Near and far field

About: Near and far field is a research topic. Over the lifetime, 15922 publications have been published within this topic receiving 220571 citations.

Identification of sound-soft 3D obstacles from phaseless data

Abstract: The inverse problem for time-harmonic acoustic wave scattering to recover a sound-soft obstacle from a given incident field and the far field pattern of the scattered field is considered. We split this problem into two subproblems; first to reconstruct the shape from the modulus of the data and this is followed by employing the full far field pattern in a few measurement points to find the location of the obstacle. We extend a nonlinear integral equation approach for shape reconstruction from the modulus of the far field data [6] to the three-dimensional case. It is known, see [13], that the location of the obstacle cannot be reconstructed from only the modulus of the far field pattern since it is invariant under translations. However, employing the underlying invariance relation and using only few far field measurements in the backscattering direction we propose a novel approach for the localization of the obstacle. The efficient implementation of the method is described and the feasibility of the approach is illustrated by numerical examples.

Experimental Study of the Spectral Properties of Near-Field and Far-Field Jet Noise

Abstract: The near and far pressure fields generated by round, isothermal and cold jets of diameter D = 38 mm with Mach numbers varying over the range 0.6 ≤ Mj ≤ 1.6 are investigated experimentally, and characterized in terms of sound spectra and levels. Properties of near-field jet noise, obtained in particular at 7.5 diameters from the jet centerline, are documented. They differ appreciably from properties of far-field noise, and form a database that can be used for the validation of the acoustic fields determined by compressible Navier-Stokes computations. The near pressure fields originating from simulations can thus be directly compared, without resorting to extrapolation methods which might lead to uncertainties in the far pressure fields. In the present paper, sound source localizations are also carried out from the near-field pressure signals. The experiments provide in addition far-field results evaluated at 52 diameters from the nozzle exit, in good agreement with the data of the literature. The classical...

A system and method for analyzing topological features on a surface

Abstract: Disclosed is a method and apparatus for using far field scattered and diffracted light (412) to determine whether a collection of topological features on a surface (408), (e.g. a semiconductor wafer) conforms to an expected condition or quality. This determination is made by comparing the far field diffraction pattern of a surface under consideration with a corresponding diffraction pattern (a 'baseline'). If the baseline diffraction pattern and far field diffraction pattern varies by more than a prescribed amount or in characteristic ways, it is inferred that the surface (408) features are defective. The method may be implemented as a die-to-die comparison of far field diffraction patterns of two dies on a semiconductor wafer. The portion of the far field scattered and diffracted light (412) sensitive to a relevant condition or quality can also be reimaged to obtain an improved signal-to-noise ratio.

Nonlinear propagation of electromagnetic waves in magnetized plasmas

Abstract: The nonlinear propagation of an electromagnetic wave along an external magnetic field in a uniform plasma is reconsidered in order to include the combined effects of the relativistic-mass and ponderomotive-force nonlinearities. The results of previous works on this subject are improved significantly.