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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]]s, effectively overcoming the [[diffraction limit]] (a kind of spatial 'uncertainty principle') associated with linear acoustics [http://asa.aip.org/books/nonlinear.html#Preface1]. Parametric arrays can be formed in water [http://asa.aip.org/books/nonuw.html], air [http://adsabs.harvard.edu/abs/1980ASAJ...68.1214T], and earth materials/rock [http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JASMAN000091000004002350000003&idtype=cvips&gifs=yes.], [http://www
Priority for discovery and explanation of the Parametric Array owes to Peter J. Westervelt, winner of the [[Lord Rayleigh]] Medal [http://www.ioa.org.uk/medals.asp](currently Professor Emeritus at [[Brown University]], although important experimental work was contemporaneously underway in the former Soviet Union [http://asa.aip.org/books/nonuw.html]. According to Muir [16, p.554] and Albers [17], the concept for the parametric array occurred to Dr. Westervelt while he was stationed at the London, England, branch office of the Office of Naval Research in 1951. According to Albers [17], he (Westervelt) there first observed an accidental generation of low frequency sound ''in air'' by Captain H.J. Round (British pior of the superheterodyne receiver) via the parametric array mechanism. The phenomenon of the parametric array,seen first experimentally by Westervelt in the 1950's, was later explained theoretically in 1960, at a meeting of the [[Acoustical Society of America]]. A few years after this, 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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