Oblique Incident Plane Wave Research Articles

Estimation of acoustical materials sound absorption coefficient under oblique incidence plane wave and diffuse field via Allard model inversion

A technique for estimating the sound absorption of materials under oblique incidence plane wave and diffuse field excitations is proposed. It requires a mobile loudspeaker and a pair of fixed microphones above a layer of absorbing material. Starting from sound pressure measurements made above the material surface for multiple source positions and by inverting Allard’s propagation model, the proposed method allows for the identification of two effective (or equivalent) parameters, namely, the complex effective density and the complex wave number. Obtaining these parameters makes it possible to estimate the sound absorption coefficient for a plane wave with any angle of incidence or under a diffuse acoustic field by summation over the angles. The results are compared to theoretical values calculated using the Johnson-Champoux-Allard model and to reference measurements obtained using an impedance tube, a small cabin, a large reverberant room, and a sound field synthesis method. One of the limitations of this method lies in the assumptions associated with the Allard model to describe the sound field above the material (assumed to be isotropic, homogeneous, of constant thickness, and to behave like an equivalent fluid). The main advantages are (1) that the sound absorption coefficient can be estimated under oblique incidence plane wave and diffuse acoustic field with values that are always in a physical range (between 0 and 1), and (2) that samples of the order of the square meter without specific preparation can be tested which reduces the constraints associated with impedance tube or reverberant room measurements.

Assessment of acoustical materials sound absorption coefficient under oblique incidence plane wave and diffuse field using a virtual source antenna

The measurement of the absorption coefficient of acoustic materials is usually performed in an impedance tube under normal incidence on a small sample, or in a reverberant room under diffuse field incidence on a large sample of material. Both methods are prone to well-documented experimental limitations and errors. The approach proposed here uses a virtual source antenna (a point source moved at successive positions) over a material sample typically 1 to 4 m2 in size, and a fixed microphone pair directly above the material surface. Several methods have been tested to reconstruct the absorption coefficient under oblique plane wave or diffuse field incidence. Among these, the inversion of the Allard propagation model over an absorbing plane, in order to extract the material properties (complex wavenumber and density) from measured microphone transfer functions, is presented here. Absorption coefficients are shown for various acoustic materials.

High-efficiency optical frequency mixing in an all-dielectric metasurface enabled by multiple bound states in the continuum

We present nonlinear optical four-wave mixing in a silicon nanodisk dimer metasurface. Under the oblique incident plane waves, the designed metasurface exhibits a multiresonant feature with simultaneous excitations of three quasi-bound states in the continuum (BIC). Through employing these quasi-BICs with maximizing electric field energy at the input bump wavelengths, significant enhancement of third-order nonlinear processes including third-harmonic generation, degenerate and nondegenerate four-wave mixing are demonstrated, giving rise to ten frequencies in the visible wavelengths. This work may lead to a new frontier of ultracompact optical mixer for applications in optical circuitry, ultrasensitive sensing, and quantum nanophotonics.

A novel retroreflective planar Luneburg lens for the radar cross section enhancement

A novel retroreflective planar Luneburg lens for the radar cross section (RCS) enhancement is proposed in this paper. The planar Luneburg lens consists of 10 layers of metasurfaces, which are used to realize the layered permittivity distribution of three-dimensional (3D) gradient-index Luneburg lens. First, a single-layer planar metasurface element is designed. The element consists of one single substrate layer and a metallic layer, and two metal square ring patches are etched on the substrate. Then, by using the transformation optics (TO) and the metasurface element, the equivalent layered permittivity distribution with 2–25 relative permittivity is realized, and elements of the planar Luneburg lens illuminated by the 30° oblique incident plane wave are designed. Finally, a 10-layer retroreflective planar Luneburg lens at 5 GHz is designed, fabricated and measured. At 5 GHz frequency, its measured RCS values in most of the incident angle domain from −60° to 60° are enhanced in different degrees. The most notable enhancements occur in the incident angle domain around −27° and 27°, while the maximum measured RCS enhancement is more than 25 dB. This is the first time that the planar Luneburg lens is used to increase its RCSs with many incident angles.

