Monopole Source Research Articles

Chuprov invariant for vector-scalar fields of multipole sources in a shallow sea

A computational and theoretical study of the properties of the well-known waveguide invariant S.D. Chuprova (IC) was carried out in a plane-parallel Pekeris waveguide. Unlike earlier works, in which predominantly non-directional (monopole) sources were used as a source, and sound pressure fields (scalar fields) were studied, in this work not only scalar, but also vector fields formed in the waveguide by directional - combined multipole sources with directivity in both horizontal and vertical planes. A differential equation has been obtained that makes it possible to fairly accurately calculate the IC values under different conditions of signal propagation and different depths of sources and receivers. This makes it possible, in a simpler way than “full computer modeling,” to predict the invariance (stability) of the IC when varying both the hydrophysical conditions in the waveguide and the geometry of the experiment. It is shown that the directionality of sources in the horizontal plane has virtually no effect on the properties of the IC, and the directionality in the vertical plane leads to a shift in the fan structure of the signal amplitude fields, but has little effect on the IC values. The properties of the fan structure change in a similar way when using vertical projections of the oscillatory velocity vector - despite the fact that another analytical relation, different from scalar fields, is used to calculate the IC, the IC value is close to (+1) at all frequencies and distances, except those at which new modes or dislocations arise. At these frequencies and in these zones, alternating emissions with different signs and magnitudes occur. It is concluded that the stability of IC allows the application of signal processing algorithms developed for scalar fields and non-directional sources to vector-scalar fields generated, including using directional sources.

A study on aerodynamic pass-by noise based on equivalent source method

Pass-by noise generated by high-speed trains is an important influencing factor on the surrounding environment and is getting more and more attention with the increase of speed. In the past, the aerodynamic pass-by noise prediction was usually conducted with the wind tunnel mode, ignoring the Doppler effect. In this paper, an equivalent source method considering the Doppler effect is proposed to study the aerodynamic pass-by noise. A cylinder case is considered first to verify the effectiveness of the method. Then a DSA 380 pantograph of a Chinese high-speed train is considered. The pantograph is divided into three parts, and each part is equivalent to a monopole or dipole sound source. Results show that the upper part of the pantograph is closer to a dipole source, while the middle and lower parts are closer to a monopole source. When the measurement point is directly above the pantogragh, the contribution of the upper part is the largest, while for both sides measurement points of the pantograph, the contribution of the upper part is the smallest.

Accounting for the vehicle influence on tyre noise through acoustic simulation based on equivalent monopole sources

Road traffic noise poses a risk to human health and amongst the different noise sources, tyre-road interaction plays a fundamental role. Due to the regulations introduced to limit noise levels, tyre manufacturers are committed to design acoustically optimized tyres. In this context, a numerical-experimental approach for including vehicle influence on tyre noise has been investigated. As a first step, equivalent monopole sources are identified based on the measurements of the radiated sound field of a rolling tyre in a semi-anechoic chamber. In a second step, the same sound sources are then employed for the prediction of sound propagation when tyres are mounted on a vehicle, which is included in the acoustic simulation. Finally, a validation against indoor experimental measurements is performed, yielding promising results.

Optimization of an active device for global control of low-frequency wall reflections in a semi-anechoic room

An active device has been designed and built at LMA to control low-frequency reflections on the absorbing walls of a semi-anechoic room: the sound pressure reflected by the walls in the presence of an unknown source is estimated by linear filtering of the total sound pressure near the walls, then neutralized using acoustic sources placed on the walls. Here, we present 2D numerical simulations, involving monopole sources and series of damped analytic modes, which have enabled us to optimize the active device, in particular to (i) deal with resonance frequencies at which cancelling the diffracted pressure near the walls is not sufficient to cancel it inside the room, (ii) minimize the number of the total pressure measurement points required to estimate the pressure diffracted by the walls at each location, and (iii) select the strategy for minimizing the virtual signals corresponding to the diffracted pressure. Simulations show that with the optimized device, the control remains efficient up to frequencies corresponding to one wall source per wavelength and a little less than two pressure sensors per wavelength. Experiments are underway to compare measurements with simulations.

