The Audio Spotlight using non-linear dsp yields sound spotlight

Just FYI
http://sound.media.mit.edu/~pompei/spotlight/
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[http://sound.media.mit.edu/~pompei/spotlight/title.jpg]Machine Listening Group
[http://sound.media.mit.edu/]
Digital Life Consortium [http://www.media.mit.edu/dl]A beam of light can be
controlled in many ways - it can be aimed at one person in a crowd, spread to
fill a room, or projected to create rich, distant imagery.

We can now do these very same things with sound.

News

First, thanks for the good words from everyone who wrote in after the Slashdot
story appeared; I've been active there for years, and was happy to see my work
covered.

You may enjoy the small gallery [gallery] of items I've collected. More will be
added when I find the time.
[beamsim.jpg]

>
The Audio SpotlightTM, invented and developed at the MIT Media Lab
[http://www.media.mit.edu/], is a device which uses subtle nonlinear properties
of the air to create an extremely narrow Sound BeamTM. This beam of sound
behaves just like a beam of light - 'shining' it at a specific listener allows
only that person to hear it, and projecting it against a surface creates an
acoustic 'image' at the point of reflection. It is the first device that
provides total control over both the locationanddistributionof high quality
sound, something impossible to achieve with traditional loudspeakers.
> The circular transducer is very thin, and can be constructed in a variety of
sizes and configurations as needed. A typical Audio Spotlight transducer has an
active area of approximately 1 foot diameter, and, depending on size and
frequency content, projects an approximately three-degree wide beam of sound
audible to well over 100 meters. Harmonic distortion has been reduced to close
to that of a traditional loudspeaker, sound level is quite appreciable (on the
order of 80-90dBA) at several meters, and frequency response, depending on size,
extends down to a few hundred Hertz, and upwards beyond the range of hearing.
Continued research is being conducted on all facets of the technology.While
still under development, we are testing applications of the device in
collaboration with several of our Media Lab Sponsors
[http://www.media.mit.edu/Sponsors]in preparation for eventual commercial
release.
PUT SOUND WHEREVERYOU WANT IT.TM

[joe_array.jpg]
Photo (c) 1998 Web ChappellF. Joseph Pompei
pompei@media.mit.edu [pompei@media.mit.edu]>

Usage> The Audio Spotlight can be used in two major ways: As directed audio, sound is
directed at a specific listener or area, to provide a private or area specific
listening space. As projected audio, sound is projected against a distant
object, creating an audio image. This audio image is literally a projected
loudspeaker - sound appears to come directly from the projection, just like
light.

[uses2.jpg]

> The Audio Spotlight consists of a thin, circular transducer array and a
specially designed signal processor and amplifier. The transducer is about half
an inch thick, nonmagnetic, and lightweight. The signal processor and amplifier
are integrated into a unit about the same size as a traditional audio amplifier,
and has similar power requirements.
Technology> Because it is impossible to generate extremely narrow beams of audible
sound without extremely large loudspeaker arrays, we instead generate the sound
indirectly, using the nonlinearity of the air to convert a narrow beam of
ultrasound into a highly directive, audible beam of sound.[beam.jpg]
> The device transmits a narrow beam of ultrasound (blue), which, due to the
inherent nonlinearity of the air itself, distorts (changes shape) very slightly
as it travels. This distortion creates, along with new ultrasonic frequencies,
audible artifacts (green) which can be mathematically predicted, and therefore
controlled. By constructing the proper ultrasonic beam, this nonlinearity can be
used to create, within the beam itself, an audible sound beam containing any
sound desired. This is presently done in real-time using low cost circuitry, a
specially designed amplifier, and transducers developed at MIT specifically for
this project.Hyperdirectivity

The directivity, or narrowness, of an acoustic wave generated by a circular
transducer is proportional to the ratio of the diameter of the transducer to the
wavelength of the sound. So a transducer much larger than the wavelength of the
sound creates a very narrow beam.

Audible sound contains wavelengths reaching lengths of several feet, so a
reasonably sized loudspeaker will alwaysproduce a very wide, non-directional
source at lower frequencies. The Audio Spotlight, in contrast, outputs short,
millimeter sized ultrasonic waves, which form a very narrow beam even in a small
transducer, which in turn generates audible sound. The nature of the nonlinear
transformation also essentially eliminates sidelobes in the resulting beam, and
maintains relatively uniform directivity across the entire audible frequency
range.

