Directional Audio Transmission

The directivity (narrowness) of any wave producing source depends on the size of the source, compared to the wavelengths it generates. Audible sound has wavelengths ranging from a few inches to several feet, and because these wavelengths are comparable to the size of most loudspeakers, sound generally propagates omni directionally. Only by creating a sound source much larger than the wavelengths it's producing can a narrow beam be created.

Clearly, having loudspeakers twenty meters wide is not very useful. To make a narrow beam of sound from a small acoustic source, we instead generate only ultrasound. The ultrasound, whose wavelengths are only a few millimeters long, are much smaller than the source, and travel in an extremely narrow beam.

The ultrasound, which contains frequencies far outside our range of hearing, is completely inaudible. But as the ultrasonic beam travels through the air, the inherent properties of the air cause the ultrasound to distort (change shape) in a predictable way. This distortion gives rise to frequency components in the audible bandwidth, which can be accurately predicted, and therefore precisely controlled. By generating the correct ultrasonic signal, we can create, within the air itself, essentially any sound desired.

Note that the source of sound is not the physical device you see, but the invisible beam of ultrasound, which can be many meters long. This new sound source, while invisible, is very large compared to the audio wavelengths it's generating. So the resulting audio is now extremely directional, just like a beam of light.

Often incorrectly attributed to so-called "Tartini tones", the technique of using high-frequency waves to generate low-frequency signals was in fact pioneered by physicists and mathematicians developing techniques for underwater sonar over forty years ago.

Recent years have seen many people developing equipments which can transmit audio like a spot of light to the listeners who are far away. This technique is being used by the military and the police for focusing highly irritable noise to disburse crowd and put down the enemy.

History

The technique of using a nonlinear interaction of high-frequency waves to generate low-frequency waves was originally pioneered by researchers developing underwater sonar techniques dating back to the 1960's ¹. These early acoustics researchers successfully derived the formal mathematical basis for this effect and developed innovative sonar systems with more directivity and bandwidth than would otherwise be available. They called this device a parametric array.

In 1975, the first publication ² appeared which demonstrated that these nonlinear effects indeed occur in air. While these researchers had not attempted to reproduce audio, they nonetheless proved that such a device may be possible.

Over the next two decades, several large companies, including Matsushita (Panasonic), NC Denon, and Ricoh attempted to develop a loudspeaker based on this principle, and published a paper describing one attempt in 1983 ³. While they were successful in producing some sort of sound, problems with cost, feasibility, and extremely high levels of distortion (>50% THD) caused the almost total abandonment of the technology by the end of the 1980's.