Startled, you turn around to realize that nobody is about to violate your personal space. In fact, there's no one close enough to be able to deliver a whisper. You see somebody walking down the sidewalk and shout, "Did you hear that?" to which they answer, "Hear what?"
Just as you decide that the last of your sanity has finally snapped and that you should probably talk to somebody about meds, your eyes are drawn to the billboard we mentioned earlier ... which reads:
98 percent of Americans don't yet realize advertisers have the balls to go there.
The good news is that the voice in your ear was no ghost, nor an auditory hallucination. It's just a device sitting seven stories up, utilizing a technique called sound from ultrasound, which basically enables it to shoot sound over distances to project it as a ghost voice right into your ear.
A company called Holosonics has developed a device called the Audio Spotlight as a tool that allows them to direct commercial audio to a specific spot without anyone else in the area being disturbed. And when we say "developed," we mean "applied a hazardous military-grade technology." The Audio Spotlight is basically a LRAD (long-range acoustic device), which is used in the field to, we quote, "send messages, warnings and harmful, pain-inducing tones over longer distances than normal loudspeakers." It's kind of a ... sound laser? We guess?