Numerical modeling of axial fan noise in fuel-cell electric truck: from component to full-vehicle analysis

This paper presents a comprehensive approach to predict the noise generated by an axial fan in electric vehicles, which is more noticeable due to the absence of combustion engines. Such cooling fans require stronger cooling performance due to increased demand for heat dissipation, resulting in even higher noise levels. A numerical model for fan noise in a fuel-cell electric vehicle (FCEV) is proposed with two steps: component-level and full-vehicle level models. At component level, Lattice Boltzmann Method (LBM) is employed for aeroacoustics simulation, determining the component sound power level (SWL). At full vehicle level, statistical energy analysis (SEA) is utilized to estimate the sound pressure level (SPL) inside the vehicle cabin, particularly at the driver's ear locations. The noise sources in the full vehicle model, were characterized from the component level SWL, as monopole sources. The model is validated by comparing the numerical predictions with the experimental measurements of the SWL in a hemi-anechoic chamber and the SPL in the cabin. Good agreements were obtained between numerical results and measurement results, demonstrating the effectiveness of the model. This allows for faster and more accurate virtual NVH design validations on large and complex systems where prototype measurements are difficult to conduct.

Source characterization of a ducted source using acoustic free velocity

A common method to characterize linear time invariant ducted acoustic sources is the electro-acoustic analogy. The sources are typically defined by their internal source strength and source impedance. Various methods have emerged to determine the source characteristics, broadly categorized into direct and indirect methods. Direct methods involve using a secondary source with significantly higher amplitude to measure source impedance but cannot determine source strength. Indirect methods vary acoustic loads to obtain both source strength and source impedance, accuracy depends on appropriate load combinations. In this research, we propose an inverse method to determine the acoustic source strength from the measured acoustic transfer function and the operational acoustic pressure. This approach yields what we term as the "acoustic free velocity," corresponding to a plane monopole source that characterizes the original sound source. Subsequently, the source impedance is obtained using the acoustic free velocity in conjunction with the load impedance and acoustic pressure in the acoustic circuit. The method is demonstrated using a compressor driver source attached to a 1 3/8" impedance tube. Validation of the measured source strength and source impedance is conducted using a two-load method. Additionally, prediction of muffler insertion loss shows a good correlation with experimental measurements, affirming the efficacy of the proposed methodology.

Numerical simulation of borehole compressional wave and shear wave in 3D vug formation

The staggered grid finite difference method (SGFDM) of a monopole source is used to simulate a three-dimensional vug reservoir model and study the effect of acoustic logging responses of different vug models on radial probing depth. The results show that the first arrival of the head wave peak of the compressional wave (P-wave) and the shear wave (S-wave) is unrelated to the radius of the vug, and the amplitude of the head wave peak of the P-wave and S-wave decreases as the vug volume increases. Compared with the volume change of the vug, the radial distance from the vug wall has little influence, while the vertical source distance has large influence on the P-wave and S-wave. When there are multiple vugs in the model, the amplitudes of the P-wave and S-wave head wave peaks change sinusoidally with the angle between the vugs. The ellipsoidal vug model with the same volume has a greater influence on the P-wave and S-wave than the spherical vug model. In the ellipsoidal vug model, the axial vug size has a greater impact on the first arrival of the head wave peak, while the radial vug size significantly influences the amplitude of the head wave peak. Finally, we validate the numerical simulation conclusions by comparing them with actual logging data responses in complex formations, demonstrating the practical value of the elastic wave response simulations for vugs.

Perfect transmission through lossy layers via ideal acoustic sources

Complementary acoustic metamaterials (CAMMs) have been proposed as a means of enhancing the transmission of acoustic imaging signals through aberrating layers. Aberrating layers with high impedance contrast compared to the surrounding material disrupt the acoustic field and hence distort acoustic images. However, conventional CAMMs are passive and thus unable to compensate for signal distortions associated with loss. This work presents an approach inspired by CAMMs that is not bound by the limits of passivity to compensate for both acoustic scattering and energy attenuation by augmenting a plane wave incident on a lossy material with monopolar and dipolar source fields to allow for perfect transmission, thus rendering the lossy medium acoustically transparent. We present a general approach to derive expressions for source magnitudes that are dimensionless with respect to frequency, system geometry, and the background medium. We validate these results with three-dimensional finite element simulations, where the appropriate monopolar and dipolar source fields are generated by prescribing velocities on finite-dimensional boundaries. We show the limitations of this approach with regard to frequency, system geometry, and lossy material characteristics via a parametric study.

