maybe that's the reason...if they can hear it but teacher's can't..they're in better position. you could even hear the vibration against something else (or against nothing even) a lot louder than you could that..
I'm really surprised people can't hear that. Maybe my speakers were up too loud, but I could hear it just fine, and I'm 34.
Same age, and I could hear it as well. Sounded like the test tone they used to give in grade school to test hearing.