Like it is said, every tool can be used either for the good or for the bad. Because of a medical condition called presbycusis, an age-related loss of hearing. People above age 20 loses their sensitivity to high frequencies 18 to 20Khz. This means that only teenagers would be able to hear sounds / noises in such frequency band.
Based on this technology, security company, compoundsecurity, has developed a device to prevent / repel teenagers from anti-social gathering infront of stores. This device emits an ultrasonic sound / noise only audible to teenagers. Here is the video (in french) of the device in action.
Ironically, the same medical condition, used in a different context, is turning to the advantage of the teenagers. Some teenagers in UK has recorded this ultrasonic sound on to their handphone so that they can receive calls and sms during class without their teacher knowing about it. 
This is a clear example of how, technology, used differently can be to one’s advantage as well as one’s disadvantage.
Reference: textually.org, ringtone