Wideband Simulations of Periodic Structures by the Hybrid Spectral FDTD/TD-VFz Method

The spectral finite-difference time-domain method is combined with the discrete-time time-domain vector fitting (TD-VFz) algorithm to analyze three-dimensional (3-D) periodic structures excited by oblique incident plane waves over a wide frequency band. The time signal reconstruction with TD-VFz produces an untruncated response over an infinite time window allowing an accurate spectral result, especially near the cutoff frequency separating the spectrum propagating wave region of the evanescent wave region.

Oblique Incidence Optimal Design of Microwave Dielectric-Magnetic Absorbing Composites

The impedance matching equation of a single-layer dielectric-magnetic absorber excited by an oblique incidence plane wave is resolved with an expression providing the optimal thickness and highlighting the composite crucial parameters. The effective propagation constant of the nonuniform refracted plane wave into the absorber is represented by a new formulation as a function of the true refraction angle which is a real number unlike the complex quantity resulting from the Snell’s law, usually applied in the literature. The proposed design process is applied to compute microwave absorption performances of different dielectric-magnetic materials, considering both normal and oblique incidence. Furthermore, the incidence angle effect is investigated.

ANALYTICAL MODELLING BASED ON RESONANCE SCATTERING THEORY EXPLAINING THE NEEDLE VISIBILITY INCONSISTENCY IN B-MODE ULTRASOUND

In an ultrasound-guided needle insertion, physicians should adjust a certain insertion angle and the position of the transducer to ensure that the initial point and final target are in-plane inside the imaging plane. One of the crucial problems in this interventional procedure is poor and inconsistent needle visibility in B-mode ultrasound. In this research, some potential physical parameters, i.e., ultrasound frequency, the incident angle of the ultrasound beam, needle density, and dimension, are investigated through analytical modeling based on the resonance scattering model to understand their behavior in affecting needle visibility. 25G non-echogenic needle is used as a model object and assumed as stainless-steel hollow cylinder insonified by oblique incident plane wave varied within the frequency of 0–10 MHz and incident angle of 0°–45°. The results suggest that those physical parameters simultaneously affect the occurrence possibility of the resonant modes, which eventually affect the total scattering pressure field Ps in a non-linear way. These observed behaviors in the form of the spectrum map of resonance scattering pressure amplitude can be used to adapt a more beneficial combination of those physical parameters to obtain a higher possibility of good needle visibility through practical insertion application and potential echogenic technology or adaptive beamforming.

A 4-Bit Multistate Frequency-Selective Surface With Dual-Band Multifunction Response

In this letter, a novel 4 b multistate frequency-selective surface is proposed. Four p-i-n diodes are used to maneuver multifunction behavior of the structure with seven different functional states. The 4 b binary state operations 0000, 1100, 0011, and 1111 offer transverse electric (TE)/transverse magnetic (TM) waves both transmit, TE transmit/TM shield, TM transmit/TE shield, and TE/TM both shield, respectively, at 2 GHz frequency. The passband at 2 GHz frequency has <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-\text{10}$</tex-math></inline-formula> dB fractional bandwidth of 117%. The binary states 1101 and 1110 are symmetric, which select the TE wave transmit/TM wave shield to the passband at 3.45 GHz frequency with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-\text{10}$</tex-math></inline-formula> dB fractional bandwidth of 49%, whereas binary states 1011 and 0111 allow the TM wave to transmit at 3.45 GHz and shield the TE wave. The four symmetric binary states 0101, 0110, 1010, and 1001 provide transmission band at 3.45 GHz frequency for dual-polarized plane wave incidence. The transmission of both the passbands can be shielded by selecting the binary state equal to 1111. An equal circuit model is used to elaborate the contribution of different passive structural elements in terms of lumped components. A prototype of the structure is fabricated, and transmission coefficients for different states at normal and oblique incidence of the electromagnetic plane wave are measured, which verifies the simulated responses of the structure.