[directivity.jpg]
The figure to the right compares the directivity of the Audio Spotlight (yellow)
to that of an ordinary loudspeaker (purple).at 400 Hz. Note that the directivity
of the Audio Spotlight is only three degrees, compared to the essentially
omnidirectional directivity of the loudspeaker.

In order to obtain such narrow directivity from a traditional loudspeaker
system, one would need a loudspeaker arrayfifty meters across!

A loudspeaker is like a light bulb, but the Audio Spotlight is like a laser.

History> The use of nonlinear interaction of high frequency sound to generate
directive low frequency sound sources has been a well researched subject in the
field of underwater acoustics since the early 1960's. Often misattributed to
so-called "Tartini Tones", the effect is more accurately described as a
parametric array, a term introduced by Westervelt [1]
[http://sound.media.mit.edu/~pompei/spotlight/#1]. In the past several decades,
many underwater sonar researchers have used the effect to both generate
directive low frequency sonar beams, detect underwater sound (parametric
receiving array), and extend the bandwidth of underwater transducers.The first
published demonstration of an airborne parametric array was in 1975 by Bennett
and Blackstock [2] [http://sound.media.mit.edu/~pompei/spotlight/#2]. Rather
than using inaudible ultrasound, they instead used very intense, high frequency
audible sound to produce simple difference tones. While their goal was not a
practical audio reproduction device, they nonetheless effectively demonstrated
that the parametric array would work in air in addition to underwater.

[yone.jpg]In the early 1980's, several Japanese companies, such as Nippon
Columbia, Ricoh, and Matsushita, attempted to develop the parametric array for
the reproduction of broadband audible sound. They typically deployed large
arrays containing hundreds of piezoelectric transducers, such as the one to the
right [3] [http://sound.media.mit.edu/~pompei/spotlight/#3], to transmit simple
AM modulated audible signals. While successful in reproducing sound, tremendous
problems with cost, robustness, and extremely poor sound quality (up to 50%
total harmonic distortion) caused them to abandon the technology as unfeasible.

[piezo.jpg]More recently in mid 1996, an American company produced their own
version of this device and proclaimed it 'a revolution' in audio. In fact, this
device, contrary to their claims and unbeknownst to the popular press, was very
similar to those described in audio journals a decade earlier (shown to the
left), and of course suffered from the very same problems of poor sound quality
and lack of robustness that plagued the earlier researchers [4]
[http://sound.media.mit.edu/AUDITORY/asamtgs/asa97snd/2pEA/2pEA5.html]. Since
then, there has been no published evidence of progress towards a practical
device.

Background> Since his days as a part-time musician and young acoustics engineer at
Bose in the early 1990's, Mr. Pompei recognized that a key ingredient missing
from audio reproduction was the ability to reliably spatialize sound.While in a
natural environment, sound occurs all around us, giving us a tremendously strong
impression of our environment, the reproduction of sound over loudspeakers, at
best, provides a very vague and limited spatial impression. Similarly, what was
missing from music, he decided, was the ability to choreograph musical
instruments in space, just as you would dancers.While pursuing as a Master's
student techniques related to '3D Audio' technologies, he realized that this
method would simply not work in an uncontrolled acoustic environment - if the
listener moved out of the small 'sweet spot', the illusion would vanish, and
there were no practical remedies to this problem, so long as traditional
loudspeakers were used. The solution, then, was to not rely on psychoacoustic
illusions, but instead to create sound independently of the loudspeaker. One of
several ideas he had at the time was the use of interacting ultrasound beams to
produce audible sound.

> After briefly researching the idea, he discovered the numerous papers describing
the underwater parametric array and the earlier attempts of its application as
an audible sound source. From these papers, he saw that there were two key
concepts which were overlooked in the previous attempts, mitigating their
success: * Preprocessing
* Earlier attempts used simple AM modulation to generate the ultrasound
signal, which does create audible byproducts, but also substantial
distortion. The nonlinear transformation from ultrasound to audible sound is
much more complex than AM demodulation. Therefore, in order to reduce
distortion, this specific transformation needed to be mathematically modeled,
inverted, and then applied as a preprocessing algorithm. The lowest-order
preprocessing method, used in the earliest MIT prototypes, was derived from a
simple model [5] [http://sound.media.mit.edu/~pompei/spotlight/#5]proposed in
1965.
Transducer Design
* The transducers used in previous attempts were common piezoelectric
transducers used for ultrasonic ranging. These transducers are highly
resonant, and do not have sufficient bandwidth to reliably reproduce the
preprocessed ultrasonic signal. Thus, even with a preprocessing algorithm,
substantial distortion would continue to result until we developed
transducers capable of reliably reproducting the broadband preprocessed
signal.

