You're listening to a pitch at 8 kilohertz, which most people can hear. But as the frequencies get higher, older ears hear less.
So now, students are downloading those higher frequencies on their phones and using them as ring tones during class. They're called "mosquito" ring tones, named after a pest that makes more of a buzz than a sound.
Here's what some Valley students said about the ring tones:
"They're kind of loud and obnoxious. You hear them in class, like all the time."
"It just like buzzes. It kinda bothers your ear."
"They're very annoying."
"They put it on during class. And our teacher didn't hear it. And I was like what the heck, that's weird."
It's true, many teachers can't hear them, even 26-year-old Megan Denman a history teacher at Hoover High School. "They said it's this new ringtone that adults aren't supposed to hear. And I couldn't hear it so that made me a little nervous. "
Still, teachers say the students aren't as sneaky as they think. "They'll give each other up. They can hear the sounds and their heads will turn toward whatever the direction the mosquito or the ringtone is coming from," said teacher Curtis Sisk.
British business owners first started using the high frequency sounds about three years ago to keep kids from loitering. But kids have turned the tables and are using them to their own advantage.
As a test, we put several parents and a young adult in a room and set off the ring tones. They're available on the internet along with the age groups that should hear them.
At first, everyone could hear. But as the frequencies got higher, around 15 kilohertz, we started losing a few.
It turns out, the age estimates are pretty close. "I was surprised I couldn't hear, and it was right at the cut off!!" said parent Carmel Harrington.
Teens say the most popular frequencies are between 17 and 18 kilohertz, which only one of our parents heard.
But if you can't hear them, Fresno audiologist Nancy McKeown said don't worry because there's no reason you need to hear those frequencies. "We don't even have equipment that can test that high. We don't consider it a problem because it doesn't affect communication. It's a part of the normal aging."
In fact, many devices can't even transmit the higher frequency sounds, including your analog T.V.
The most common reason adults slowly lose their ability to hear is exposure to loud noises. So if kids want to continue hearing these tones, they should protect their ears.
Meanwhile, adults can look at the bright side. "It also kind of gives parents a chance to not have to listen to excessive ringing, and the texting and 'beep - beep - beep'. All the noises. I don't really mind it."
So either you hear them or you don't; and if you can't, you might want to count your blessings because like the pests they're named after, teens say the mosquito tones can be annoying.