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. -97-