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The ''parametric array'' is a nonlinear transduction mechanism that generates narrow, nearly sidelobe free beams of low frequency sound, through the mixing and interaction of high frequency sound wave waves, effectively overcoming the diffraction limit (a kind of spatial 'uncertainty principle') associated with linear acoustics [http://asa.aip.org/books/nonlinear.html#Preface1].
Practical applications are numerous and include underwater sound (sonar, depth sounding, sub-bottom profiling),medical ultrasound, underground sesimic prospecting, active noise control [http://www.mecheng.adelaide.edu.au/anvc/abstract.php?abstract=378], and directional high-fidelity commercial audio systems [[http://en.wikipedia.org/wiki/Sound_from_ultrasound]].
Priority for discovery and explanation of the Parametric Array owes to Peter J. Westervelt,winner of the Lord Rayleigh Medal [[http://en.wikipedia.org/wiki/Lord_Rayleigh]] (currently Professor Emeritus at Brown University [http://www.physics.brown.edu/]), although important experimental work was contemporaneously underway in the former Soviet Union [http://asa.aip.org/books/nonuw.html]. The phenomenon of the parametric array was seen first experimentally by Westervelt in the 1950's and explained theoretically in 1960, at a meeting of the Acoustical Society of America. A few years later a full paper [2] was published as an extension of Westervelt's classic work on the nonlinear Scattering of Sound by Sound, as described in [8,6,12].
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