Teen Torture (App Store link) is a new iPhone app that promises to annoy teenagers with sound that they can hear, but you can’t.
Teen Torture – Annoy your kids with high frequency sounds that they can hear, but you can’t. As you age you lose the ability to hear high frequencies(called presbycusis). This concept was put to use with the Mosquito alarm, which is an electronic device which emits an ultrasonic sound, similar to a mosquito (~17.4 kHz at 85dB). The device is typically heard by people less than 25. The device is used for preventing behavior such as gang loitering, graffiti, vandalism, drug usage, drug distribution, and violence. Students took advantage of this by using Mosquito Ringtones that only they could hear but the teachers could not.
The problem with these devices is that the correlation between age and hearing loss isn’t absolute – a lot of older people can hear these frequencies just fine.