One of my co-workers stops by my office and asks if I will take a “test”. Sure, why not? She has me listen to an audio file from her computer.

“Did you hear anything?” she asks.
“Yes, it was pretty annoying.”
She smiles, “I don’t hear it.”

She then explains what it is – below is an excerpt from NPR.

British shopkeepers tired of teenage loiterers have turned to the Mosquito teen repellent, which emits a high-pitch frequency that most teenagers can hear — but not most adults.

But now teens have struck back against the Mosquito: They are using the same sound to communicate without adults’ knowledge.

Apparently, if you’re under 30 or so you can hear this sound, otherwise nothing. I’m the only person in my office who can hear it, so needless to say I knew when someone new was being tested next door. I wonder if it fades away or if one day you just don’t get that frequency?

Here is the sound …can you hear it?

Here and here are a couple other related sites.