Over by the Harold Washington library, I’ve noticed they have speakers set up that emit a very high-pitched noise. What are they for? It’s painful to hear them while waiting for the light to change. I’m guessing it’s to chase pigeons away? It’s only moderately successful.
My WAG: It’s a machine that makes a high-pitched noise that repels only young people?
I couldn’t hear it, but it drove my kids nuts.
http://www.cbsnews.com/stories/2005/12/05/earlyshow/living/main1095665.shtml
I’ve heard of “mosquito” sounds to discourage loitering, but it seems ironic that a library would want to discourage young people from getting anywhere near it.
maybe they’re trying to drive them inside the library. 
Oh, I hate these things. They put them in high streets and shopping centres here to discourage groups of young people from hanging about. Unfortunately a) I’m in my thirties, with money to spend, but they drive *me *away from the area too because they hurt my ears, and b) it brought the kids’ attention to the idea that adults can’t hear high frequencies, so now they all have “mosquito” tones on their phones, which then go off in class. It does freak them out that I can hear it, though, despite my advanced age!