Virtual Source Research Articles

An indirect geometry crystal time-of-flight spectrometer for FRM II

We present a concept for an indirect geometry crystal time-of-flight spectrometer, which we propose for a source similar to the FRM-II reactor in Garching. Recently, crystal analyzer spectrometers at modern spallation sources have been proposed and are under construction. The secondary spectrometers of these instruments are evolutions of the flat cone multi-analyzer for three-axis spectrometers (TAS). The instruments will provide exceptional reciprocal space coverage and intensity to map out the excitation landscape in novel materials. We will discuss the benefits of combining a time-of-flight primary spectrometer with a large crystal analyzer spectrometer at a continuous neutron source. The dynamical range can be very flexibly matched to the requirements of the experiment without sacrificing the neutron intensity. At the same time, the chopper system allows a quasi-continuous variation of the initial energy resolution. The neutron delivery system of the proposed instrument is based on the novel nested mirror optics, which images neutrons from the position of the pulse cutting chopper representing a bright virtual source onto the sample. The spot size of less than 1 cm × 1 cm at the virtual source allows the realization of very short neutron pulses by the choppers, while the small and well-defined spot size at the sample position provides an excellent energy resolution of the secondary spectrometer.

Multichannel Analysis of Surface Waves based on Common Virtual Source Gathers of Seismic Ambient Noise Cross-Correlations: A Case Study at an Earth Dam in Brazil

The S-wave velocity (Vs) is a valuable parameter for assessing the mechanical properties of subsurface materials for geotechnical purposes. Seismic surface wave methods have become prominent for estimating near-surface Vs models. Researchers have proposed methods based on passive seismic signals as efficient alternatives to enhance depth of investigation, lateral resolution and reduce field effort. This study presents the Multichannel Analysis of Surface Waves (MASW) utilizing Common Virtual Source Gathers (CVSGs) derived from seismic ambient noise cross-correlations, based on Ambient Noise Seismic Interferometry concepts. The method is applied to passive data acquired with an array of receivers at the Paranoá earth dam in Brasília, Brazil, to construct a pseudo-2D Vs image of the massif for interpretation. Our findings showcase the adopted processing flow and combination of methods as an effective approach for near-surface Vs estimation, demonstrating its usability also for large earth dam embankments.

Radon-domain acoustic and elastodynamic interferometric redatuming of VSP data

Summary The virtual source method (VSM) is a useful tool for imaging and monitoring below complex and time-varying overburden. When it is applied to VSP geometries, the redatumed virtual source data often suffer from artifacts and aliasing due to the violation of theoretical acquisition conditions and partial focusing of virtual multiples, which further degrade seismic imaging quality. While the conventional Radon-domain VSM (R-VSM) mitigates these issues to some extent, it is still possible to improve upon the imaging. This study develops a simple and effective high-resolution Radon-domain VSM (HR-VSM) to effectively address these issues, offering superior noise and artifact suppression compared to traditional VSM and R-VSM. HR-VSM shows a strong performance across a wide range of VSP configurations and data types, especially in applications to multi-component and passive seismic data, demonstrating its versatility and effectiveness in seismic exploration and showing potential for use in earthquake seismology. To extend subsurface illumination, we combined correlation-type and convolution-type HR-VSM for automatically redatuming virtual surface seismic profile (SSP) shot gathers with a complete receiver array covering all surface source locations, without the additional pre-processing steps typically required by VSM. It was also tested on reverse VSP (RVSP) synthetic active data and passive seismic data, proving effective in constructing virtual shot gathers and showing potential for extending its use to earthquake data and ambient-noise seismology. Moreover, HR-VSM can perform elastodynamic interferometric redatuming, enabling the redatuming of pure P-P and pure P-SV waves from multi-component VSP data. We applied HR-VSM to redatum virtual single well profile (SWP) data containing fault-related P-P and P-SV reflections from VSP data, which were further used for fault imaging. Finally, HR-VSM was used to generate virtual crosswell data with receivers in both boreholes, eliminating the need for downhole sources. The obtained virtual crosswell containing direct waves, as well as P-P and P-SV reflections, enable constructing a velocity model and imaging the structure between wells.

