I was watching "Forensic Heroes II" the other day, and something interesting caught my attention.
There's this ringtone that is rather popular in HK, widely used by teens at school when their teacher wouldn't even hear anything at all.
The reason behind this is that the ringtone is made of very high pitch "noise", where anyone older than 25 wouldn't be able to hear it. (Which wasn't exactly right since my sis and bro managed to hear it as well)
However, listening to it frequently will cause damage to your ears, thus it's not recommended to be used as a ringtone. =)
You can try to let your dad/ mom/ uncle/ auntie listen to it, and i'm sure they won't be able to hear anything. Hehe.