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.

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.

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.

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