https://en.wikipedia.org/wiki/The_Mosquito
Teens also have used this frequency as a ringtone in class as most teachers couldn't hear it.
Teens also have used this frequency as a ringtone in class as most teachers couldn't hear it.
Plus, there's a lot of teachers in their 20s who can hear it just fine.
[0] https://www.ultrasonic-ringtones.com/. Unsurprisingly, I can't hear above 17k anymore. I'm not terribly upset.
For my part, I used a Morse code ringtone that happened to sound similar to the howling noise the radiators made, which everyone had learned to simply tune out.