Sound Source Distribution Research Articles

The Forward and Inverse Problem Based on Magneto-Acoustic Tomography with Current Injection

The Magneto-acoustic Tomography with Current Injection (MAT-CI) is a new biological electrical impedance imaging technique that combines Electrical Impedance Tomography (EIT) with Ultrasonic Imaging (UI), which possesses the non-invasive and high-contrast of the EIT and the high-resolution of the UI. The MAT-CI is expected to acquire high quality image and embraces a wide application. Its principle is to put the conductive sample in the Static Magnetic Field(SMF) and inject a time-varying current, during which the SMF and the current interact and generate the Lorentz Force that inspire ultrasonic signal received by the ultrasonic transducers positioned around the sample. And then according to related reconstruction algorithm and ultrasonic signal, electrical conductivity image is obtained. In this paper, a forward problem mathematical model of the MAT-CI has been set up to deduce the theoretical equation of the electromagnetic field and solve the sound source distribution by Green’s function. Secondly, a sound field restoration by Wiener filtering and reconstruction of current density by time-rotating method have deduced the Laplace’s equation that caters to the current density to further acquire the electrical conductivity distribution image of the sample through iteration method. In the end, double-loop coils experiments have been conducted to verify its feasibility.

Identification of jet noise source using causality method based on large-eddy simulation of a round jet flow

An acoustic analogy analysis based on a decomposition of the source term in Lighthill’s equation is discussed in light of a large-eddy simulation of a subsonic turbulent jet exhausting from a baseline round nozzle at Mach number M = 0.9 with Reynolds number Re = 105. The decomposed sub-terms show the nonlinear reciprocal interactions of density, velocity, vorticity, and dilatation fields. To understand the aerodynamic sound generation mechanism, intrinsic links between turbulent flow and emitted acoustic signals are made and applied to the large-eddy simulation data. Cross-correlation functions are used for the links between the far-field sound signals and the sub-terms as well as major flow variables in the jet flow domain. The spatial distributions of cross-correlations are examined to identify the sound source distribution throughout the domain and observe the mutual interactions and cancellations between the decomposed sub-terms. The contributions of sub-terms are also studied in frequency domain.

Image Reconstruction in Magnetoacoustic Tomography With Magnetic Induction With Variable Sound Speeds.

Acoustic and electrical characteristics of biological tissue are important factors in magnetoacoustic tomography with magnetic induction (MAT-MI). Acoustic inhomogeneity significantly affects the propagations of sound waves. Differences in sound speed lead to distortions of the sound sources in the reconstruction process. The objective of this study is to develop a novel algorithm to reconstruct the sound source distribution in an acoustically inhomogeneous medium. The proposed algorithm is developed on the basis of the finite-difference time-domain method and time-reversal acoustic theory; it combines the relationship among symmetrical transducers with the back-projection algorithm. An acoustically inhomogeneous model with different regions of variable sound speeds is established to validate the proposed algorithm. From the data collected by a rotated focused transducer, first, the sound speed distribution is reconstructed, and then, the sound sources of the model are reconstructed. The reconstructed sound sources are obviously distorted when the speed differences are not considered. In contrast, the proposed algorithm yields reconstructed sound sources that are consistent with the model in terms of shape and size. Thus, the proposed algorithm is capable of accurately reconstructing the acoustic sources distribution in an acoustically inhomogeneous medium. This method provides a solution reducing the influence of acoustic inhomogeneity in MAT-MI. The distributions of sound speed can be obtained during the process of reconstructing the sound source. Consequently, the imaging of the acoustic speed and the electrical conductivity of biological tissues can be implemented simultaneously in MAT-MI.

A discrimination method for convergence zones using wideband noise signal

The strong similarity of signals from different convergence zones in deep water brings about high sidelobes in the ambiguity surface using the conventional match-field processing, which leads to localization ambiguity while the target is located in convergence zone. Aimed at the question, this paper presents a discriminate approach of convergence zone based on the l 1-optimization match-field processing based on compressive sensing theory, which takes advantage of the difference in fluctuating features of received signals at different convergence zones in frequency-domain due to the sparsity of space distribution of sound source. Compared with the conventional match-field processing, the approach has a high-resolution performance and can efficiently suppress side-lobe ambiguity problem under the condition of limited sampling. The validity of the approach is verified by means of simulation and the experimental data analysis of towed array.

