teens just come to loiter and hang out with friends, deterring older customers from coming to the mall from the disorder and roudiness. The high frequency pitches seemed a quick solution, but it's unlikely to become implemented because of the large conditionals.