Field In Reverberation Chamber Research Articles

Estimation of Response of Composite Shells to Acoustic Excitation Using Sea and Its Experimental Verification

Shells made up of composite material are extensively used in aerospace structures as they offer excellent specific strength and stiffness properties over their metallic counterparts. These elements experience high frequency and high intensity acoustic loads during the launch. Response behaviour of structures in the high frequency region is analyzed in a Statistical Energy Analysis (SEA) framework. Though the methodology of estimating the response using SEA exists, its applications to composite cylindrical shells and how they compare with experimental results are seldom reported. Present work fills this gap. Acceleration and strain responses of a typical composite sandwich cylinder under diffused acoustic field are estimated in SEA framework. Modal densities and coupling loss factors are estimated using expressions. The dissipation loss factors are determined experimentally. Same cylinder is subjected to diffused acoustic field in reverberation chamber and the accelerations and strains are measured. The work presents how much the theoretically estimated responses compare with the experimentally obtained results. It is seen that the acceleration responses compare reasonably well, but needs improvement. The strain-velocity relations compare very well. Acceleration and strain bounds such that 99 % of all measurements are within are also predicted which could be used in the structural design.

Effects of Frequency Stirring on Reverberation Chamber Testing: An Analysis as a Radiation Problem

The effects of frequency stirring on the statistics of the currents induced on an equipment-under-test (EUT) placed in a reverberation chamber field are analyzed. A reciprocal approach that relates the received currents in a susceptibility test to the free-space input impedance presented by the EUT when excited at the test point is used. An analytical expression is then derived to give the distribution functions of the induced currents under frequency stirring that accounts for the frequency dependence of the EUT radiation characteristics. A numerical study of shielding-box EUTs with coupling apertures shows that the changes in the EUT test-port input impedance with frequency yield non-Rayleigh current statistics when frequency stirring is used. It is unlikely that a single distribution family with a limited number of parameters can be fit to the current statistics with EUTs of arbitrary volume, coupling aperture, and measurement probe configurations.

Elliptic Stochastic Fields in Reverberation Chambers

A theoretical and experimental characterization of complex elliptic random fields inside a reverberation chamber is performed. The real and imaginary parts of the pseudovariance and noncircularity quotient are found to be simultaneously nonzero and to have comparable magnitudes, in general. The I/Q statistical anisotropy and correlation have a spectral envelope that is robust under varying experimental conditions and decays at $-$ 5 dB per decade, on average. Expansion coefficients for a circular random base field are extracted for a widely linear field representation. Joint, marginal and conditional probability density functions are derived for both the unconditional and the directional energy and for the phase. An iterative stochastic algorithm is developed for the systematic extraction of the sampling distributions. These distributions provide increased model accuracy and improved agreement with empirical distributions, provided the constellation of sample stir data conforms to a symmetric complex elliptic distribution.

Uniqueness of Plane Wave Integral Representation for Idealized Fields in Reverberation Chambers

A plane wave integral representation for fields in reverberation chambers has been found useful for the idealized case of well-stirred fields. The representation satisfies Maxwell's equations, and the statistical properties of the fields are obtained by letting the angular spectrum be a random variable with appropriate statistical properties. This representation yields the expected idealized ensemble (stirring) averages, in particular, spatial uniformity and isotropy. The purpose of this letter is to show that this representation is unique in that no other field representation will yield these desired ensemble averages.

On the Low-Frequency Optimization of Reverberation Chambers

An original method allowing to define a particular frequency called “quasi-ideal reverberation chamber (RC) minimum frequency” is presented. This frequency corresponds to the frequency where an RC is considered as working in a “quasi-ideal regime.” According to the definition of an ideal RC, the method is based on the assessment 1) of the electromagnetic (EM) field distributions obtained in the working volume (in comparison with the distributions obtained in the ideal model defined by Hill [D. A. Hill, “Plane wave integral representation for fields in reverberation chambers,” IEEE Trans. Electromagn. Compat., vol. 40, no. 3, pp. 209--217, Aug. 1998.]) and 2) of the number of uncorrelated samples obtained over one mode stirrer revolution. The method is considered as helpful in order to optimize the functioning of an RC at low frequencies for broadband EM compatibility immunity and emission tests. Indeed, it is shown from experimental and numerical results that the insertion of an optimal loading in the chamber allows to decrease the frequency where the chamber works quasi-ideally. The influence of complexifying the mode stirrer geometry is also discussed.

A Bivariate Normalization Approach for Characterizing Reverberation Chambers

We suggest a novel bivariate normalization (BN) approach to evaluate distributions of electromagnetic fields in reverberation chambers. The BN approach is simpler and more cost effective than the conventional marginal probability density functions (MPDF) method. In our approach, we transform the complicated MPDF into simplified probability density functions using a normalization technique. In addition, we can also present cumulative distribution functions as simple closed forms with no help of any numerical integration. According to the experiment and the goodness-of-fit test, the BN method shows good agreement with the conventional one, which proves accuracy and validity of our proposal.