A reciprocity-aware, low-rank regularization for multidimensional deconvolution

A novel factorization-based, low-rank regularization method is introduced to solve multidimensional deconvolution (MDD) problems in the frequency domain. In this approach, each frequency component of the unknown wavefield is represented as a complex-valued square matrix and approximated as the product of a rectangular matrix and its transpose. The benefit of such a parameterization is twofold. First, each frequency matrix is guaranteed to be symmetric. From a physical standpoint, the recovered wavefield inherently adheres to the principle of reciprocity; in other words, the response to a receiver from a virtual source does not change when the two are swapped. Therefore, we refer to this low-rank parameterization as reciprocity-aware in that it respects the underlying physics of wave propagation associated with the MDD solution. Second, the size of the unknown matrix is greatly reduced compared with that of the original wavefield of interest (and halved compared with the existing factorization-based low-rank approximations). We further show that the associated objective function can be successfully optimized using the accelerated proximal gradient algorithm and present a robust strategy to define the initial guess of the solution. Numerical examples on synthetic and field data demonstrate the effectiveness of the proposed method in compressing the retrieved Green’s function while preserving its accuracy.

An efficient device model for ferroelectric thin-film transistors

Ferroelectric thin-film transistors (Fe-TFTs) have promising potential for flexible electronics, memory, and neuromorphic computing applications. Here, we report on a physics-based efficient device model for Fe-TFTs that effectively describes memory switching and device I–V characteristics. This model combines a stochastic multi-domain description of FE switching dynamics with a virtual source treatment of TFT device characteristics. It demonstrates that the memory window of Fe-TFTs depends on the amplitude and duration of the applied voltage pulses, thus suggesting quantitative means of programming and control. Additionally, we introduce a machine-learning-enabled method to automatically generate optimal voltage pulses for accurately programming multiple intermediate FE states, which is crucial for multi-bit memory and neuromorphic computing applications. To showcase the model’s applications, we simulate a 4×4 crossbar array circuit based on Fe-TFTs, highlighting its utility in performing multiply-accumulate computing operations. This small array can achieve a high speed of ∼1.28 tera operations per second (OPS) and a power efficiency of ∼0.43 W/PetaOPS. The model developed here is valuable for exploring the capabilities of Fe-TFTs in future flexible memory and computing technologies.

A simple, effective and high-precision boundary meshfree method for solving 2D anisotropic heat conduction problems with complex boundaries

A simple, effective and high-precision boundary meshfree method called virtual boundary meshfree Galerkin method (VBMGM) is used to tackle 2D anisotropic heat conduction problems with complex boundaries. Temperature and heat flux are expressed by virtual boundary element method. The virtual source function is constructed through the utilization of radial basis function interpolation. Calculation model diagram and discrete model diagram of real boundaries, and schematic diagram of VBMGM are demonstrated. Using Galekin method and considering boundary conditions, the integral equation and the discrete formula of VBMGM are given in detail. The benefits of the Galerkin, meshfree, and boundary element methods are all presented in VBMGM. Seven numerical examples of general anisotropic heat conduction problems (including three numerical examples with complex boundaries and four numerical examples with mixed boundary conditions) are computed and contrasted with precise solutions and different numerical methods. The computation time of each example is given. The number of degrees of freedom used in many examples is half or less than that of the numerical method being compared. The suggested method has been demonstrated to be effective and high-precision for solving the 2D anisotropic heat conduction problems with complex boundaries.

Gradient descent optimization of acoustic holograms for transcranial focused ultrasound

Acoustic holographic lenses, also known as acoustic holograms, can change the phase of a transmitted wavefront in order to shape and construct complex ultrasound pressure fields, often for focusing the acoustic energy on a target region. These lenses have been proposed for transcranial focused ultrasound (tFUS) to create diffraction-limited focal zones that target specific brain regions while compensating for skull aberration. Holograms are currently designed using time-reversal approaches in full-wave time-domain numerical simulations. Such simulations need time-consuming computations, which severely limits the adoption of iterative optimization strategies. In the time-reversal method, the number and distribution of virtual sources can significantly influence the final sound field. Because of the computational constraints, predicting these effects and determining the optimal arrangement is challenging. This study introduces an efficient method for designing acoustic holograms using a volumetric holographic technique to generate focused fields inside the skull. The proposed method combines a modified mixed-domain method for ultrasonic propagation with a gradient descent iterative optimization algorithm. The findings are further validated in underwater experiments with a realistic 3D-printed skull phantom. This approach enables substantially faster holographic computation than previously reported techniques. The iterative process uses explicitly defined loss functions to bias the ultrasound field’s optimization parameters to specific desired characteristics, such as axial resolution, transversal resolution, coverage, and focal region uniformity, while eliminating the uncertainty associated with virtual sources in time-reversal techniques. The proposed techniques enable more rapid hologram computation and more flexibility in tailoring ultrasound fields for specific therapeutic requirements.