Quasi-normal mode theory of the scattering matrix, enforcing fundamental constraints for truncated expansions

We develop a quasi-normal mode theory (QNMT) to calculate a system's scattering $S$ matrix, simultaneously satisfying both energy conservation and reciprocity even for a small truncated set of resonances. It is a practical reduced-order (few-parameter) model based on the resonant frequencies and constant mode-to-port coupling coefficients, easily computed from an eigensolver without the need for QNM normalization. Furthermore, we show how low-$Q$ modes can be separated into an effective slowly varying background response $C$, giving an additional approximate formula for $S$, which is useful to describe general Fano-resonant phenomena. We demonstrate our formulation for both normal and fixed-angle oblique plane-wave incidence on various electromagnetic metasurfaces.

A New Hybrid Chamber for Generating a Spectrum of Oblique Incident Plane Waves at the DUT

A novel type of hybrid measurement facility comprising a chamber antenna array (CAA) inside an overmoded waveguide (WG) is proposed and analyzed numerically. The reflecting walls of the metal rectangular WG are used in conjunction with a CAA to synthesize obliquely incident plane-wave (PW) fields at the device under test (DUT). This enables increased flexibility in emulating almost any PW multipath testing conditions in the WG chamber without the high cost and complexity of classical anechoic measurement systems employing relatively large phase-steered PW generators (PWGs). A modeling framework is proposed that has been used to devise first-order design rules (e.g., the number of independent propagating modes, dimensions of the WG, CAA, and DUT). Afterward, an optimally beamformed CAA example is presented to numerically validate the quality of the on- and off-axis PW fields in the test zone (TZ). This study shows design tradeoffs between the amplitude ripple in the DUT region, the total power focused in this region, the DUT size, and the angle of incidence.

Agile design of cross-section enhancement of a conducting plate radar through active metasurface

The design concept of employing the dynamically reconfigurable scattering pattern to enhance radar cross-section is proposed and examined. In combination with varactor loading, a physical unit cell geometry with an embedded bias network for the varactors is utilized to build the metasurface. Biased by an applied direct-current source with voltage gradient, the proposed active metasurface can exhibit electrically tunable reflection phase distributions for either normal or oblique incidence plane wave, so as to achieve flexible redirection of the angle of reflection. As a result, the agile effect for monostatic or bistatic cross-section enhancement is then facilitated. Taking a conducting plate as an example, three different incidence and reflection scenarios are considered for calculation and full-wave simulation. Reconfigurable scattering patterns produced at the operating frequency of 10 GHz by the presented design are observed, which indicates its capability of real-time control of the angle of reflection. In conjunction with an experimental measurement, the effective enhancement of monostatic and bistatic cross-sections are validated.

Vanadium dioxide embedded frequency reconfigurable metasurface for multi-dimensional multiplexing of terahertz communication

Multi-dimensional multiplexing based on the broadband metasurface is a promising candidate for the next generation terahertz (THz) communication system, which has become a research focus for data transmission rate and channel capacity enhancement. This paper proposes a THz frequency-reconfigurable metasurface hybridized with vanadium dioxide (VO2) for communication multiplexing on both dimensions of orbital angular momentum and frequency. Theoretically, 4 × n channel (n can be any positive integer) orthogonal coaxial beams carrying different data flow can be simultaneously generated based on the proposed metasurface in the tunable operating frequency band. The simulation results verify that the THz incident waves can be converted into orthogonal coaxial beams with different topological charges or frequencies, propagating perpendicular to the metasurface, when eight-channel oblique incident plane waves with varying angles or at various frequencies are reflected by the metasurface. The multi-dimensional multiplexing can be achieved in the frequency range of 0.29–0.39 THz and 0.24–0.34 THz with the VO2 switching between its fully insulating and metallic state. The proposed metasurface is expected to enable multi-band and broadband applications and has significant potential in high-speed and high-capacity THz communication.

