Imagine that you walk into your friendly neighborhood big box store and instead of being greeted by a smiling retiree, you hear whispered voices prompting you to buy things. You spin around to see who's speaking, but there's no one there and none of the other shoppers seem alarmed. Have you finally gone nuts? No, but the ]advertising industry has.
A company called Holosonics has developed a technology called the Audio Spotlight system, which uses tiny speakers to focus sound into a very narrow beam. Ultrasonic frequencies are too high for the human ear to hear, but as the sound travels from the Audio Spotlight system's speakers, air distorts the sound and makes it audible. It's perfect for in-store advertising, but you'd have to be standing in the right place to hear it.