Virtual source total focusing method for surface defects using leaky Rayleigh waves

Leaky Rayleigh waves are sensitive to surface defects and beneficial for automated detection due to their non-contact characteristics. However, the amplitude of the leaky Rayleigh waves is low due to the attenuation of waveform conversion and acoustic waves propagation, which limits the imaging quality for long-distance detection. This study introduces a novel method combining leaky Rayleigh waves detection with virtual source total focusing method. The virtual source (VS) technology enhances the emission energy of leaky Rayleigh waves. Then, the total focusing method (TFM) is applied to acquire high-accuracy images of defects. To reduce noise and artifacts, a weighting function based on the coherence factor (CF) is developed to weight the TFM superimposed signals, thereby achieving high-quality image reconstruction. Experimental results show that compared with the conventional TFM method, the proposed method can effectively improve the amplitude of imaging signals while reducing system noise and imaging artifacts. The lateral accuracy of defects is improved, with the average lateral error of defect size being 0.178 mm. The signal-to-noise ratio (SNR) of ultrasound images is increased by 27.59 dB, and the array performance index (API) of ultrasound images is decreased by 33.78 %. The proposed method provides a new and effective approach for quantitatively assessing the surface defects of metal components using leaky Rayleigh waves.

Virtual sound source construction based on adaptive crossfade processing with electro-dynamic and parametric loudspeaker arrays

Among the various high-presence sound field reproduction technologies, we focus on wave field synthesis (WFS) using a loudspeaker array, as it can construct a virtual sound source (VSS) with few restrictions. A VSS using parametric array loudspeakers (PALs) can provide more accurate direction perception than VSS using electro-dynamic loudspeakers (EDLs) because PALs hold sharp directivity by utilizing ultrasounds. However, the distance perception tends to be degraded. We previously proposed a VSS construction based on WFS with hybrid control of an EDL and a PAL arrays. This method enhances distance perception by exploiting the difference in radiation characteristics between PALs and EDLs. However, the difference in frequency responses between PAL and EDL is noticeable to the listener when switching between them. In this paper, to enable more smooth transition of the EDL and PAL arrays, we propose a VSS construction method based on adaptive crossfade processing with EDL and PAL arrays. For an arbitrary VSS construction position, the coefficients of crossfade for each frequency are determined by an adaptive filter based on measured impulse responses of VSSs with EDL and PAL arrays. We demonstrated the effectiveness of the proposed method through evaluation experiments.

Deep learning-enhanced blind separation of incoherent and spatially disjoint sound sources

Separating incoherent sound sources within complex acoustical fields presents a significant challenge in acoustic imaging. Existing methods, such as Principal Component Analysis (PCA) applied to the Cross-Spectral Matrix (CSM), yield 'virtual' sources based on statistical orthogonality. However, this approach often fails to accurately identify distinct physical sources, primarily due to its reliance on statistical orthogonality solely. A state-of-the-art method involves computing a rotation matrix to enforce criteria such as least spatial entropy, or spatial orthogonality among sources, a process that, while effective, significantly increases computational complexity and time. This work introduces a hybrid approach combining PCA and deep learning for predicting spatially disjoint source maps from virtual sources. By simulating sound sources in random quantities and locations, we train a neural network tailored to this task. We address the order mismatch between PCA-derived virtual sources and pre-simulated labels by framing source separation as a set prediction problem, utilizing the Hungarian loss for efficient mismatch resolution. This method simplifies the separation process, offering faster post-training computations and eliminating the need for complex optimizations. Validation in simulated environments and real-world datasets has shown the model's effectiveness in source separation for acoustic imaging, indicating the potential of integrating deep learning with existing methods.