Recording of extended sound fields using spherical microphone arrays and a-priori knowledge of the sound source positions

The acquisition of comprehensive sound field information is a central topic in spatial acoustics. In conventional systems, recording devices must be located at the listener’s viewpoint. This research introduces a sound field recording technique for spherical microphone arrays which makes use of a-priori information regarding the distribution of sound sources. The proposal generates sound field descriptions for viewpoints that are located away from the recording device, as long as there are no sound sources between the target viewpoint and the microphone array. Sound field descriptions, a set of spherical harmonic expansion coefficients, are generated from the array signals. A translation operator, calculated from approximate source positions known in advance, is defined so as to shift the plane wave decomposition of the partial fields associated with each expansion coefficient. The proposed method is compared with an existing technique based on a different kind of translation operator traditionally used in implementations of the boundary element method. These operators require no a-priori information, but their region of validity is limited by the spherical harmonic expansion order. The conventional method achieves greater accuracy in the proximity of the recording array; however, the proposed method can generate sound field descriptions covering a much larger region.

Automating source localization and separation using sonic detection and ranging

The sonic detection and ranging (SODAR) technology developed previously (Wu and Zhu, JASA, 2013) is automated for performing blind sources localization and separation. In particular, source localization results are displayed in terms of space-frequency or space-time correlations. In other words, one can either view distributions of sound sources in three-dimensional (3-D) space versus any user-defined frequency bands, or distributions of sound sources in 3-D space versus time history. The sound pressure level (SPL) values associated with the identified sources are also calculated and displayed in these space-frequency and space-time correlation graphs. To acquire a better understanding of the distribution of sound sources together with their SPL values in 3-D space, a 3-D viewer using Google SketchUp software is employed to view these results in both space-frequency and space-time correlations. With this 3-D viewer, one is able to rotate and look at any source from any perspective, and zoom in and out to examine the details of relative positions of individual sources. The information on source locations together with windowing and filtering technologies enable us to separate the individual source signals. Examples of using this blind sources localization and separations in a non-ideal environment that involves random background noise and unspecified interfering signals are demonstrated.

On a Generalized Multipole Expansion with Application to Propeller Design Synthesis

The radiation of sound by surfaces in motion in a non-uniform flow, including the effects of reflections from obstacles on noise, is specified by an extension of the Kirchhoff integral that leads to a generalized multipole expansion that extends the classical series of spherical harmonics to account for the effects of ( i) the moving medium and ( ii) the rotating sound sources. The corresponding radiation integrals are evaluated analytically for an arbitrary source distribution along the blades of a propeller at an angular inflow. The effects of the incident flow and propeller rotation on the amplitude and phase of sound, e.g. through the retarded time, lead to an extension of the generating function for Legendre polynomials; this provides the representation of the acoustic field as the generalized multipolar series. Each generalized multipole sound field is shown to consist of a fundamental blade passng frequency (BPF) plus all harmonics, with directivities specified by integrals of Bessel functions, that are evaluated analytically. The theory is validated by comparison with noise measurements using two different model propellers in two distinct aeroacoustic wind tunnels. The experimental results are used to illustrate the principle of propeller noise synthesis, relating the radiated sound field to the sound source distribution.

Distribution and propagation of sound sources in the vocal tract

In the current study, an array of 128 microphones is strategically positioned outside a vocal tract model that is placed above an excised canine larynx. Acoustic holography technique is then used to identify the distribution of sound in the vocal tract by using simultaneous measurements from the microphone array. The results show that with no downstream constriction (i.e., open mouth) the energy coming from the higher harmonics is concentrated near the vibrating folds. When downstream constriction is added (i.e., partially closed mouth), the energy in the higher harmonics is shifted further downstream toward the mouth. These results suggest that acoustic holography can be used as a new tool to assess how sound sources propagate in the vocal tract during normal speech and consequently determine the severity of certain speech disorders such as hypernasality and nasal emission.

