Diffraction Effects of Apertures in the Extreme Nearfield.
Technical memo. for period ending May 82,
PENNSYLVANIA STATE UNIV UNIVERSITY PARK APPLIED RESEARCH LAB
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A model is developed that describes, at least qualitatively, the sound field within a wavelength of an aperture in an opaque, plane screen insonified by harmonic waves. The normal component of the particle velocity is assumed to be the same in the aperture as if no screen were present and zero everywhere else in the plane of the screen. Using this assumption, an expression for the transmitted sound field is developed in the form of the inverse Fourier transform of the product of the Fourier transform of the normal component of the particle velocity in the plane of the screen and the Fourier transform of the Green function appropriate for Neumann boundary conditions on a plane. This expression can be evaluated quickly using a fast-Fourier-transform FFT algorithm. The validity of the above assumption is demonstrated in the extreme nearfield by comparison with experiment for the case of a diffraction grating of straight strips separated by a distance less than a wavelength and insonified by plane waves. The model is then used to study the diffraction of cylindrical waves by a grating of straight strips. A rapid method of calculating exactly the sound field of an oscillating piston of arbitrary configuration in a rigid baffle is also indicated.
- Physical Chemistry