Active broadband sound transmission loss control through an arbitrary thick smart Piezo-laminated cylinder

The feedback control strategy is utilized for the enhancement of the sound transmission loss (STL) through an infinite thick hollow Piezo-laminated cylinder of arbitrary thickness insonified by an oblique progressive incident plane wave. The close form formulation is accomplished by utilizing the sound wave equation, the three-dimensional (3D) exact theory of elasticity and piezoelasticity. The active control strategy is achieved by sending a control signal from the distributed piezoelectric sensor layer through a controller with certain feedback gain to drive the external piezoelectric actuators. The mechanical stress generated by the piezo-actuator layer cancels the parts of the input incident wave to enhance the STL. Comprehensive numerical studies are carried out to investigate the poling direction and feedback gain effects on the STL. The results show that in the case of radially polarized piezoelectric materials, the feedback control provides significant sound enhancement especially, when the angle of the incident wave is close to normal, however, in the case of axially polarized piezoelectric materials, the control feedback strategy reduces the unfavorable oscillatory motion of the cylinder in addition to providing a superior sound isolation enhancement, particularly for near-grazing incident angles. The accuracy of the proposed solution is confirmed by comparing the presented results with the ones from the simplified models in the literature as well as the commercial finite element package Comsol Multiphysics.

In-situ measurement method of sound transmission loss under an oblique incidence of plane wave

In this study, two-dimensional finite-difference time-domain analysis was used to investigate sound transmission loss (STL) of porous materials with different incident angles. This method has provided evidence for the feasibility of measuring STL in situ using one particle velocity–pressure sensor and one pressure sensor. In the numerical analysis, the total sound pressure and the vertical component of the total particle velocity at the incident side of the material were used to separate incident and reflected components. The STL was calculated using the separated sound pressure at the incident side and the sound pressure at the transmitted side of the porous material. The STL calculated from simulation agrees with theoretical values at all incident angles investigated.

Analytical and numerical models compared with measurements for analysis of electromagnetic radiation through apertures of a metallic enclosure

Generally, most of the electronic equipments need a metallic enclosure in order to mechanically protect and electrically shield the interior printed circuit boards (PCBs) and subsystems. But in practical situations, the apertures or slots of various forms are essential parts of the shielding enclosure for thermal dissipation, CD-ROMs, connectors, I/O cabling and so on. The performance of shielding enclosures for high-speed digital systems is compromised by these inevitable discontinuities on enclosure. To minimize the electromagnetic interferences and susceptibility risks by these discontinuities, the shielding enclosures with apertures should be designed based on thorough analysis about the electromagnetic coupling mechanism through apertures. In this paper, the effect of apertures and oblique incident plane wave on shielding effectiveness of the enclosure is studied with the Circuital Approach, and the Finite-Difference Time-Domain (FDTD) method, the simulated SE data are verified by experimental technique. Good agreements are found between these approaches.

Quasi-Elliptic Bandpass Frequency Selective Surface Based on Coupled Stubs-Loaded Ring Resonators

In this paper, a novel design of frequency-selective surface (FSS) based on coupled stubs-loaded ring resonators (SLRRs) is proposed. The proposed FSS exhibits quasi-elliptic bandpass filtering characteristic at C-band. And each unit cell of the structure is composed of two SLRRs coupled by a circular aperture. A novel method of transmission line (TL) model is proposed to investigate the operating principle of a single SLRR under normal incidence plane wave. Moreover, the operating mechanism of the proposed FSS is further researched with the aid of the equivalent circuit model (ECM) analysis method. Due to the symmetric and low-profile physical structure, the FSS keeps stable frequency response under oblique incident plane wave for both transverse electric (TE) and transverse magnetic (TM) polarizations. Finally, the designed FSS is fabricated and measured. The measured results agree well with the simulated results. The measured results indicate that the proposed FSS exhibits a dual-polarized and stable bandpass frequency response with two transmission zeros (TZs) at around 5.71 and 6.87 GHz under different incident angles up to 40°.

Sound transmission through a thick-walled FGM piezo-laminated cylindrical shell filled with and submerged in compressible fluids

Estimation of sound transmission loss (TL) due to the piezoelectric effects, as an attenuation of acoustic waves, is studied for a thick-walled piezo-composite cylindrical shell excited by an oblique incident plane wave. The cylinder is filled with and submerged in an acoustic media. The three-dimensional (3D) exact theory of elasticity and piezoelasticity are engaged to model the cylindrical shell, while the classical Helmholtz equation governs the propagation of waves through the internal cavity and external surrounding fluid. A state space method, as well as the transfer matrix technique is utilized to describe the deformation and stress in the cylindrical shell. TL is calculated by exact integration over the shell’s outer surface. The validity of the current analytical solutions is cross-checked with various data from the simplified case found in the relevant literature as well as a finite element package known as Comsol Multiphysics. Parameter studies are conducted to investigate the effects of piezoelectric material properties, piezoelectric polarization direction, shell thickness ratio, electrical boundary conditions and functionally graded piezoelectric material (FGPM) on the sound transmission loss due to the piezoelectricity. New results and findings provide guidance of piezoelectric coupled with thick shell design for passive wave absorption.

