There’s been quite a bit about these new gadgets in the British press recently - I can’t seem to find a story on them online, though.
Basically, the idea behind them is that when installed in a spot where teenage yobs congregate, they emit a high-pitched annoying noise, that can only be heard by people under 20. Adults over that age don’t notice it.
How do they work?
They emit a high-pitched noise that can usually only be heard by young people. Sensitivity to high-pitched noises decreases steadily as one gets older. As a young person in my office, I can hear odd sounds made by computer fans that my 50-something coworker can’t, and I’m sure my youngest brother (who is 13) can hear noises that I can’t.
Is there an increase in sensitivity to low-pitched noises as you get older, too? Or is it just a general decline?
Seems to be just a general decline; exposure to loud noise kills the hairs in your ears that vibrate in response to sound, and they don’t seem to regenerate. The more loud noise the ear is exposed to, the faster the decline (hence the hearing-loss warnings on earphones and the like) although there is also a natural die-off with age.
In effect, as most people get older, their hearing tends to drop off in both sensitivity to volume as well as sensitivity to higher pitches.
Is the purpose of this thing to discourage loitering or something?
I’m 27 and it still hurts when a TV is on without a signal input.
Yes. The idea is that young punks will be discouraged from congregating in front of the store, while older people will be able to walk in and out without noticing anything’s amiss.
Older people without children, that is. So there goes your mid-30s young family demographic.
“Mommy, I hate this store!”
“Shut up and come inside.”
Parents rarely listen to their children. I don’t think that would matter.
I’m 36 and can still hear up to 16-18kHz (based on fiddling with sound generators at work, not on controlled tests or anything). I’m sure I’m not the only one. I for one think that intentionally flooding a place with noise pollution is a mindbogglingly stupid idea. If you want to convince the young punks to move on, play Mozart at them. They’ll either go away, or become cultured and not a problem. 
Of course what the geniuses who thought this up have failed to realise is that the youngsters won’t be able to hear their pesky noise over the din from their iPods. 
Source: http://www.breitbart.com/news/2006/03/24/060324175603.gg50bf2u.html
Maybe Slim Whitman hits instead?
A couple of unmanned railway stations in the UK managed to drive gangs of teenage vandals and yobs away by playing classical music over the PA system.
Customers would more than likely be going into the store, rather than hanging about outside, it is easy enough to produce a sound that is acceptable for a short while but is irritating to hear over a period of time.
A high pitched whistle that changes tone would be a start, the sound will not carry far so it won’t annoy the neighbourhood.
If the sound stayed at a constant pitch, the listener would become conditioned to it, and the brain would simply block it out, but if it changes and is non-repetitive then the brain has a much harder job of ignoring the information coming in through the earholes.
Hmmm… Humans aren’t moles. There’s no arbirary cutoff for the age of loitering anti-socialites or hearing ability, plus even punk kids are smart enough to defeat such a device: iPods, or even destroying the speaker…In my youth, I might have been inclined to hang out in the area just to defy the attempt to keep me away.
Unlike the classical music example, there’s no “plausible deniability” on this one. I appreciate the need, but this is not an acceptable solution.