Identification of the Sectional Distribution of Sound Source in a Wide Duct

덕트 내 음원 면에서의 음압과 입자 속도분포를 상세히 알 수 있다면, 주된 소음원들의 위치와 강도를 분석하여 전파특성을 잘 이해할 수 있고, 이에 따라 저소음화 설계에 유용한 정보로 활용가능하다. 이를 위한 기존의 방법들은 대개 단면상 위치와 무관한 일정 변수로 나타내는 제한점이 있다. 본 논문에서는 음원의 단면 분포를 높은 공간분해능으로 관찰할 수 있는 방법에 대해 연구하였다. 모드 합성법을 기반으로 감쇠파의 영향과 근접장 측정을 포함하는 행렬식을 유도하였으며, 컴프레션 드라이버에 의해 일부 단면이 가진된 유동이 없는 덕트 시스템에서 검증하였다. 감쇠파모드 개수의 증가에 따라 음압 스펙트럼을 더욱 정확하게 근사화 할 수 있었으며, 26개의 감쇠파 모드를 포함한 수렴 결과로부터 관심 헬름홀쯔 수 영역에서 -25 dB 이하의 오차로 예측할 수 있었다. 수렴된 모드 진폭들을 이용하여 kR = 1에서 음원 면에서의 음원변수 분포를 관찰한 결과, 실제 음원이 설치된 국소 위치에서 높은 음압과 입자 속도 값을 분명히 나타내는 것을 보였다. 또한, 감쇠 모드의 역추산시에 정규화기법을 도입하여, 과결정된 반경방향 모드에 의해 발생된 무의미한 피크들을 효과적으로 제거할 수 있었다. If one identifies the detailed distribution of pressure and axial velocity at a source plane, the position and strength of major noise sources can be known, and the propagation characteristics in axial direction can be well understood to be used for the low noise design. Conventional techniques are usually limited in considering the constant source characteristics specified on the whole source surface; then, the source activity cannot be known in detail. In this work, a method to estimate the pressure and velocity field distribution on the source surface with high spatial resolution is studied. The matrix formulation including the evanescent modes is given, and the nearfield measurement method is proposed. Validation experiment is conducted on a wide duct system, at which a part of the source plane is excited by an acoustic driver in the absence of airflow. Increasing the number of evanescent modes, the prediction of pressure spectrum becomes further precise, and it has less than -25 dB error with 26 converged evanescent modes within the Helmholtz number range of interest. By using the converged modal amplitudes, the source parameter distribution is restored, and the position of the driver is clearly identified at kR = 1. By applying the regularization technique to the restored result, the unphysical minor peaks at the source plane can be effectively suppressed with the filtering of the over-estimated pure radial modes.

Measurement of sound source distribution around a circular cylinder in a uniform flow by combined particle image velocimetry and microphone technique

This study examined an experimental method for locating aerodynamic sound sources from a bluff body in a stream. The sound sources are evaluated from cross-correlation analysis of the pressure fluctuations on and around the flow field and the sound pressure fluctuations. Measurements are carried out in the sound field emitted from the circular cylinder at Reynolds number Re=4×104. The pressure fluctuations around the circular cylinder are evaluated by measuring the instantaneous velocity field using particle image velocimetry combined with the pressure Poisson equation, while the sound pressure fluctuations are measured simultaneously by a microphone in the far field. The experimental results showed that sound sources were found near the flow separation on both sides of the cylinder and in the near wake. The experimental results were validated by direct measurement of the pressure fluctuations on the cylinder surface using a pressure transducer. The sound sources obtained from the cross-correlation analysis agreed well with those of the direct measurement of pressure fluctuations on the cylinder surface and those around the cylinder in previous studies.

