A new form of audio marketing works by beaming inaudible hypersonic waves that suddenly become audible when something solid, such as your body, walks into the beam:

The technology works by beaming waves of hypersonic sound at a pitch that can't be detected by the human ear. However, when those sound waves strike an object, such as a human body, they suddenly become audible in the immediate area of the object -- allowing personalized marketing that can target one person in a crowd of hundreds.

In December, the A&E Television Network used the technology to deliver the "Who's there? What's that?" message from a billboard on a busy New York street to advertise a new show about ghost hunters called "Paranormal State." It's also being used in grocery stores in France and in some museums, and could soon be turning traditional advertising on its ear here in Canada too, said Ken Hardy, a marketing professor at the University of Western Ontario.