Yes, Hypersonic sound is the way to go. We were messing about with some Hypersonic speakers at the UK Laser Meet a couple of years ago, this resulted in me buying one off eBay. They are *really* spooky as you tend to hear the sound inside your head rather than through your ears. As has previously been said, they're quite focusable like a laser but for sound.
Quote: "There is a theory which states that if ever, for any reason, anyone discovers what exactly the Universe is for and why it is here it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another that states that this has already happened.”... Douglas Adams 1952 - 2001