Optimization of Microphone Array Geometry with Evolutionary Algorithm

The complex relationship between array spatial gain pattern and microphone distribution limits the effective use of traditional methods to optimize the element placements of microphone arrays. This paper presents a genetic algorithm (GA) for microphone array optimization problems with the purpose of maximizing beamforming SNR for given possible distribution of sound sources. Functions based on the statistical geometry descriptors are applied as the objective functions of GA. Three optimization experiments involving linear and planar arrays in continuous and discrete design spaces are performed under various acoustic scenes. Results demonstrate that proposed algorithm sorts out these superior geometries with significant SNR improvement over randomly generated arrays and regular arrays. High successful rate, rapid convergence speed, and fast processing time observed in all the experiments demonstrate the feasibility of this algorithm as a practical tool for microphone array optimization.

Image Processing-Based Technique for Computation of Axial Sound Source Distribution in a Chocked Screech Jet

A proposed technique to characterize the acoustic and flow fields during screech noise emission of a screech sonic jet is presented. Time-resolved visualization of the flow and acoustic fields under investigations are obtained using a high-speed schlieren optical system. Digital image processing of the acquired visualization is conducted and information on both the acoustic and flow fields is subsequently derived with Fourier analysis. The amplitude and phase of the screech tone in the near field are used to compute the screech-producing sound source distribution along the jet axis using the point source model proposed by Lee and Westley . The technique enables simultaneously the measurements of the screech tone frequencies emitted from the jet and characterization of the associated acoustic and flow fields.

A Simple Model for Study of Sound Generation Mechanism in Mixing Layers

Applying Lighthill's acoustic analogy equation to temporally-developing mixing layers, we derived a direct relation between the near-field dynamics, in terms of pressure work, and the far-field sound. The sound radiation predicted by the new model was compared to the direct numerical simulation result, and they agreed well in all stages of vortex dynamics development in mixing layers: roll-up, pairing, merging, saturation, and viscous damping. Using the new formulation, we explained the mechanism for sound generation from the considered flow, and depicted general topological structures for the distribution of sound sources. Though the dynamics is different for vortex roll-up and vortex pairing, the same mechanism for sound generation is suggested by similar topological structures of the sound sources defined here.

A comparative study of high-speed train noise prediction models

The present train noise prediction model in Chinese standards (HJ 453-2008 and HJ 2.4-2009) is only aimed for conventional train. Aerodynamic/rolling noise contribution of high-speed train noise are different from those of conventional train noise, thus, the Chinese prediction model might not be adaptable for the prediction of high-speed train noise. The aim of this paper is to discuss the noise prediction models of high-speed train above 300 km/h. Based on the comparative analysis of different models, our work mainly focus on noise source identification. Firstly the sound pressure level distribution on the surface of high-speed train body is investigated. In order to overcome the shortage of single line source, the noise source is divided into three line sources of different heights parallel to each other according to the energy distribution of sound source. Then sound pressure level generated by the three line sources at the receiving point is calculated separately. Finally the predicting outcomes are compared with measured results to verify the reliability of the model.

G100044 等価的音源同定に基づく音場推定法開発のための基礎研究

Under severe acoustic environments such as rocket launching sites, we need to pay particular attention to sound pressure field. However, no numerical method is currently available enough for accurate mapping of sound pressure level especially in case of large sound field due to unclear location and distribution of sound sources. It is then desired to develop new methods to realize accurate estimation of sound pressure field. In this paper the authors propose a new estimation method for the technical desire. The method is basically built up based on the boundary element method and a new equivalent sound source identification for modeling actual sound sources. Furthermore, since the number of the velocity parameters to be determined is more than that of sound pressure measurement points in practice, the singular value decomposition method is used in the solution process of the inverse problem in the identification. Experiment is also carried out for verification using sound of 300Hz and 600Hz in a semi-anechoic camber. As a result, it is proved that the estimation method has enough potential ability to estimate acoustic fields with a practical accuracy.

