I love this. As a telecom engineer and as a parent, this is great!
A short while back, someone decided to play high frequency noise outside shops where teenagers (damn them and their trendy youth stylee) were congregating to the annoyance of the older, more mature, dignified residents. The idea being that the high pitched noise would deter the yoof from loitering, while the oldies couldn’t hear the sound anyway – being deaf to the high pitch.
Now, some kids are using the same technology as ring tones so that they can hear calls and texts come in while in class (where of course phones and other fun stuff are banned). What a great turn around! 😀