Sounds from Ultrasound: History
The technique of using a nonlinear interaction of high-frequency waves to generate low-frequency waves was originally pioneered for sonar applications by researchers developing underwater sonar techniques dating back to the 1960's . These early acoustics researchers successfully derived the formal mathematical basis for this effect and developed innovative sonar systems with more directivity and bandwidth than would otherwise be available. They called this device a parametric array.
In 1975, the first publication  appeared which demonstrated that these nonlinear effects indeed occur in air. While these researchers had not attempted to reproduce audio, they nonetheless proved that such a device may be possible.
Over the next two decades, several large companies, including Matsushita (Panasonic), NC Denon, and Ricoh attempted to develop a loudspeaker based on this principle. A paper describing one attempt was published in 1983 . While they were successful in producing some sort of sound, problems with cost, feasibility, and extremely high levels of distortion (>50% THD) caused the almost total abandonment of the technology by the end of the 1980's.
While a graduate student developing '3D Audio' at Northwestern University in the late 1990's, Joseph Pompei had similar ideas of using ultrasound as a loudspeaker, largely to overcome deficiencies he saw with traditional methods of sound reproduction. After performing extensive research on the idea, he discovered the large body of knowledge in the field of nonlinear acoustics, as well as the earlier attempts at using ultrasound as an audible source. Soon after arriving at MIT, his insight led him to identify – and subsequently rectify – the barriers which had plagued the earlier researchers. Through a combination of careful mathematical analysis and solid engineering, he was able to construct the very first, and still only, practical, high-performance audio beam system .
Audio Spotlight systems have been in use in thousands of installations all over the world since 2000. Customers include American Greetings, Best Buy, Boston Museum of Science, Cisco Systems, the Field Museum, the Guggenheim, Harvard Peabody Museum, Jack Morton Worldwide, Kaiser Permanente, Motorola, Science World BC, Tate Modern, Walt Disney, Western Union and the Yale Art Gallery.
1 Westervelt, P. J., JASA v35 535-537 (1963)
2 Bennett, M. B., and Blackstock, D. T., JASA v57 562-568 (1975)
3 Yoneyama, M., et al., JASA v73 1532-1536 (1983)
4 Pompei, F. J., Proc.105th AES Conv, Preprint 4853 (1998)