    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] 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 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 (<30W),
    construction was straightforward, and distortion had been
    reduced by several orders of magnitude compared to all earlier
    attempts.

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