The contribution of dynamic cues to externalization of virtual source in virtual auditory display

It is well known that static virtual auditory display with headphone is inclined to create the auditory perception of in-head-localization, especially for target virtual sources in the median plane. By introducing the dynamic variation of binaural pressures caused by head turning, dynamic virtual auditory display eliminates the in-head-localization effectively. The present work investigates the contributions of dynamic interaural time difference (ITD) and dynamic interaural level difference (ILD) in binaural pressures to the externalization of auditory perception. By modifying the head-related transfer functions used in a dynamic virtual auditory display, binaural signals with dynamic ITD or dynamic ILD alone are created and presented by headphone. A psychoacoustic experiment is conducted to evaluate the extent of perceived externalization. The experiment results prove that, for the target virtual source in the median plane, dynamic ITD and dynamic ILD contribute to the externalization of low and high frequency components of the stimuli respectively. For wideband stimulus with low and high frequency components, both dynamic ITD and dynamic ILD contribute externalization but dynamic ITD contributes more than that of dynamic ILD.

Azimuth perception of virtual sources in automotive environment : speech and musical stimulus

The spatial reproduction of musical media in car environment presents a number of challenges, due to the fact that the loudspeakers are not symmetrical in relation to the listeners and to the unique acoustic characteristics. In particular, the positioning of virtual sources is difficult to apprehend, unlike in conventional stereophonic configurations. In an initial study, it was shown that an azimuth perception model, the extended energy vector, produced results that were consistent with experimental data. As the first experiment was carried out using only a short burst of pink noise, it seemed interesting to compare this model with other data from more natural signals. New experiments were therefore carried out using audio samples such as speech and instruments. The subjects, seated in the driving seat, pointed their noses in the perceived direction of the signal, their head movements being recorded with a headtracker. The model will be compared with these new experimental data.

An Underwater Target Location Algorithm Combined with Schmidt Orthogonalization

Abstract In underwater target localization research, where challenges such as low signal-to-noise ratio and multiple interferences exist in conventional matched field processing, this paper presents a novel approach combining compressed sensing with Schmidt orthogonalization. Initially, an orthogonalization procedure is applied to the observation matrix, yielding a new matrix with reduced column coherence and maximal adherence to the Restricted Isometry Property (RIP) constraint. Subsequently, the Sparse Adaptive Matching Pursuit (SAMP) algorithm from compressed sensing is employed to reconstruct the signal, leading to precise underwater target localization. Simulation experiments substantiate that this combined approach outperforms the conventional matched field algorithm by yielding a higher signal-to-noise ratio for target acoustic source localization and markedly reducing the count of interference virtual sources. Consequently, the performance of localization for targets is substantially enhanced.

СКЛАДОВІ ПОТУЖНОСТІ ТИРИСТОРНОГО РЕГУЛЯТОРА НАПРУГИ З АКТИВНИМ НАВАНТАЖЕННЯМ

A study of the power balance of the semiconductor voltage regulator on an active load with phase control by two-operation thyristors was carried out. The suitability of its use for simultaneous obtaining of the reactive power resource for distribution electric networks has been confirmed. According to the concept of intelligent electrical networks, such devices belong to the class of flexible alternating current transmission systems, and they can be the basis for building virtual energy sources. The power components for individual components of the regulator, including the thyristor block, were studied. The spectral method of describing currents and voltages at the regulator sections is applied. The obtained results of theoretical studies were confirmed by an example with numerical calculations, which made it possible to clarify the physical essence of energy transformations during voltage regulation. Bibl. 8, fig. 3.

Efficient Photon Beam Diffusion for Directional Subsurface Scattering.

