To encourage discussion and share ideas and information about this topic, I have setup a Yahoo Group which I invite you to visit and join, either via your browser at http://groups.yahoo.com/group/audiobeam/, or email a topic to audiobeam@yahoogroups.com . The Yahoo Group contains some files and links to sites already and one member, me. Although with your participation it is likely to grow.

Personally, I am very intrigued by this emerging technology and want to share information and ideas with other interested people. I am also trying to contact the researchers and project developers to join the Group, so that we may have direct access to news and updates.

Hope to hear from you on the Group pages.

John Wesley Barker
Music Producer, Composer & Musician

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from: http://www.spie.org/web/oer/october/oct00/indfocus.html
NB: This webpage contained 2 illustrations which have were removed to reduce file size.

Audio spotlight exploits nonlinearity in air
by Sunny Bains

Ultrasound speakers that allow passengers to hear their own audio selection have recently been incorporated into a concept car developed at Chrysler. Developed by researchers at the MIT Media Lab (Cambridge, MA), the speakers use similar principles to those behind frequency-doubling within nonlinear optical crystals. Similar technology has also been developed at Sennheiser Electronic in Germany, which won a German innovation prize for their work last year. The two groups are now competing to produce commercial products.

Ultrasound has the advantage of being highly directional (compared to audible sound) because of its relatively short wavelength. Decades ago, researchers realized they could exploit this property to create directional sound beams by using intermodulation of ultrasound beams in air. This is possible because at high enough intensity (above 125 dB -- as opposed to the normal volume of 30 to 90 dB) the air stops behaving linearly. When this happens, the propagation of one beam modulates the propagation of the other. As for frequency doubling in optics, various beams result: sum and difference (and their harmonics) and the original beams. By choosing the ultrasound frequencies to be relatively close to each other, the difference beam can be audible.

By creating a complex ultrasound waveform (using a parametric array of ultrasound sources), many different sources of sound can be created. If their phases are carefully controlled, then these interfere destructively laterally and constructively in the forward direction, resulting in a collimated sound beam or audio spotlight. Today, the transducers required to produce these beams are just half an inch thick and lightweight, and the system required to drive it has similar power requirements to conventional amplifier technology.

Though demonstrated first in the seventies, early implementations had two major problems. The first was that they used relatively low-frequency ultrasound, near the audible range. Because of the high intensities needed to get the nonlinear response, these beams could be considered dangerous. The other problem is the distortion caused by using very basic demodulation strategies to produce the audio signal. Using the simplest envelope function created an unwanted signal that varied with the square of the audio modulation depth. Turn it up and it distorts, turn it down and it becomes inaudible and/or highly inefficient.

Media Lab PhD student Joseph Pompei, who started his career as the youngest engineer at Bose Corp. has developed the technology by using computational techniques to pre process the audio signal in order to minimize this distortion.1 To do this, he investigated the basic physics of the problem and found that the problem could be eliminated simply mathematically.

The problem was that to implement the solution would require the use of an infinite series of harmonics, most of which could not be reproduced on a real array. On the other hand, the correct solution could be approximated using a wideband ultrasound source. Pompei compared the various options experimentally, testing the sound produced by the array in an anechoic chamber using a high-frequency linear microphone. His system was publicly demonstrated at an Audio Engineering Society conference in 1998.

Both Pompei and his rivals in Germany are now pursuing the many potential applications of the technology. Not only can beams of sound be directed at particular places (allowing one person, and not his or her neighbor, to hear), but the beam can also be directed to a listener by reflection. This allows for the creation of virtual speakers, where objects can appear to have sound coming from them while being entirely passive. Sennheiser engineers suggest that this could be exploited, for instance, by having an audio spotlight follow the location of a flock of birds flying by on a movie screen.
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Sunny Bains is a scientist and writer based in London, UK.
Reference
1. F. Joseph Pompei, The Use of Airborne Ultrasonics for Generating Audible Sound Beams, 105th Conv. AES, San Francisco, CA, 26-29 September 1998.

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http://groups.yahoo.com/group/audiobeam/
and join