I found this website long ago and it just came up in a conversation i had recently so here’s the site: http://www.ultrasonic-ringtones.com/
These ringtones, also known as mosquito ringtones, are popular amongst teens nowadays because of their purported usefulness – teens can hear the phone ring but adults can’t.
This is true to some extent since teens have a better hearing ability (most of the time) and can therefore hear a wider range of sounds so when the ringtones reach a certain high frequency, only those whose hearing abilities are good enough can hear the ringtones.
You can click through the various ringtones in the website to see how good your ears are. A word of warning though, some sounds can be so high pitched that your ears might throb a long time after you stopped listening to it. =p I learnt this the hard way. =p
Anyway, just another trivia: These ringtones were initially used in certain shops to deter teens and kids from loitering near the shops as the high pitch sound irritates and repels these youngsters but not the adults with purchasing power (since adults can’t hear a thing).