As a side project during his Master's work, he continued his development of
these ideas, studying nonlinear wave interactions and ultrasonic transducer
design, eventually deciding to pursue the area as the focus of a doctoral
dissertation. Of all the universities that he applied to, he decided that the
free-wheeling nature of the MIT Media Lab [http://www.media.mit.edu/] was the
ideal environment for developing the idea.The first full size prototype was
demonstrated in April 1998 to our Media Lab Sponsors, and performed beyond all
expectations. The first demonstration was a John Coltrane solo, whose saxophone
was heard loud and clear, projected like a spotlight all around a movie theater,
and flying right over the audience. Power consumption was nominal (A paper [6] [http://sound.media.mit.edu/~pompei/spotlight/#6]describing the
results of the first prototype, as well as a live demonstration, were presented
at the 105th Convention of the Audio Engineering Society in September, 1998, and
received a standing ovation. While the parametric array itself is not
patentable, MIT has applied for patents on key aspects of the technology which
make it a practical device.

[400Hz_small.jpg][distortion.jpg]This directivity plot of a prototype clearly
illustrates the extreme narrowness of the beam. (Published in [6]).During the
summer of 1998, we compared distortion of prior devices with our prototype. Note
that distortion has been reduced nearly to that of a traditional loudspeaker.
(Published in [6]).

Since then, development has been remarkably productive, with engineering and
mathematical advances resulting in more sound output, better sound quality, and
reliable performance.

"Everything you do with light, you can now do with sound."TM

REFERENCES:
[1] Westervelt, P. J., J. Acoust. Soc. America, v35 535-537 (1963)
[2] Bennett, M. B., and Blackstock, D. T., J. Acoust. Soc. America, v57, 562-568
(1975)
[3] Yoneyama, M., et al., J. Acoust. Soc. America, v73, 1532-1536 (1983)
[4] Blackstock, D. T., J. Acoust. Soc. America, v102 3106(A) (1997) link
[http://sound.media.mit.edu/AUDITORY/asamtgs/asa97snd/2pEA/2pEA5.html]
[5] Berktay, H. O., J. Sound Vib., v2, 435-461 (1965)
[6] Pompei, F. J., J. Audio Eng. Soc., v47, 726-731 (1999)
(originally in Proc. 105th AES Conv., Preprint 4853 (1998) )

ABOUT THE INVENTOR:
Beginning his career in acoustics at 16 while in high school, starting as the
first high school co-op and becoming the youngest engineer at Bose Corporation
[http://www.bose.com/], Frank Joseph Pompei continued working part-time and
summers for Bose while earning a degree in Electrical Engineering with an
Electronic Arts Minor from Rensselaer Polytechnic Institute
[http://www.rpi.edu/]. Recognizing the importance and underutilization of
spatialized sound, he decided to pursue research in psychoacoustics and
application of auditory localization at Northwestern University
[http://www.nwu.edu/], earning a Master's degree. Acutely aware of the
limitations of traditional loudspeakers, he had the idea of using ultrasound as
an acoustic projector, and is now developing such a device at the MIT Media Lab
[http://www.media.mit.edu/], continuing his education in pursuit of a Ph.D.Mr.
Pompei is honored to have been chosen as a British Telecom [http://www.bt.com/]
fellow for his second year in a row.

FOR MORE INFORMATION:
A technical paper [6] describing the basic device (along with a live demo) was
presented at the Audio Engineering Society's 105th Convention (September, 1998)
[http://www.aes.org/events/105]. Pleasecontact them [http://www.aes.org/]
directly with preprint requests. The same paper was just published in the
September 1999 issue of the Journal of the Audio Engineering Society."Official"
press/public inquiries: Contact our Communications and Sponsor Relations
[http://www.media.mit.edu/CASR] team.

Or, you can email me [pompei@media.mit.edu].

All content (c) 1999 F. Joseph Pompei, MIT Media Lab, except where noted.
B&W photo of early parametric array (c) 1983 Acoustical Society of America.
Reproduction, archiving, and/or redistribution of any part of this document
prohibited without written permission from Mr. Pompei or the MIT Media Lab.
Patents Pending.