Extension of the source scanning technique to curved rectangular panels for the vibroacoustic characterization of structures using wall-pressure plane waves

The Source Scanning Technique (SST) is an experimental method used to synthesize the vibroacoustic response of a given structure under random excitations such as the Diffuse Acoustic Field (DAF) and the Turbulent Boundary Layer (TBL) using a single monopole source. It was previously developed and validated for simple plane structures such as Flat Rectangular Panels (FRPs). However, in industrial cases, the structures of interest can be more complex. In this paper, the feasibility of the extension of SST to Curved Rectangular Panels (CRPs) is investigated. To numerically evaluate the performances of SST when generating wall-pressure plane waves (WPPWs) on CRPs, analytical developments are proposed to estimate the transfer functions between the monopole source and the CRPs. Two- and three-dimensional cases are considered and validated using the boundary element method. Then, parametric studies are carried out to determine the optimal parameters for implementing the SST process in two- and three-dimensional cases considering a conformal geometry, i.e., the array of monopoles has the same geometry as the structure of interest. The numerical results obtained are then supported by experimental investigations using a robotized system to move the monopole source. The transfer functions obtained using the proposed analytical models are compared to the measured ones, yielding relatively good agreement between the results.

Acoustic emissions based estimation of the temporal changes in microbubble radius during ultrasonic excitation

Our ability to study microbubble dynamics in vivo and link them to distinct mechano-biological effects hinges on our ability to accurately estimate the temporal changes in MB radius during ultrasonic excitation. Here, we hypothesize that real-time passive cavitation detection (PCD) monitoring combined with linear acoustic wave propagation theory can accurately estimate stable MB radius dynamics under in vivo conditions. To test our hypothesis, we employed numerical simulations, based on Rayleigh–Plesset modeling, followed by experimental validation, using calibrated PCDs with concurrent optical imaging of MB dynamics using high frame rate microscopy. Our method termed linear acoustic wave propagation and superposition (LAWPS) algorithm, combines Fourier series expansion with Euler’s relationship to estimate the acoustic emission (AE) from single MB, which is considered as a monopole source (i.e., R0 + ΔR < λ). LAWPS algorithm, which is independent of MB properties and, thus, can be linearly reversed to calculate MB oscillation radius from AE, was able to accurately capture the radiated pressure generated from a Rayleigh–Plesset MB modeling. Crucially, inverse LAWPS algorithm onto AE generated by Vokurka et al. resulted in the original MB model. Experimental observation using monodisperse MBs was able to accurately estimate the temporal changes in MB radius during 0.5 MHz ultrasonic excitation

Variational study of a model for near-perfect transmission through lossy media with acoustic sources

Complementary acoustic metamaterials have been proposed as a means of compensating for the high impedance mismatches of aberrating layers that disrupt the acoustic field and hence distort acoustic images. Recently, a complementary acoustic metamaterial featuring active components was shown in principle to compensate for both the impedance mismatch and energy attenuation of lossy materials, but a physical realization of the concept has not yet been implemented. Here, we present results from a one-dimensional acoustic model showing how a plane wave incident on a lossy material can be augmented by point monopole and dipole sources to allow for near-perfect transmission, thus rendering the lossy medium acoustically transparent. We present general expressions for source magnitudes that are dimensionless with respect to frequency, material thickness, and the background medium. We explore the sensitivity of the performance to variations in each of the model parameters, considering both theoretical and practical limitations to the proposed method. We show that these findings are consistent with three-dimensional finite element simulations.

General acoustic Willis media modeled as conventional materials with embedded sources

Willis coupling vectors provide additional degrees of freedom to manipulate acoustic waves unavailable in conventional materials, but the additional material properties make it difficult to simulate the interaction between sound and Willis media especially when inhomogeneous media with arbitrary geometries are involved. To address this challenge, we establish the equivalence between the wave equation inside a three-dimensional, inhomogeneous, and anisotropic Willis medium to the well-known acoustic wave equation in conventional materials with embedded continuous distributions of monopole and dipole sources. We validate the equivalence in numerical simulations showing that conventional materials with embedded sources replace physical Willis metamaterials anisotropic in the Willis and mass density tensors without modifying the scattered sound distribution. Moreover, the equivalence offers insights into the physics of Willis materials. For example, it shows that various combinations of Willis coupling vectors can generate identical sound scattering for any excitation. It also shows whether effective material parameters extracted from single Willis cell simulations remain valid in bulk metamaterials obtained by replicating periodically that cell. These results suggest that the equivalence model can advance the design of Willis metamaterials and provide a tool to better understand the physics of Willis media.

