    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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