Real-time subsurface scattering techniques are widely used in translucent material rendering. Among advanced methods that rely on the bidirectional scattering-surface reflectance distribution function (BSSRDF), screen space algorithms exhibit limited translucency, while existing large-distance methods are inefficient and yield poor illumination details. To address these limitations for better large-distance scattering, we develop a novel algorithm by extending the photon beam diffusion (PBD) model within the light view and screen space. Unlike surface irradiance in prior methods, we incorporate the refracted beam in the medium into real-time scattering estimation, presenting a new consideration for photon beam utilization. Concretely, we store all photon beam samples in light view textures and utilize an adaptive sampling pattern for beam sample selection in large filtering kernel sizes. This can reduce the sample count based on surface attributes. In screen space, virtual sources are derived from samples to estimate PBD contributions, with an approximation that preserves boundary conditions. To avoid possible overestimation, we implement correction factors that scale contributions, effectively aligning our results with path-tracing references. Through these reformulations, our efficient PBD generates results closest to references among existing methods. The experiments accurately represent better front-face illumination details and backlit translucency effects, while significantly accelerating performance compared to previous large-distance methods.

Wave Optical Modeling of the SEM Column From Source to Specimen.

Probe formation in scanning electron microscope (SEM) is often reduced to objective lens action modeling based on a point-spread function or Fourier transforms. In this study, we present the first complete wave optical modeling of the whole SEM column based on plane-by-plane propagation of the electron beam wavefunction without simplifying the optical system. We identify the challenges in plane-by-plane beam propagation and show how sampling limitations produce aliased results. Through a careful selection and combination of propagators, we have developed a general wave optical propagation method that is able to overcome the aliasing problem to achieve the appropriate probe widths. Using a two-step propagator, we show that it is possible to model the electron beam distribution throughout the column from the virtual source plane to the specimen plane. We also show that our results from the wave optical simulations are consistent with the geometrical theory of probe formation. Finally, as a direct application of this method, we demonstrated that the combined effect of aberrations in the condenser lens and the probe forming objective lens cannot be accurately represented using only the objective lens. Designing beam shaping experiments and studying the effect of partial coherence can be some novel applications.

EchoSee: An Assistive Mobile Application for Real-Time 3D Environment Reconstruction and Sonification Supporting Enhanced Navigation for People with Vision Impairments.

Improving the quality of life for people with vision impairments has been an important goal in the research and design of assistive devices for several decades. This paper seeks to further that goal by introducing a novel assistive technology platform that leverages real-time 3D spatial audio to promote safe and efficient navigation for people who are blind or visually impaired (PVI). The presented platform, EchoSee, uses modern 3D scanning technology on a mobile device to construct a live, digital 3D map of a user's environment as they move about their surroundings. Spatialized, virtual audio sources (i.e., virtual speakers) are dynamically placed within the digital 3D scan of the world, providing the navigator with a real-time 3D stereo audio "soundscape." The digital 3D map, and its resultant soundscape, are continuously updated as the user moves about their environment. The generated soundscape is played back through headphones connected to the navigator's device. This paper details (1) the underlying technical components and how they were integrated to produce the mobile application that generates a dynamic soundscape on a consumer mobile device, (2) a methodology for analyzing navigation performance with the application, (3) the design and execution of a user study investigating the effectiveness of the presented system, and (4) a discussion of the results of that study along with a proposed future study and possible improvements. Altogether, this paper presents a novel software platform aimed at assisting individuals with vision impairments to navigate and understand spaces safely, efficiently, and independently and the results of a feasibility study analyzing the viability of the approach.

Super-Resolution Ultrasound Imaging Using the Erythrocytes-Part I: Density Images.

A new approach for vascular super-resolution (SR) imaging using the erythrocytes as targets (SUper-Resolution ultrasound imaging of Erythrocytes (SURE) imaging) is described and investigated. SURE imaging does not require fragile contrast agent bubbles, making it possible to use the maximum allowable mechanical index (MI) for ultrasound scanning for an increased penetration depth. A synthetic aperture (SA) ultrasound sequence was employed with 12 virtual sources (VSs) using a 10-MHz GE L8-18i-D linear array hockey stick probe. The axial resolution was [Formula: see text]m) and the lateral resolution was [Formula: see text]m). Field IIpro simulations were conducted on 12.5- μ m radius vessel pairs with varying separations. A vessel pair with a separation of 70 μ m could be resolved, indicating a SURE image resolution below half a wavelength. A Verasonics research scanner was used for the in vivo experiments to scan the kidneys of Sprague-Dawley rats for up to 46 s to visualize their microvasculature by processing from 0.1 up to 45 s of data for SURE imaging and for 46.8 s for SR imaging with a SonoVue contrast agent. Afterward, the renal vasculature was filled with the ex vivo micro-computed tomography (CT) contrast agent Microfil, excised, and scanned in a micro-CT scanner at both a 22.6- μ m voxel size for 11 h and for 20 h in a 5- μ m voxel size for validating the SURE images. Comparing the SURE and micro-CT images revealed that vessels with a diameter of 28 μ m, five times smaller than the ultrasound wavelength, could be detected, and the dense grid of microvessels in the full kidney was shown for scan times between 1 and 10 s. The vessel structure in the cortex was also similar to the SURE and SR images. Fourier ring correlation (FRC) indicated a resolution capability of 29 μ m. SURE images are acquired in seconds rather than minutes without any patient preparation or contrast injection, making the method translatable to clinical use.