Numerical study of the scattering of acoustic waves by an elliptic vortex.

The scattering of the acoustic waves generated by a monopolar source propagating through a two-dimensional elliptic vortex, fixed or convected by a uniform flow, is studied by solving the Linearized Euler Equations in Cartesian coordinates using the Discontinuous Galerkin Method. For a fixed vortex position, the number, amplitudes, and angular spreads of the acoustic interference beams resulting from the sound scattering are found to significantly depend on the orientation of the vortex major axis with respect to the direction of the incident waves and on the vortex maximum tangential velocity. In particular, additional interference beams are obtained at large observation angles for a more elliptical vortex. For a convected elliptic vortex, the interference beams are curved as the angle between the incident acoustic wave and the vortex major axis varies when the vortex travels in the downstream direction. As expected, the scattering of the acoustic waves leads to spectral broadening in this case. Moreover, the widths and the frequencies of the lateral lobes obtained in the spectra on both sides of the peak at the source frequency are different for elliptic and round vortices.

Two-dimensional finite difference-time domain simulation of moving multipole sources

In this paper, the implementation of a moving multipole sound source in the two-dimensional (2D) finite difference-time domain method is described. The fundamental solution of the moving multipole source is theoretically derived by spatial differentiation of the fundamental solution of a moving monopole source in the 2D field. It was found theoretically that the directivity of a moving multipole source depends on the velocity and order of spatial differentiation. Numerical experiments were performed on the 2D sound field for moving multipole sources, and the results showed that the effect of the moving velocity on the amplitude of the multipole source is increased with the order of spatial differentiation. It was also found that the higher the order of spatial differentiation, the sharper the directivity in the moving direction and the larger the front-to-back ratio of the directivity. The present method can be accurately applied to the moving multipole sound sources.

The equivalent source method is an indirect boundary element method with an implicit Voronoi mesh

The Equivalent Source Method (ESM) consists of modeling the acoustical field radiated or scattered from an object by a set of radiating monopole sources located below its surface. While highly efficient for modeling the scattered field by a smooth object, ESM becomes less effective in representing acoustic diffraction from sharp edges. In this article, ESM is understood as an approximation to the single layer integral equation. In this framework, ESM belongs to the family of Indirect Boundary Element Methods (BEM) with an implicit surface mesh slightly below the object surface, and a zero order integration scheme on each element of the mesh. This implicit mesh is well approximated by the Voronoi diagram associated to the source distribution. This formalism establishes a strong link between these two widely used methods (ESM and BEM) and allows the construction of hybrid methods that can combine performances of both methods. On the first hand, a new hybrid method able to model edge diffraction while remaining as computationally efficient as ESM is developed. In this method, labeled “BEM Voronoi”, equivalent sources are positioned on the surface’s object rather than below it and singularities are handled with the integral formulation. On the other hand, a set of new hybrid methods which combines ESM with higher order integration scheme is constructed. Performances of the new methods developed in this paper are numerically compared to each other and to Finite Element simulations (used as a reference) for modeling scattering from an object excited by a plane wave. Each hybrid model is tested on multiple configurations involving two sets of boundary conditions (rigid and elastic objects respectively) successively immersed in two different external environments (water and air) and for both smooth and a sharp-edged objects. The results show that the hybrid method “BEM Voronoi” better handles edge diffraction than conventional ESM, at a much lower computational cost (implementation of BEM Voronoi is 50 times faster than conventional BEM implementation).

Sound power determination by intensity-Are field indicators and criteria in ISO 9614 meaningful?