The Fresnel Coefficient of Thin Film Multilayer Using Transfer Matrix Method TMM

When the light incident on a multilayer planar mound, it is reflected, transmitted, and absorbed in the method that derived the Fresnel equations. The metal used in this work is the thin film of gold put on the glass. The properties of the preparation of waveform for dielectric charged surface Plasmon polariton structure of the bearing surface of the carrier plasma were studied using a simple transport matrix method(TMM), The Transfer Matrix Method (TMM) used to calculate the reflection, transmission & absorption for both TE (transverse wave) and TM (magnetic browser for the oblique incident plane wave. The relation between the incident angle with the Fresnel coefficient plotted at the different wavelength regimes (300nm,700nm,1200nm) was plotted using the program of the TMM written in Matlab (R2013). The best result for this Fresnel coefficient in the visible incident plane wave with wavelength = 700nm.

Model approximation for sound transmission from underwater structures in high-frequency range

Sound-insulation model provides a straightforward way to describe sound transmission behaviours of the thin-walled structures in engineering applications. The sound transmission characteristics depend on the parameters of incident wave, such as incident wave amplitude and incident angles. However, this model is limited when the sound source is located in an enclosed space (e.g., noise source in underwater cabins), because it is difficult to obtain incident angles especially in the high-frequency range. In this paper, we develop a simply analytical model that can effectively study the sound transmission from an enclosed shell with internal acoustic excitation. In order to extend the application of the sound-insulation model to a submerged shell, the structural vibration equation is firstly simplified to the plate vibration equation. Then, the sound pressure near the inner surface of the shell is decomposed into an expansion of orthogonal cavity eigenmodes, and each cavity mode is replaced by two pairs of incident plane waves. Finally, the acoustic transmission loss can be obtained by substituting the parameters of incident waves into the sound-insulation model. Numerical results show that the sound transmission for the fundamental cavity mode (0, 0, 0) can be explained by the normal incidence in the sound-insulation model, while every other modes corresponds to a group of oblique incident plane waves whose incident angles decrease monotonically with the increase of frequency. In addition, it can be observed that the total reflection phenomenon in the sound-insulation model is consistent with the low radiation efficiency of the high order modes in the shell model.

On the matching medium for microwave-based medical diagnosis**This work was supported in part by the University of Queensland under the University of Queensland Postdoctoral Research Fellowship Scheme, Chalmers University of Technology under the Karl G Eliasson Supplementary Fund, and Swedish Research Council under Grant number 2010-7262-78830-59.

In microwave-based medical diagnosis, reflections of electromagnetic waves from the air–skin interface can be reduced with a proper choice of matching medium such that electromagnetic radiation can be effectively penetrated into and through human bodies. Due to the heterogeneous and lossy nature of tissue and the fact that the electromagnetic problem varies as a function of matching medium, frequency, incident angles and polarization, it is not trivial to analyze the entire propagation phenomena. In this study, transmission of electromagnetic wave through a multi-layer planar human tissue model with different matching medium is considered. The reflected, absorbed and transmitted power components are computed to quantify the transmission performance of the matching medium. Our results show that although reflection can be minimized through a proper choice of matching medium, it does not necessary result in an optimal transmission as the transmitted signal is severely attenuated inside the tissue. Our results also show that the oblique incidence plane wave components, which in general correspond to the non-zero spatial frequency components of a near field distribution, can better penetrate through the tissue when a low-permittivity matching medium is used. These findings suggest that a better signal integrity can be obtained when a low-permittivity matching medium ( between 5 and 30) is used.