One co-worker could hear all these even sitting around the corner......made me wonder if she would respond to a dog whistle, but I didn't tell her that....
http://journal.plasticmind.com/ears/mosquito-tone-or-how-to-tell-youre-a-youngun/

http://journal.plasticmind.com/ears/mosquito-tone-or-how-to-tell-youre-a-youngun/