Some company or another creates a 17KHz tone which is supposed to repel teenagers (so they stop hanging-out in front of the 7-11). Older people don't hear as well so most people over 20 can't hear it. Annoying teenagers then take said tone and use it for a cellphone ringtone, so parents, teachers, and adults in general can't hear it. Heres the tone played twice: http://www.jamster.co.uk/storage/vie...c_Sound_16.mp3 I'm 27 and I can only hear it if my computer speakers are ALL the way up, and I'm within 3 feet of the speakers. I guess CDs 20,000Hz ceiling limit would be lost on me, eh? (saying nothing of SACD's 40KHz limit). PS: Its really annoying if you can hear it.