So annoyng youths are discouraged from hanging around shop fronts by equally annoying high pitched whine that only they can hear. Annoying youths promptly take said noise and make a high pitched ring tone that they can hear but older folk (teachers mainly) can't. Well done I applaud their inventiveness Anyway point is that I can hear it. Age over 40 and I can still hear 20Khz sound. My younger colleagues can't make it much past 13Khz. So why am I different? Maybe it is the multiple perforations of ear drums in my 20's and the scared drum is some how tauter. Maybe cos I was driven mad by ultrasonic graphics tablet that no one else could hear when I was younger. Most likely it was because an ear infection blocked my ears causing partial deafness from 18 to 21 and thus saved them from the pounding loud music gigs, parties and night clubs should have given them. So just for once I win. So used am I to finding some part of my body that doesn't work properly anymore I am chuffed to bits that at least my ears do. http://www.ochenk.com/entry.php?id=63
If you want to check your own hearing. Of course you will need someone young in room to prove that there is sound when you can't hear anything as it could be your speakers.
UPDATE: Have tried this out on different PC at home with wife. Seems that I must have been hearing a harmonic at 20Khz cos on set up at home 15Khz is as high as I can go. The wife can hear 16Khz...So scrap that post..really my moment of smugness truly shattered.