capt_noonie, I read about something recently that made me think of you. Apparently there is some device that's like a dog whistle, but for teenagers!
I guess adults tend to lose the ability to hear very high frequencies, but teenagers with their young ears still can. So you can buy this device that emits a sound very annoying to those with young ears... basically teenager repellent. I don't know how dogs / young kids deal with it, though.