A couple of interesting sound focusing technologies have made headlines recently. One uses ultrasonics and a flat speaker disc to "beam" sound to a designated target listening area over a distance of many meters, while the other uses a line array of tiny speakers to create a personal "listening zone" for users of portable devices.
The first technology, Audio Spotlight from Holosonics, generates an ultrasound beam modulated with an audio signal, and then uses the predictable nonlinear behavior of the transmission medium itself - in this case air - to demodulate the audible signal. Here's an entertaining short video [3:12] showing the technology in use, as its developer has some fun bewildering patrons in a public library:
I remember reading about the Audio Spotlight technology several years ago. Since then it is apparently being used successfully in a variety of museum and display applications to create localized sound for visitors standing in front of exhibits while avoiding any noise outside of the immediate display area.
Now, however, it's also being used for commercial advertising and recently made its appearance in an outdoor billboard ad for a new paranormal-themed A&E television series. As unwitting passers-by entered the target area on the sidewalk below they suddenly found themselves hearing a voice that came out of "nowhere," as the following video [1:30] shows:
A prototype of another sound focusing technology was recently described in a paper at the 154th Acoustical Society of America meeting in New Orleans. The "personal sound system" is envisioned as allowing users of portable consumer products to privately listen to speaker-produced audio from their devices without headphones or bothering people nearby.
The prototype uses nine half-inch speakers arranged in an array, and achieves regional sound focusing by adjusting the magnitude and phase of the signals in each loudspeaker - or "acoustic contrast control" - to control constructive and destructive interference of the sound waves. In this example, the frequency range of interest was 800 Hz to 5 kHz and the difference in sound level inside and outside the listening zone was 20 dB.