Evolutionary Algorithm for Microphone Array Optimization

This paper presented a genetic algorithm (GA) to optimize element placements of microphone array with the purpose of maximizing beamforming SNR for given possible distribution of sound sources. A function based on relationship between statistical geometry descriptors and array performance matrices was applied as the objective function of GA. Optimization experiments of 1D linear array and 2D planar array were performed to demonstrate that this algorithm can effectively sort out superior arrays with significant SNR improvements over randomly generated arrays and regular arrays. High successful rate, rapid convergence speed, and fast processing time observed in all the experiments demonstrate the feasibility of this algorithm as a practical tool for microphone array design.

Spatial Localization of Concurrent Multiple Sound Sources Using Phase Candidate Histogram

Mobile robots communicating with people would benefit from being able to detect sound sources to help localize interesting events in real-life settings. We propose using a spherical robot with four microphones to determine the spatial locations of multiple sound sources in ordinary rooms. The arrival temporal disparities from phase difference histograms are used to calculate the time differences. A precedence effect model suppresses the influence of echoes in reverberant environments. To integrate spatial cues of different microphones, we map the correlation between different microphone pairs on a 3D map corresponding to the azimuth and elevation of sound source direction. Results of experiments indicate that our proposed system provides sound source distribution very clearly and precisely, even concurrently in reverberant environments with the Echo Avoidance (EA) model.

Prediction of aeroacoustic sound using the flow field obtained by time-resolved particle image velocimetry

Based on vortex theory, we experimentally and directly predict sound sources distributing in the flow field and determine the sound pressure level as a result of the spatial integration of sound sources. In employing this direct evaluation method for the aeroacoustic sound, the problem is that a large integration area is required to minimize errors caused by the sudden truncation of the integration area; we overcome it by adopting and applying a modified formula that neglects the quadrupole sound under the condition that the dipole sound is dominant at a low Mach number. Through the flow field measurement using a time-resolved particle image velocimetry (TR-PIV) technique, we will clearly demonstrate the feasibility of our method and the distribution of dipole sound sources in the vicinity of a body even if a comparatively small integration area must be taken. In this basic study, a circular cylinder with a diameter of 6.0 mm is used; the spatially integrated sound pressure is compared with the actual sound pressure which is measured with a microphone. Further, the sound sources evaluated using only the flow field are determined, which give us detailed information about the amplitude and phase of the sound source structure. This direct evaluation method for the dipole sound is applicable to a more complex body.

Flow field detection using acoustic travel time tomography

Acoustic travel time tomography uses the dependence of sound speed on temperature and flow properties along the propagation path to measure these parameters. An algorithm is introduced that is capable of resolving the two-dimensional flow field within a specified measuring area. Different flow fields have been simulated to explore the reconstruction properties of the algorithm. Furthermore, an experiment within the Barrel of Ilmenau has been carried out, which demonstrates that the acoustic tomographic method is able to detect flow fields in a closed convection chamber. The simulations reveal that the vector tomographic algorithm can resolve the flow field characteristics speed and direction. Furthermore it is demonstrated that the distribution of sound sources and receivers has to be homogeneous around the measuring site. For the reconstruction of flow distributions, this requirement is even more important compared to temperature reconstructions, since information along inclined sound ray paths within each grid cell are needed to reconstruct the flow vector properly.

Sound source imaging of low-flying airborne targets with an acoustic camera array

Two-dimensional images of sound source distribution from near-ground airborne sounds are created using an array of 32 microphones and time-domain beamforming. The signal processing is described and array configurations spanning a square area with a side length of 3.45 m, approximately five wavelengths for a 500 Hz sound, are examined. Simulations of a 32-element under-populated log 6 × log 6 spaced array are given for sound sources centered over the array at 250 Hz, 500 Hz, and 1000 Hz. Stochastically optimized array geometry with a simulated annealing algorithm is discussed and a 32-element array optimized for a 500 Hz source is given along with a simulated image for direct comparison with the log 6 spaced array. Images from field testing a 32-element under-populated log 6 × log 6 spaced array are provided for a small aircraft flyover. Results show that this type of acoustic camera generates accurate images of sound source location. Suggested uses include monitoring small aircraft flying too low to be detected by radar as well as monitoring ecological events, such as bird migration.