Field Penetration Through a Wire Mesh Screen Excited by a Reverberation Chamber Field: FDTD Analysis and Experiments

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Field penetration into a metallic enclosure with a large aperture protected by a wire mesh screen is analyzed in this paper, using an in-house finite-difference time-domain code with a thin-wire approximation that accurately accounts for the cross between wires of the mesh. All results are experimentally validated using a reverberation chamber. Different enclosure dimensions and mesh sizes are considered, highlighting their implication on shielding performances. </para>

Corrections to “Boundary Fields in Reverberation Chambers” [May 05 281-290

In the above paper, the plane-wave integral representation for well-stirred fields in reverberation chambers was extended to apply near the chamber walls. Several analytical terms for the mean of squared field magnitudes were derived. The formulas are inaccurate for describing the conditions near the chamber walls and need to be corrected, since they lead to non-physical negative values for the mean of squared magnitudes for small input values kx, ky, or kz.

Boundary Fields in Reverberation Chambers

A previous plane-wave integral representation for fields in a reverberation chamber is extended to analyze fields near the boundaries of a rectangular chamber. The field point can be located near a single wall, a right-angle bend, or a right-angle corner. Multiple image theory is used to ensure exact boundary conditions. Expressions for the ensemble average of the squares of the electric and magnetic field components illustrate the evolution of the fields from the chamber boundary to the center of the chamber where the fields are statistically uniform. The results are relevant to the useful test volume of rectangular reverberation chambers or, equivalently, how far test objects must be located from chamber walls to obtain a uniform test field.

Plane wave integral representation for fields in reverberation chambers

A plane wave integral representation is presented for well-stirred fields in a reverberation chamber. The representation automatically satisfies Maxwell's equations in a source-free region and the statistical properties of the fields are introduced through the angular spectrum, which is taken to be a random variable. Starting with fairly simple and physically appropriate assumptions for the angular spectrum, a number of properties of the electric and magnetic fields and the power received by an antenna or a test object are derived. Many of these properties and test object responses are in agreement with other theories or with measured results. An important result for radiated immunity testing is that the ensemble (stirring) average of received power is equal to the average over plane wave incidence and polarization.

Sound Field in Reverberation Chamber at Discrete Frequencies

In order to find the low-frequency limit of sound power measurements in reverberation chambers at discrete frequencies, the sound field in a 70-ft3 model reverberation chamber has been investigated. Results of both the sound-pressure measurements and averaging along various paths of microphone travel at various frequencies are presented. The influence of the wall absorption on the sound-pressure level fluctuations, averaging and accuracy of sound-power measurements has been investigated. Results of correlation relations are also presented and the validity of existing formulas for sound-pressure variance is discussed.

Frequency Dependence of Different Ways to Obtain Diffuse Sound Fields in Reverberation Chambers

In the reverberation chamber of the Physikalisch-Technisehe Bundesanstalt (PTB), the absorption coefficient of two absorbing arrangements has been measured for testing different states of equipment of the room with diffusing elements. The room has fixed convex cylindrical segments on two walls and the ceiling. In addition, movable spherical segments can be fastened to the walls and/or diffusing elements can be suspended in the space. With uncovered mineral-wool slabs as the testing material, additional space diffusors change the resulting absorption coefficient more or less in the frequency range above 200 cps. Decrease or increase of the absorption coefficient depends on the test-area size and on the number of diffusors. In the second arrangement, a cover of linoleum was fixed on the same slabs, thus performing an efficient low-frequency absorber with a resonance at 140 cps. With diffusors of 2 m2 each present in the space (their total area amounting to at least 20 m2), reasonable results were gained. The available cylindrical and spherical segments on the boundaries could not produce sufficient randomness in the low-frequency range.

Measurement of Sound Diffusion in Reverberation Chambers

This paper describes a variety of methods for the measurement of the diffusion of sound fields in reverberation chambers. Diffusion is defined on the basis of the angular distribution of sound energy flux, in accordance with the definition that has found its visual expression in the “sound hedgehog” of Meyer and Thiele. The theoretical foundations of the methods proposed here are: normal mode expansion, the sampling theorem (both in time and two-dimensional space), and either Fourier or correlation analysis. The quantities to be measured are sound pressures and, in some cases, sound pressure gradients at a number of sampling points on the measuring wall. Results of these measurements are suitably transformed to give the sound energy fluxes for all possible angles of incidence. The accuracy of measurement is determined by the Q (frequency times reverberation time) of the chamber and is typically of the order of 1°. This extra-ordinary directivity is achieved without substantial perturbation of the sound field. Methods applicable to both single frequencies and finite frequency bands are described.

Random Sound Field in Reverberation Chambers

The production of random sound fields in reverberation chambers is very necessary for the determination of sound absorption coefficients and sound transmission losses. Various techniques for randomizing sound fields have been proposed in the past. The aim of this paper is to present experimental results comparing the suitability or otherwise of some of the practices of randomizing sound fields, viz., altering the room geometry by a rotating vane, by fixed scattering surfaces on the walls or in space, or by absorbing patches on the walls. To assess the state of randomness of the sound field three methods were used: the correlation coefficient between two observation points, the standard deviation of reverberation time measurements, and the measurement of the sound absorption coefficient of a standard sample.