Ultrasonic Imaging of Deeper Bone Defect Using Virtual Source Synthetic Aperture with Phased Shift Migration: A Phantom Study.

Ultrasound imaging for bone is a difficult task in the field of medical ultrasound. Compared with other phase array techniques, the synthetic aperture (SA) has a better lateral resolution but a limited imaging depth due to the limited ultrasonic energy emitted by the single emitter in each transmission. In contrast, the virtual source (VS) synthetic aperture allows a simultaneous multi-element emission and could provide a higher ultrasonic incident energy in each transmission. Therefore, the VS might achieve a high imaging quality at a deeper depth for bone imaging than the traditional SA. In this study, we proposed the virtual source phase shift migration (VS-PSM) method to achieve ultrasonic imaging of the deeper bone defect featured in the multilayer structure. The proposed VS-PSM method was validated using standard soft tissue phantom and printed bone phantom with artificial defects. The image quality was evaluated in terms of contrast-to-noise ratios (CNR) and amplitudes of scatters and defects at different imaging depths. The results showed that the VS-PSM method could achieve a high imaging quality of the soft tissues with a significant improvement in the scattering amplitude and without a significant sacrifice of the lateral and axial resolution. The PSM was superior to the DAS in suppressing the background noise in the images. Compared with the traditional SA-PSM, the VS-PSM method could image deeper bone defects at different ultrasonic frequencies, with an average improvement of 50% in CNR. In conclusion, this study demonstrated that the proposed VS-PSM method could image deeper bone defects and might help the diagnosis of bone disease using ultrasonic imaging.

A novel simplified simulation method for the precooler based on the virtual source term: modeling, validation and application

Precooling technology is one of the promising approaches for expanding the flight envelope of conventional turbine engines and enhancing flight performance at high Mach numbers. Within this domain, a crucial challenge is to rapidly and accurately obtain the flow and heat transfer characteristics of the precooler. Currently, full-size simulation of the precooler is impractical, while simplified and fast simulation methods employed in previous research have exhibited certain deficiencies, including excessive simplification and unconvincing validation. In this study, a novel simplified simulation method for the annular precooler based on the virtual source term model was proposed. This method comprehensively considered the axial and radial non-uniformity of airflow caused by the coupling effect between the precooler and the inlet channel in the actual flow field. Subsequently, systematic validations of the reliability and superiority of this method were conducted, along with its application in the overall performance calculation of the precooled engine. The results demonstrate that the simplified method proposed in this paper showcases higher accuracy than the methods employed in previous research. Compared with the real model, the relative errors of the overall parameters are within ± 1 %, and the relative errors of the distortion indices for total temperature and total pressure are 5.1 % and −1.5 %, respectively. Meanwhile, the number of computational mesh cells is reduced by more than 95 %. Moreover, the first application of the simplified method in the overall performance calculation of the precooled engine reveals that the traditional low-dimensional model underestimates both the total pressure loss of the precooler and the fuel consumption for precooling, resulting in larger calculation deviations in the thrust and equivalence ratio, respectively. The values of thrust calculated with the high-dimensional virtual model show an average decrease of 2.01 % compared to those derived from the low-dimensional model. In addition, a higher Mach number results in lower effectiveness, higher air inlet temperature, and reduced resistance of the precooler to heat exchange imbalance caused by flow non-uniformity. These factors lead to an escalating relative deviation of the equivalence ratio between the two models, reaching 26.45 % at Mach 5.