The three parts of ISO 9614 describe methods for the determination of the sound power level of noise sources. According to these standards, measured sound power levels must be qualified by comparing several sound field indicators to given criteria. This procedure is investigated by analytical calculations with monopole and dipole sources. Their sound fields are superposed with extraneous free and diffuse sound fields. When the ISO 9614 method is applied to these cases, it turns out that the signed pressure intensity indicator is well suited to qualify the measured sound power level. In contrast to this, the unsigned pressure intensity indicator and the field non-uniformity indicator fail to describe the quality of the determined sound power level. This theoretical finding is verified by a large measurement program.

On the noise reduction mechanisms of over-tip liners.

The application of acoustic liners near/directly over a sound source has gained significant interest for the excess noise reduction achieved with Over-the-Rotor (OTR) liners compared to the conventional liner installations at the intake of an aero-engine. However, the mechanism of noise reduction achieved in the OTR liners is not clearly understood. This paper aims to explain this mechanism by considering a static monopole source placed over a finite liner insert with a zero background mean flow. This has been investigated numerically using COMSOL Multiphysics in a half-space domain and compared with reference analytical solutions for infinite lined walls. One of the key findings of the paper is the underlying physics of the source modification mechanism, which has been found to be the interference between the primary noise source and a secondary noise source forming on the liner surface. It is identified through an optimal impedance study that this back-reaction mechanism is dominant when the source is located for a normalised tip gap, e/λ<0.25, where e is the distance between the source and the liner surface and λ is the acoustic wavelength. Within this region, there exists an optimum normalised liner length, L / e providing a maximum insertion loss.

Analyzing monopole sources modeling with structural variations and material contrast: An analytical perspective

This study presents a comprehensive analysis of modeling acoustic radiation from monopole sources within waveguides, considering variations in structural parameters, including backward passage and the use of sound-absorbent materials. The investigation employs analytical techniques, including mode-matching methods and Fourier transforms, to address the intricate boundary value problem governing acoustic behavior in waveguides. Eigenfunction expansions are utilized to describe acoustic responses in various segments, leading to systems of linear algebraic equations that are numerically solved after truncation. The research examines the influence of critical factors such as frequency, thickness of absorbent materials, and chamber length on power components, including reflection, absorption, transmission, and propagation, across different regions of the waveguide. Notably, the choice of sound-absorbent material emerges as a significant factor affecting scattering characteristics, with A-Glass outperforming E-Glass and Steel Wool in various scenarios. These insights offer valuable knowledge for optimizing acoustic systems in engineering applications, facilitating the design and enhancement of sound control mechanisms within waveguides.

Numerical simulation and prediction of aerodynamic noise of high-speed trains

In order to improve the efficiency of high-speed train aerodynamic noise analysis and provide a feasible method for aerodynamic noise prediction, the aerodynamic noise and its influencing factors are analyzed from the perspective of the total energy (sound power) radiated to the far field per unit time by the aerodynamic fundamental noise sources (monopole, dipole and quadrupole sources). A full-scale and a 1:8 scale-down computational fluid dynamics model of a high-speed train are established. The far-field sound pressure level at several receivers and velocities is calculated by using the transient large eddy simulation and the FW-H equation. The numerical simulation results are used to predict the aerodynamic noise under specified working conditions. The research work can achieve the prediction of aerodynamic noise at other velocities using noise data at known velocities on the same noise source case, as well as the prediction of aerodynamic noise of full-scale model using data of scale-down model, and is applicable to either bogies as local noise sources or the complete vehicle as a noise source. The maximum error between the prediction and simulation result is 0.33 dBA under various working conditions, which meets the engineering calculation requirements.

Measurement of Compound Sound Sources with Adaptive Spatial Radiation for Low-Frequency Active Noise Control Applications

The proposed compound sound sources for low-frequency noise control applications are composed of dipole sources. Their spatial radiation, which is critical in the modal field of small, closed spaces, is intended to be controlled with independent driving signals of each dipole. The need for small and efficient low-frequency elementary monopole sources led to the proposed vented sub-woofer loudspeaker design with low force factor (low-Bl) drivers. The investigated sources are set up in quadrupole configurations and measured in terms of polar near field response patterns to verify the theoretical predictions. The measurement results consist of the validation of the proposed compound sound sources on the implementation of active noise control problems in the low-frequency range. Also, their small size and modular construction make them interesting for use in other applications.