> Above 20,000 Hz, she says, an alarm “starts sounding not really urgent, but like a squeak.” Not sure it's even right to call it a squeak. Many adults hear no

> Above 20,000 Hz, she says, an alarm “starts sounding not really urgent, but like a squeak.” Not sure it's even right to call it a squeak. Many adults hear nothing at 20,000 Hz.