Just last night, I made a new friend who unfortunately has a dog, but a well trained and well behaved dog that actually didn't annoy the hell out of me. She has an ultrasonic device that looks like a little birdhouse that she keeps near her door that she's been using on her dog to train it not to bark at people that come to the door.

I saw it in action without knowing what it was, at first. Someone my friend had been expecting came to the door, so she got up and answered the door and the dog, of course, followed her. I think I heard one bark. Shortly after, the dog comes slinking back into the living room and hides behind the couch, while cautiously eyeing the door. I first thought the dog just didn't like the visitor, for some reason. Turns out, that was the dogs reaction to the ultrasonic device, because my friend had turned it on just after opening the door. The dog clearly doesn't like whatever frequency is coming out of the thing, so I was immediately curious about this device.

I probably wouldn't believe something like this worked if I hadn't seen a dogs reaction in person. Now I want to try one out on the barkers in my apartment complex!

It looks like this one. For $30, I'm tempted to buy one just to take it apart and see what frequency it puts out. So I can build a more powerful version