Technology
The directivity (narrowness) of all wave producing sources 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 omnidirectionally. Only by creating a sound source much larger than the wavelengths it produces can a narrow beam be created. In the past, loudspeaker manufacturers have created large speaker panels or used reflective domes to provide some directivity but, due to the sound’s large wavelengths, the directivity of these devices is still extremely weak.
These fundamental limitations of traditional speakers are shown in detail below. For a more detailed, mathematical explanation, please read the full article.
The ultrasound has wavelengths only a few millimeters long, which are much smaller than the source, and therefore naturally travel in an extremely narrow beam.
Of course, 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 change shape in a predictable way. This gives rise to frequency components in the audible band, 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 pioneered over forty years ago. Over the past two decades, many others have attempted – and failed – to use this technique to make a practical audio source.
Through a combination of careful mathematical analysis and engineering insight, the patented Audio Spotlight sound system has become the very first, and still the only, truly directional audio system which generates high quality sound in a reliable, professional package.
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. A paper describing one attempt was published 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.
While a graduate student developing ’3D Audio’ at Northwestern University in the late 1990′s, Joseph Pompei had similar ideas of using ultrasound as a loudspeaker, largely to overcome deficiencies he saw with traditional methods of sound reproduction. After performing extensive research on the idea, he discovered the large body of knowledge in the field of nonlinear acoustics, as well as the earlier attempts at using ultrasound as an audible source. Soon after arriving at MIT, his insight led him to identify – and subsequently rectify – the barriers which had plagued the earlier researchers. Through a combination of careful mathematical analysis and solid engineering, he was able to construct the very first, and still only, practical, high-performance audio beam system [4].
Audio Spotlight systems have been in use all over the world since 2000. Customers include American Greetings, Best Buy, Boston Museum of Science, Cisco Systems, the Field Museum, the Guggenheim, Harvard Peabody Museum, Jack Morton Worldwide, Kaiser Permanente, Motorola, Science World BC, Tate Modern, Walt Disney, Western Union and the Yale Art Gallery.
Directivity Demonstrations
The simulations on this page serve to demonstrate the following:
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Fixed Source Size, Varying Wavelength
The first set of simulations is of a fixed source size (0.4m/16″), with varying wavelength. From the statements above, we expect to see an omnidirectional response for a large wavelength relative to source, and higher directivity as wavelength decreases.
Figure 1: Ideal sound fields from a 16″/0.4m source, with varying wavelength. Directivity is weak for wavelengths similar to source size, but increases for very small wavelengths (high frequencies).
This is precisely what is observed; for wavelengths much larger than the source, the sound field is omnidirectional. Modest directivity is shown for wavelengths similar to the source size, and as the wavelength decreases to a small fraction of the source size, the loudspeaker is always naturally directional.
Fixed Wavelength, Varying Source Size
For an alternate perspective, simulations were done at a fixed wavelength, while varying source size. As it is where the ear is most sensitive and most real-life content is centered, 1kHz is used for analysis, which has a wavelength of 0.35m. One would expect loudspeakers much smaller than this wavelength to have an omnidirectional response, while loudspeakers much larger than 0.35m (14″) would be more directional.
Figure 2: The wavelength is fixed at 0.4m (1kHz) while the source size is varied. A source smaller than or comparable to the wavelength is mostly omnidirectional, while a large source becomes more directive.
For a small source ¼ of the wavelength, the sound field is again omnidirectional. As the source size increases, directivity increases as expected. But notable directivity exists only when the size source is much larger than the wavelength.
Phasing and Focusing
Phased arrays have been used for decades to steer and/or focus acoustic energy, as well as other wave sources (such as radar). The basic principle is to apply a suitable delay (or phase offset) to each small portion of the source, such that the contributions from each element of the array sum constructively along the desired direction (for a steered array), or at a particular point (for a focused array). As isolation is typically the goal of directivity (and steering decreases directivity), the focused case will be simulated. Phasing a linear array is acoustically equivalent to building a perfectly curved loudspeaker array, or using a perfect reflecting surface.
For this simulation, we will allow a very large loudspeaker size of 1m (39″), and a 1kHz tone. Distance is analyzed continuously from 0.5m to 2.5m. A small circle marks the focal point.
Figure 3: Sound fields from a 1m source playing 1kHz, perfectly focused to distances ranging from 0.5m to 2.5m. Focus is apparent at short distances, but has no clear effect at distances longer than about 0.5m.
A reasonable degree of focus is noted for the 0.5m case, but the sound fields for focal lengths beyond this are nearly identical to the unfocused case. This makes sense qualitatively; one cannot expect to focus effectively at distances much further than the size of the source array.
Note also that while focusing is effective for very short distances, the cost is directivity – outside the focal point, sound expands far more than it would have with an unfocused array. Therefore, as a practical matter, focusing arrays are not effective for providing localized listening zones.
Conclusions
Loudspeaker sources of a variety of configurations are used to illustrate that even under ideal circumstances, directivity and localization of sound is extremely limited. No degree of phasing, focusing, or other manipulation can improve these results, as they are an inherent limitation of traditional acoustics.
The Holosonic® technique for sound generation in the Audio Spotlight is a fundamentally different way of creating sound, and is not limited by the barriers facing traditional acoustics illustrated above. This is because the source of sound from an Audio Spotlight is not the physical loudspeaker panel – it is the volume of air in front of it. This makes the actual sound source far larger than the physical loudspeaker seen – it just happens to be invisible, and made from ultrasound. This enables the Audio Spotlight to fundamentally provide far more directivity and isolation than any loudspeaker can ever provide.
Today is: Monday, Sep 27, 2010 
