Can you hear this sound?
Apparently it's used to repel teenagers from shopping malls etc. It's a very annoying high frequency sound that drives them away, but doesn't affect older shoppers. That's bullshit - I can hear it and I'm 37. It's a bloody horrible, invasive high-pitched whistle that actually hurt my left ear and made me feel a bit dizzy. I had to turn it off after a few seconds. See if you can hear it.