Parametric array: Difference between revisions

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Professor (now Emeritus) Peter J. Westervelt, in his original treatment of the scattering of sound by sound, noted that sound fields propagating and interacting in real (and therefore, nonlinear) media can emit, ampify, and absorb sound. This profound observation has yielded a number of important applications, most notably the parametric acoustic array [2]. The parametric array allows the generation of narrow, nearly sidelobe free beams of low frequency sound, through the mixing of higher frequency sound wave beams, effectively overcoming the diffraction limit (a kind of spatial 'uncertainty principle') associated with linear means of sound generation and scattering. Applications include underwater sound [www.bayshoretechlabs.com], medical ultrasound, underground sesimic prospecting, and directional high-fidelity commercial audio systems.

Priority for discovery and explanation of the Parametric Array owes to Westervelt, although experimental work was contemporaneously underwater in the former Soviet Union. The phenomenon of the parametric array was seen first experimentally by Westervelt in the 1950's and theoretically presented first in 1960 at a meeting of the Acoustical Society of America, as an extension of Westervelt's classic work on the nonlinear Scattering of Sound by Sound.

In theWestervelt's original papers on the Scattering of Sound by Westervelt, it was concluded that two non-collinear sound beams do not scatter to produce sum or difference frequency fields to points lying outside of the interaction region of the beams. It is somewhat remarkable that this result, although quickly confirmed by Bellin and Beyer [38 ], and by experimental work by a number of other researchers, this result has been somewhat conroversial even up to the present day, being contradicted by a number of theoretical studies. However, the mostmore recent and highly accurate experiment by Roy and Wu [6] hasdid demonstratedconclusively uniquivocallydemonstrate that Westervelt’s original non-scattering result is correct, however, contrary theoretical predictions have been published and haveit neverwould appear, to this author’s knowledge,have never been corrected or retracted to bring them into line with experiment.

The foundation for Westervelt's theory of sound generation and scattering in nonlinear acoustic media owes to the equation of Lighthill [36,37 ]. The application of Lighthill’s theory in the nonlinear acousic realm yields the Westervelt-Lighthill (nonlinear acoustic wave) Equation, or WLE [1].